Embed Size (px)
Citation preview
ASSESSMENT OF SOIL QUALITY FOR
ORGANIC COCOA CULTIVATION IN
SOUTHERN SAO TOMÉ
Aantal woorden: 29.082
Erika Buggenhout Stamnummer: 01106960
Promotor: Prof. dr. Geert Baert
Copromotor: Prof. dr. Kathy Messens
Masterproef voorgelegd voor het behalen van de graad master in de richting Master of Science in de
biowetenschappen: land- en tuinbouwkunde
Academiejaar: 2017-2018
“De auteur en de promotor geven de toelating deze scriptie voor consultatie beschikbaar te
stellen en delen van de scriptie te kopiëren voor persoonlijk gebruik. Elk ander gebruik valt
onder de beperkingen van het auteursrecht, in bijzonder met betrekking tot de verplichting de
bron uitdrukkelijk te vermelden bij het aanhalen van resultaten uit deze scriptie.”
“The author and the promoter give permission to use this thesis for consultation and to copy
parts of it for personal use. Every other use is subject to the copyright laws, more specifically
the scource must be extensively specified when using the results from the thesis.”
24th of august 2018
The author The promoter
Abstract
São Tomé has a rich history with cocoa and might have a promising future with this cash crop
as well, since cocoa demands are globally increasing. This study investigated the potential
suitability for a cocoa plantation in the south of the tropical island São Tomé. Equally the
possibilities for agroforestry and organic cocoa were examined. A soil and vegetation survey
was performed in the study area. The obtained data were compared to the crop requirements
for cocoa, using land evaluation methods to assess the suitability of the area. Also local
climate data were evaluated. The study revealed that large regions where found unsuitable due
to mainly slope, soil depth and stoniness. About one third (150.49 ha) of the initial study area
was found to be potentially suitable for a cocoa agroforestry plantation. This means that
physical characteristics are sufficient (although there are some obstacles) and chemical
shortcomings can be corrected. The very humid climate might be a challenge with regards to
fungal diseases.
Keywords: cocoa, São Tomé, soils, land evaluation
Samenvatting
São Tomé heeft een rijke cacao geschiedenis en heeft misschien ook een belangrijke toekomst
met deze cash crop, aangezien de vraag naar cacao wereldwijd stijgt. Deze studie onderzocht
de mogelijke geschiktheid voor een cacaoplantage in het zuiden van het tropische eiland São
Tomé. Er werd ook gekeken naar de mogelijkheden voor agroforestry en biologische cacao.
Een bodem- en vegetatie onderzoek werd uitgevoerd in het studie gebied. Daarna werden de
bekomen data vergeleken met de gewas behoeften voor cacao, door middel van land evaluatie
technieken uiteindelijk de geschiktheid van het gebied te bepalen. Ook de lokale klimaat data
werden geëvalueerd. Het onderzoek toonde dat grote gebieden ongeschikt waren, meestal
door te steile helling, ondiepe bodem of te stenig. Ongeveer een derde (150,49 ha) van het
originele gebied werd als potentieel geschikt omschreven, voor een cacao agroforestry
plantage. Dit houd in dat de fysieke eigenschappen toereikend zijn (al zijn er nog steeds
enkele obstakels) en de chemische tekortkomingen kunnen gecorrigeerd worden. Het erg
vochtige klimaat kan mogelijks een uitdaging zijn in het gevecht tegen schimmelziektes.
Kernwoorden: cacao, São Tomé, bodems, land evaluatie
Preface
With this master dissertation I will finalise my degree of Master of Science in biosciences:
agriculture and horticulture, major Tropical plant production. I did not accomplish this on my
own, and therefore I wish to express my sincere gratitude to the people who made this
possible and helped me along the way.
First of all I am very grateful to SOCFIN for giving me this amazing opportunity to help in
one of their projects. Thank you Regis Helsmoortel for your support and for making this
interesting period on São Tomé possible. Special thanks to Fanny Roussel for teaching me
various important things, and Claire Houssiau for coaching me along the way. My adventure
on the tropical island wouldn’t have been as fantastic without the people from Agripalma.
Thank you José Cortez Pereira for your wonderful hospitality, and Vitor and Pedro for the
entertaining company during my stay. I would also like to thank Amancio, for being my
partner in crime in the field, for the help during navigation and sampling and for sharing with
me bits of his great knowledge of the area and the vegetation.
A very sincere thank you goes to the person who made this dissertation possible in the first
place: Prof. Dr. Geert Baert. Your accessibility, prompt replies, useful advise and the
encouragements made the process of this master dissertation a lot more agreeable. Thank you
for your well-timed help when things went less smooth and guidelines when things weren’t
very clear.
Thank you Maud Buggenhout for proofreading this work and for all the other tips and tricks
you gave me during my studies.
At this moment, nothing is more appropriate than a major thank you to my parents, who have
been a vital support throughout my entire education. Thank you for your unconditional faith
in my capacities. One thing is certain: without them, I would not be here.
Last but not least I would like to thank the some special friends who kept supporting me and
helped me through the harder periods. With Lisa and Helena cheering on the sidelines, I
accomplished more than I initially thought I had inside me. Thanks girls. And of course thank
you Sam, for your corrections, but foremost for your patience and care. It might sound a little
cliché, but I couldn’t have done this without you.
Erika Buggenhout
4
Table of contents
1 Introduction ......................................................................................................................... 8
2 Literature review ............................................................................................................. 10
2.1 São Tomé ................................................................................................................... 10
2.1.1 Location and climate .......................................................................................... 10
2.1.2 Vegetation .......................................................................................................... 11
2.1.3 Soils .................................................................................................................... 11
2.1.4 History of cocoa production in Sao Tomé ......................................................... 13
2.2 Cocoa ......................................................................................................................... 15
2.2.1 Distribution and production area ........................................................................ 15
2.2.2 Origin ................................................................................................................. 16
2.2.3 Morphology ........................................................................................................ 17
2.2.4 Genetic varieties ................................................................................................. 18
2.2.5 Pests and diseases ............................................................................................... 19
2.2.6 Crop requirements ............................................................................................. 22
2.2.6.1 Climatic requirements for cocoa cultivation .................................................. 22
2.2.6.2 Soil and landscape requirements for cocoa cultivation .................................. 24
2.2.7 Nutrient requirements ......................................................................................... 30
2.2.8 Shade .................................................................................................................. 36
2.2.9 Agroforestry and organic cocoa ......................................................................... 38
2.2.9.1 Cocoa agroforestry ......................................................................................... 38
2.2.9.2 Organic cocoa ................................................................................................ 40
2.2.10 Establishment of a cocoa plantation ................................................................... 43
2.2.10.1 Suitable shade trees ..................................................................................... 44
2.2.10.2 Planting densities & yields ....................................................................... 45
2.2.11 Cadmium restrictions ......................................................................................... 45
3 Materials and methods ...................................................................................................... 47
3.1 Agripalma, oil palm business on Sao Tomé .............................................................. 47
3.2 Location of the research area ..................................................................................... 48
3.3 Field work .................................................................................................................. 48
3.3.1 Soil observation by augering .............................................................................. 48
3.3.2 Soil observation in soil profiles .......................................................................... 49
3.3.3 Vegetation .......................................................................................................... 51
3.4 Laboratory analyses ................................................................................................... 52
3.4.1 Routine soil analyses .......................................................................................... 52
5
3.4.2 Cadmium ............................................................................................................ 52
3.5 Data processing.......................................................................................................... 53
4 Results and discussion ...................................................................................................... 54
4.1 Climate suitability .................................................................................................... 54
4.1.1 Climatic data ...................................................................................................... 54
4.1.2 Climatic evaluation for cocoa cultivation in study area ..................................... 56
4.2 Soil and landscape suitability .................................................................................. 60
4.2.1 Soil and landscape data ...................................................................................... 60
4.2.2 General evaluation soil profiles and composite samples ................................... 60
4.2.2.1 Chemical soil characteristics .......................................................................... 62
4.2.2.2 Physical soil characteristics ............................................................................ 66
4.2.2.3 Landscape characteristics ............................................................................... 70
4.3 Fertilization in organic cocoa plantation ................................................................... 71
4.3.1 Introduction ........................................................................................................ 71
4.3.2 Organic materials ............................................................................................... 73
4.3.3 Agrominerals ...................................................................................................... 77
4.4 Evaluation of the vegetation .................................................................................... 80
4.4.1 Tree density and canopy openness ..................................................................... 81
4.4.2 Biodiversity ........................................................................................................ 83
4.4.3 Tree height (and strata) ...................................................................................... 86
4.5 Cadmium restrictions ............................................................................................... 90
4.6 Overall evaluation of the potential suitability for cocoa agroforestry ...................... 92
5 Conclusions and recommendations ................................................................................... 95
6 Bibliography ..................................................................................................................... 97
7 Appendix ......................................................................................................................... 105
6
List of figures
Figure 1: Flowchart of land evaluation ................................................................................................... 8
Figure 2: Topographic map of São Tomé.............................................................................................. 10
Figure 3: Precipitation map of São Tomé (Aguilar, 1997) .................................................................... 11
Figure 4: Geological sketch map of Sao Tomé island (source: Caldeira & Munha, 2002) ................... 13
Figure 5: Merchantable cocoa production per year in São Tomé (adapted form Aguilar, 1997) .......... 14
Figure 6: Main cocoa-producing countries in the world (map from ICCO) (Hartemink, 2005) ........... 15
Figure 7: Production in tonnes of the top 10 cocoa bean producing countries (average 1994 – 2016)
(FAOSTAT, 2017) ................................................................................................................................ 16
Figure 8: Evolution of cocoa bean production, period 2007-2017 (ICCO, 2017) ................................. 16
Figure 9: Schematic representation of the cocoa tree, excluding further branching and leaves (van
Vliet & Giller, 2017) ............................................................................................................................. 17
Figure 10: Cocoa tree with pods and beans a pod ................................................................................. 18
Figure 11: Black pod disease (left), witches’ broom disease (middle) and cocoa pod borer (right) ..... 20
Figure 12: Different types of root distribution depending on the nature of the subsoil and height of
water table ............................................................................................................................................. 27
Figure 13: Simplified nutrient cycling diagram for cocoa ecosystems (Hartemink, 2005) ................... 31
Figure 14: Results of application of fertilizers under different degrees of shade (Ahenkorah et al.,
1987)...................................................................................................................................................... 37
Figure 15: Schematic diagram of the different vegetation layers or strata in agroforests (Bieng et al.,
2013)...................................................................................................................................................... 40
Figure 16: Design 20 years after the installation of cocoa agroforest on previous high density oil palm
plantation, in savannah region, Cameroon (Jagoret et al., 2012) .......................................................... 44
Figure 17: Map of Agripalma and the research area ............................................................................. 47
Figure 18: Location of the soil observation points and of the vegetation plots. Observation points were
grouped per composite sample. ............................................................................................................. 49
Figure 19: Location of profile pits ........................................................................................................ 50
Figure 20: Soil profile, pit 6. The A, B and C horizon are indicated .................................................... 51
Figure 21: Location of the weather stations Dona Augusta and Porto Alegre, and the research area in
between .................................................................................................................................................. 54
Figure 22: Monthly precipitation at the weather station of Agripalma (Ribeira Peixe) ........................ 55
Figure 23: N/P2O5 ratio of some cocoa plantations in Ghana in comparison with the optimum (Source:
Snoeck et al., 2016) ............................................................................................................................... 65
Figure 24: Texture of the soil profiles (red for topsoil, green for subsoil) and composite samples (blue)
............................................................................................................................................................... 66
Figure 25: Spatial distribution of soil depth observed in the augerings ................................................ 68
Figure 26: Spatial distribution of soil stoniness observed in the augerings .......................................... 69
Figure 27: Weathered basalt fragments present in soil ......................................................................... 70
Figure 28: Landscape position of profile 3: steep slope and rather high surface stoniness ................... 70
Figure 29: Slope classes measured at observation points during survey ............................................... 71
Figure 30: Flow chart of fresh oil palm fruit bunch processing (solid boxes) showing points of
generation of wastes (dashed boxes) (Anyaoha et al., 2018) ................................................................ 75
Figure 31: Impact of chicken manure on cocoa production (Ruf, 2016)............................................... 76
Figure 32: Geological map of the study area and surroundings (extract of Geological map of Sao
Tome, sheet 5, on scale 1:25000) .......................................................................................................... 78
Figure 33: Phonolite chimney in Sao Tomé (Cão Grande) ................................................................... 79
Figure 34: Location of the investigated vegetation plots ...................................................................... 80
Figure 35: Legend of the vegetation of the map (1958) used as background for the map in Figure 34 81
Figure 36: Scatter plots of tree density, canopy openness and amount of big trees (DBH > 50 cm) .... 82
Figure 37: Picture of the canopy in plot 6, plot 1, plot 3 ....................................................................... 83
7
Figure 38: Cumulative curve of plant species associated with cocoa plantations in southern Cameroon
(Sonwa et al., 2007) ............................................................................................................................... 84
Figure 39: Salla salla in plot 1(upper left) and shrub layer plot 7, plot 2 and plot 6 ............................. 85
Figure 40: Individual height of the 406 trees measured in 9 vegetation plots. ...................................... 86
Figure 41: Trees per plot divided in three strata ([0-10], [11-25] and [26-40]). ................................... 87
Figure 42: Examples of streams crossing suitable areas ....................................................................... 93
Figure 43: Example of severe surface stoniness and boulders (right) ................................................... 93
Figure 44: Suggestion of potentially suitable areas based on the suitability classes per observation
point. When a point is not suitable, the reason is indicated................................................................... 94
List of tables
Table 1: Climatic requirements for cocoa (Sys et al., 1993) ................................................................. 23
Table 2: Climatic requirements for cocoa (Ritung et al., 2007) ............................................................ 24
Table 3: Soil and nutrient requirements for growth of cocoa (Sys et al., 1993; Ritung et al., 2007),
when values were different data from Ayorinde et al. (2014) were added............................................ 24
Table 4: Rating of fertility levels for cocoa in top 15 cm of soil (Landon, 2013) ................................. 29
Table 5: Nutrient inputs into and removal from cocoa systems (source: van Vliet & Giller, 2017) ..... 32
Table 6: Concentrations and amounts of nutrients in the litter fall and standing litter of cocoa and
shade trees combined (source: van Vliet & Giller, 2017) ..................................................................... 33
Table 7: Nutrient removal in crop of 1 t ha-1 dry cacao beans (7% humidity) with 1.4 t ha-1 husks
IFA, 2008) ............................................................................................................................................. 34
Table 8: Rates of application of nutrients to cocoa (in van Vliet & Giller, 2017) ................................ 35
Table 9: Conventional and organic cocoa cultivation practices (Djokoto et al., 2016) ......................... 41
Table 10: Pluviometric data .................................................................................................................. 55
Table 11: Climatic data, collected at latitude: 0.113°, longitude: 6.580°, elevation: 197 m ................. 56
Table 12: Climatic requirements for cocoa(Sys et al., 1993) ................................................................ 56
Table 13: Climatic requirements for cocoa (Ritung et al., 2007) .......................................................... 57
Table 14: Suitability of soil and landscape for soil profile data ............................................................ 60
Table 15: Soil and landscape data of soil profiles ................................................................................. 61
Table 16: Soil data of composite samples ............................................................................................. 61
Table 17: cation ratios of the profiles .................................................................................................... 63
Table 18: cation ratios of composite samples ........................................................................................ 63
Table 19: Liming materials ................................................................................................................... 73
Table 20: Chemical nutrient composition of the coffee husk compost after three months composting
(Nguyen et al., 2013). ............................................................................................................................ 74
Table 21: Properties of shredded EFB, POME and final compost (Baharuddin et al., 2009) ............... 76
Table 22: The canopy openness, tree density and amount of big trees per plot .................................... 82
Table 23: Biodiversity parameters for the nine plots (30 m x 30 m) ..................................................... 83
Table 24: Vernacular and scientific names of the trees found in the nine vegetation plots, and their use
............................................................................................................................................................... 88
Table 25: Cadmium contents (mg.kg-1) of sedimentary and igneous phosphate rocks (Van
Kauwenbergh, 2001) ............................................................................................................................. 91
8
1 Introduction
Cocoa is an important cash crop in tropical countries all over the world. On the African
archipelago São Tomé and Principe the cocoa tree and it’s valuable fruits played an
important role the last two centuries. During colonial times large cocoa plantations, locally
called ‘roças’, where spread all over the islands. In the beginning of the 20 th century the
small country was temporarily the world’s largest cocoa exporter. Various factors caused
a decline of the cocoa production, and nowadays the island’s product ion is only a fraction
of what it once was. It is however still one of the most important export products today. A
revival of the cocoa industry could contribute to the improvement of the country’s
economy.
This research was carried out at the request of SOCFIN to investigate the potential of cocoa
cultivation in the Southern part of the island of São Tomé. The objective of this research
was to provide a suitability analyses for the establishment of an organic agroforestry cocoa
plantation. The research focussed on an delimited area in the southern part of the island.
Using land evaluation methods (Figure 1), different parameters were assessed. The soil
quality assessment is the main part in this study. After all, a suitable soil is paramount for
agriculture. Since there was a certain interest in organic cocoa and agroforestry, an
additional vegetation survey was performed.
Figure 1: Flowchart of land evaluation
9
First a literature review on the principal subjects was conducted. This chapter also informs
on the crop requirements, crucial for land evaluation. After a description of the used
materials and methods, the results of the fieldwork and laboratory analyses are presented.
Each part the results is immediately discussed and evaluated. One by one climatic, soil,
landscape and vegetation data are examined and compared to the suitable values for cocoa
agroforestry. Because there are restrictions on cadmium content in the cocoa food chain, a
special chapter is devoted to cadmium concentrations. This section concludes with an
overall evaluation of the indicated study area, putting all evaluated components together.
This results in a map showing suitable areas, complemented with remarks and the
indication of potential issues and difficulties. Lastly, in the chapter conclusions and
recommendations, the most important findings are summarized together with suggestions
for further research on this specific subject.
10
2 Literature review
2.1 São Tomé
2.1.1 Location and climate
The island of São Tomé (total area of 854 km2, population +/- 180.000) is located in the Gulf
of Guinea, about 250 km east of Gabon, just above the equator. It’s the largest island of the
Archipelago São Tomé and Príncipe. It is a volcanic island, and part of the Main Cameroon
Line volcanic chain (Becker, 2014; FAOSTAT, 2017). The geological substrate consists almost
exclusively of volcanic rock, and basalt is the most common parent material.
Figure 2: Topographic map of São Tomé
As the island is located close to the equator, the climate is mostly tropic and humid. However,
the mountainous relief (see Figure 2) causes the climate to vary from semi-arid in the north to
super-humid in the south. This is mainly caused by the difference in annual rainfall, which is
three times higher in the centre and south-east than in the north (Verheye, 1986; Aguilar, 1997)
(Figure 3). The amount of annual sunshine is 1760 hours in the capital and decreases with
altitude (Aguilar, 1997). The average relative humidity across the island is about 84% with little
differences between seasons or regions (Verheye, 1986; Aguilar, 1997).
11
Figure 3: Precipitation map of São Tomé (Aguilar, 1997)
2.1.2 Vegetation
The natural vegetation of the southern region is dense tropical forest. Only on the wettest and
most rugged areas this forest remains undisturbed. The well-preserved forest area is currently
buffered by large parts of secondary forest. These secondary forests are the result of abandoned
logging and plantations. When the agricultural activities stop, a rapid regrowth takes place.
These secondary forests are in turn surrounded for the most part by plantations, combined with
shade trees. Here, the tropical forest has been replaced with industrial crops like cocoa, coconut,
oil palm and coffee in most of the coastal areas, up to an altitude of 300 m (de Lima et al., 2014;
Verheye, 1986).
2.1.3 Soils
According to Aguilar (1997) the soils of Sao Tomé are in general close to optimum quality, in
both physical (depth and drainage) and chemical terms (organic matter, absorbing complex,
exchangeable bases, nitrogen content, acidity). Many kinds of crops can be grown, given the
diversity of climate and richness of the soils. FAO mentions cocoa, banana, coconut, oil palm,
maize, taro and yam as the crops with the highest production area on the island (FAOSTAT,
2017).
12
A more thorough research on the Sao Tomean soils has been performed by Cardoso & Garcia
(1962). The main rocks are basalts, phonolites, trachytes, andesites, tephrites and volcanic tuffs.
Major soils types in Sao Tomé are (Cardoso & Garcia, 1962):
Lithosols and regosols (same names in WRB-2015 (IUSS Working Group WRB. 2015)):
young generally very gravelly soils, mostly located on steep slopes. Often rock and laterite
outcrops occur on steep slopes.
Alluvial (Fluvisols, Gleysols WRB-2015), black tropical (Vertisols, WRB-2015) and dark
brown tropical soils (Cambisols-2015): young, generally deep very fertile soils, variable
texture from very sandy to very clayey, drainage from poorly drained to well drained.
Hydromorphic soils groups de very poorly drained soils located in marshes.
Red or yellow tropical fersiallitic soils (Luvisols WRB-2015): deep, often gravelly soils,
with loamy to clayey texture, slightly acid to neutral pH, high content of basic cation and
high base saturation.
Red or yellow ferrallitic soils (Lixisols, Nitisols WRB 2015): are similar to the fersiallitic
soils, but more weathered (mainly 1:1 clay minerals, Fe-oxides and gibbsite), acid and with
lower content of basic cations (low base saturation). Those soils mainly occur in the southern
part of Sao Tomé, with very high rainfall of over 4000 mm y-1 (Figure 3).
The soils in the study area are mostly lithosols, dark brown tropical soils and ferralitic soils,
with varying gravel content.
Detailed information of the soils of Sao Tomé can be consulted in following links, texts are in
Portuguese:
Cardoso, J. C., & Garcia, J. S. (1962). Carta dos solos de Sāo Tomé e Príncipe: Junta de Investigações
do Ultramar: http://library.wur.nl/isric/fulltext/isricu_i3755_001.pdf
Soil map: http://www.jeffginger.com/old/CIPS/saotome/maps/geography/AgZonesComposite.jpg
13
Figure 4: Geological sketch map of Sao Tomé island (source: Caldeira & Munha, 2002)
2.1.4 History of cocoa production in Sao Tomé
The Island was discovered by Portuguese navigators in 1471. The São Tomé island was at that
moment completely forested and there were no signs of human settlements. In the 16th century,
14
large zones in the coastal areas were cleared to grow sugar cane. In 1822 cocoa was introduced
to the island of Principe and in 1855 to the São Toméan island (Eyzaguirre, 1986; Pape &
Rebelo de Andrade, 2013). By the mid-19th century, much of the forest, especially in the
lowlands, was replaced by shade cocoa plantations (Eyzaguirre, 1986).
Cocoa production increased from the introduction of the plant onwards until the beginning of
the 20th century. In 1913 São Tomé reached a production of 36 500 tons of cocoa and became
the world’s biggest cocoa exporter. After a peak during the 1920’s the production collapsed due
to historical, political and agricultural causes. One of them was the devastation of plantations
by the cocoa thrips (Selenothrips rubrocintus). As shown in Figure 5 production keeps dropping
until the 1980’s and is more or less stable afterwards (Aguilar, 1997; Pape & Rebelo de
Andrade, 2013).
Figure 5: Merchantable cocoa production per year in São Tomé (adapted form Aguilar, 1997)
In 1975 the island became independent. Under the new socialist government the 26 colonial
plantations were nationalized into 15 state-owned firms (Roche & Dulcire, 2007). After the
independence a lot of the Portuguese plantation owners left, together with foreign agricultural
workers which led to a lack of managerial staff, qualified labourers and thus know-how
(Aguilar, 1997). Nevertheless cocoa is still the most important commodity for the São Tome
and Principe economy with a share of 80% of the agricultural exports and contributing to the
subsistence of many people living in rural areas (Aguilar, 1997; Orlandi, 2011). The mean
annual production of cocoa beans in Sao Tomé and Principe from 2004 to 2014 was 2320,9
tons (FAOSTAT, 2017).
0
10000
20000
30000
40000
50000
60000
18
80
19
00
19
19
19
30
19
50
19
60
19
65
19
70
19
74
19
75
19
82
19
85
19
86
19
87
19
88
19
89
19
90
19
91
19
92
19
93
19
94
19
95
Pro
du
ctio
n in
to
ns
Merchantable cocoa production
15
2.2 Cocoa
2.2.1 Distribution and production area
Cocoa is mainly produced in what is called ‘the cocoa belt’. This ‘belt’ is situated within 10°N
and 10°S of the equator, as this is where the main cocoa producing countries are situated (Figure
6). This distribution is governed by the climatic and environmental requirements of the cocoa
tree.
Figure 6: Main cocoa-producing countries in the world (map from ICCO) (Hartemink, 2005)
The top 10 cocoa bean producing countries are presented in Figure 7. In 2016 world’s leader in
cocoa production was Ivory Coast with a production of 1 472 313 tonnes of cocoa beans,
followed by Ghana and Indonesia with 858 720 and 656 817 tonnes respectively (FAOSTAT,
2017).
As cocoa is the main ingredient in chocolate and the worldwide demand for chocolate is rising,
the cocoa production is also increasing over the past years, as illustrated in Figure 8. Total
annual world production of cocoa beans is approaching 5 million tons.
16
Figure 7: Production in tonnes of the top 10 cocoa bean producing countries (average 1994 – 2016)
(FAOSTAT, 2017)
Figure 8: Evolution of cocoa bean production, period 2007-2017 (ICCO, 2017)
2.2.2 Origin
Cocoa originates from the South American Amazon and Orinoco basins, on the eastern slopes
of the Andes Cordillera and in the equatorial regions (Van Himme & Snoeck, 2001). The
cultivation of cocoa goes back to ancient times. Mayas, as well as the Aztecs and Toltecs would
have cultivated it 1500 years ago in Central America (Motamayor et al., 2002). In the 16th
century the cultivation of cocoa started to spread and the fruit was also brought to Europe
(Spain) by Cortés. For three centuries cocoa production mainly took place in South and Central
America and the West Indies. After introducing cocoa to the islands of Bioko, Principe and Sao
Tomé the first cocoa beans were planted on continental Africa by the end of the 19th century.
From then on cocoa rapidly expanded in Africa and since the end of the First World War, West
Africa has dominated the cocoa market. Unlike Congo, Liberia and the islands of São Tomé
17
and Príncipe, where cocoa production has essentially been the domain of large companies,
almost all of West Africa’s huge cocoa output derives from smallholder farms. For them, cocoa
is an important cash crop (Van Himme & Snoeck, 2001).
2.2.3 Morphology
Cacao (Theobroma cacao), from the family of the Malvacea is economically the most important
species in the genus Theobroma (Vanegtern et al., 2015). It was, originally, an understorey
rainforest tree. The tree grows only 4 to 6 m in height when cultivated. The seedlings grow 1
to 2 m as a single stem and then split into a jorquette (Figure 9). A jorquette is when the stem
stops developing vertically and grows into a whorl of 3 to 5 branches. These branches have a
plagiotropic habit, whereas the stem and the suckers or “chupons” have a orthotropic growth
habit (Van Himme & Snoeck, 2001). The root system consists of a large taproot of 0.8–1.5 m
and a lateral root system in the topsoil that accounts for the uptake of nutrients and moisture
(van Vliet & Giller, 2017). Young leaves are naturally pale reddish brown (Sys et al., 1993).
Their production occurs in “flushes”. The flowers appear on flower pads or cushions on the
truck. Under optimal conditions these cushions will continue to produce flowers for 60 to 100
years.
Figure 9: Schematic representation of the cocoa tree, excluding further branching and leaves (van
Vliet & Giller, 2017)
To pollinate the flowers of the cocoa tree a pollinator is required. This is mainly a small fly
Forcipomyia sp. but also other insects like midges and ants (Vanegtern et al., 2015; Van Himme
& Snoeck, 2001). Only about 1,5% of the flowers are pollinated and only 10 to 30% of the
young fruits reach maturity. This is due to spontaneous abortion of young fruits, “cherelle wilt”.
The fruit, called a cocoa pod, contains 30 to 60 beans, covered in with white mucilage that is
high in sugar content (Figure 10). This sugar is needed for the fermentation process that takes
place after harvesting.
18
Figure 10: Cocoa tree with pods and beans a pod
The eventual product sold to the market are dry beans. Thus the harvested cocoa pods need to
be processed. This involves natural fermentation and drying of the beans. Fermentation is an
important step in the process, because it alters the organoleptic properties of the beans (Van
Himme & Snoeck, 2001).
2.2.4 Genetic varieties
It is generally accepted that the upper Amazon basin is the centre of diversity. The large genetic
variation in this area is often used to bring in useful characteristics like resistance to diseases.
In 1944 Cheesman classified all cultivated cocoa into one species, T. cacao. In the population
of this species he suggested there are three main groups: Criollo, Amazonian Forastero and
Trinitario, Trinitario being a cross between the former two. Amazon Forastero trees provide
95% of the world’s cocoa market. Criollo and Trinitario account for a small part but their beans
are collectively known as “fine and flavour”. The Criollo varieties however tend to be more
susceptible to diseases and are not vigorous. Forastero beans have more distinguished cocoa
flavour and a higher fat content. The trees are hardy and robust. Yields are higher and disease
resistance is better (Van Himme & Snoeck, 2001).
Cuatrecasas (1964) divided the two morpho-geographic groups into two subspecies: Forastero
cacao, assigned to Theobroma cacao ssp. sphaerocarpum and Criollo cacao (Theobroma cacao
ssp. cacao) (Motamayor et al., 2002; Wood & Lass, 1985). The research of Motamayor et al.
(2002) shows that the genetic distance of some Forastero individuals are equivalent to some
Forastero and ancient Criollo. They contradict the hypothesis of Cuatrecasas (1964) that Criollo
is a separate subspecies that evolved independently to South American populations in Central
America. The terms Criollo and Forastero are based on the specific pod morphologies, not on
proven genetic differences. Motamayor et al. (2002) states that Criollo probably originated from
a few individuals in South America that may have been spread by man within Central America.
19
According to Wood and Lass (1985) the Forastero group is large and contains cultivated and
wild populations of which the Amelonado populations are the most extensively planted. The
term Amelonado is also used to describe the typical pod shape: smooth, shallow furrows, melon
shaped with a blunt end and slight bottle neck. In spite of its ordinary quality, however,
Amelonado’s high level of homogeneity is much appreciated by manufacturers. There are three
main types within the Amelonado population (Wood & Lass, 1985):
Amelonado: In the State of Bahia, Brazil and in West Africa this is the predominant type. It
is strikingly uniform in all characters. It’s a hardy and productive type with light green and
smooth pods.
Comum: The plant material probably used to start the cocoa industry of Bahia derives from
the lower Amazon region. Until recently, this variety was the dominating type. The Comum
variety has a typical Amelonado shape.
West African Amelonado: This is the cocoa variety introduced by the Portuguese in Sao
Tomé, transferred from Brazil. The variety is similar to the Comum. It’s a relatively uniform
population. The plant is not vigorous and takes long to reach maturity. However, under good
conditions it yields over 3000 kg per ha, which is a very high yielding level. West African
Amelonado is susceptible to swollen root virus and vascular-streak dieback, but less
susceptible to Phytophthora pod rot than for example Trinitarios.
Criollo has a low yield and a high sensitivity to pests and disease. However, its flavour is widely
appreciated, earning higher market prices. The pods are usually pointed with a thin exocarp,
which can either be smooth or warty. This variety is often used in luxury chocolate products
(Van Himme & Snoeck, 2001).
The Trinitario population consists of hybrids which resulted from a cross between Forastero
and Criollo elements. The variety is characterised by the high level of variability due to the
heterogeneity inside the population. The interesting trees combine the vigour and productivity
of Amazonian Forastero with the quality of Criollo (Van Himme & Snoeck, 2001).
2.2.5 Pests and diseases
According to Van Himme and Snoeck (2001) the world’s potential cocoa production is
impaired by an estimated 21% due to diseases and another 25% due to pests. The importance
of pests and diseases differs per region.
The main pests in West Africa are mirids, also called capsids. The sucking insects cause young
pod wilt and rapidly affect yield. Since they also suck on young branchlets, these insects
damage the tree which has an indirect effect on the yield. Greatest damage is found in lightly
or unshaded plantations. Although proper shade management can prevent major pest outbreaks
additional chemical control with insecticides is often needed (Wessel & Quist-Wessel, 2015).
20
Given the major role West Africa plays in cocoa production, mirids can be seen as the pest with
the greatest impact on the world cocoa output (Petithuguenin, 1998). The predominant diseases
in the area are black pod disease and Cocoa Swollen Shoot Virus (CSSV).
Black pod (Figure 11), caused by Phytophthora palmivora and Phytophthora megakarya, is the
main fungal disease of cocoa, not only causing yield losses in West Africa, but also in Brazil
and Asia. The annual crop losses due to this disease may range from 30 to 90% according to
Bowers et al. (2001). The spread of black pod is linked to rainfall and air humidity. The
oospores of Phytophthora spp. survive in the soil, in leaf litter and in the debris of previous
infected pods. From there, the fungus is transmitted by direct contact, rain splash or insects and
eventually spreads to all the pods on the tree. In humid weather, the infected fruits are covered
by a mass of creamy-white sporangiophores, serving as a secondary source of infection.
Penetration of the mycelium is followed by the appearance of spots on the surface of the pod,
which turn brown. Eventually they cover the whole pod. Trunks, branches and roots can also
be attacked by black pod, where it provokes cancers. To control the black pod disease some
rules should be followed, including harvesting the mature pods more frequent, sanitary
harvesting of the infected pods, opening pods outside the plantation to limit sources of infection,
since infected pods are a main source of inoculum. Furthermore shade should be regulated to
improve aeration and reduce air humidity. Infected branches must be removed and cancers must
be cut out and the lesion treated with fungicide. Wessel (2015) states the highest incidence of
Phytophthora pod rot is found in the shaded cocoa in Cameroon. Regular removal of infected
pods and shade reduction to lower the humidity can reduce pod losses to a certain degree but
usually additional chemical control by regular spraying of fungicides is needed. Because the
fungus persists in soil for several years, the control of black pod is difficult (Van Himme &
Snoeck, 2001).
Figure 11: Black pod disease (left), witches’ broom disease (middle) and cocoa pod borer
(right)
21
The swollen shoot virus is a serious problem in West Africa, causing drops in production and
the death of trees. Already huge losses have been recorded. It’s transmitted by feeding female
mealybugs, which previously fed on the sap of an infected plant. It cannot be transmitted by the
seed. The disease cannot be cured and according to Van Himme and Snoeck (2001) there are
no resistant varieties. However, the Upper Amazonian types are less prone to swollen shoot
than the West African Amelonado. To control this disease trees can be treated with systemic
insecticide against the mealybugs and contact insecticides against the ants. Ants protect the
mealybugs and in this way also contribute to the spreading of the disease. Also appropriate
farming methods make cocoa more vigorous and resistant. To stop the virus from spreading, all
infected as well as neighbouring trees should be uprooted (Van Himme & Snoeck, 2001).
The prominent pests and diseases in Southeast Asia are the Cocoa Pod Borer and vascular streak
dieback. The Cocoa Pod Borer, also known as Cocoa Moth (Conopomorpha cramerella),
attacks cocoa pods, resulting in losses of quantity and quality. According to van Himme and
Snoeck (2001) they prey particularly on young cacao trees exposed to full sunlight. They
excavate tunnels in the branches and without intervening, the tunnel will extend to the trunk
causing the entire branch or even the main stem of the young cocoa tree to die. Proper plantation
maintenance and cutting away dead branches counteract the pod borer, as well as systemic
insecticide if necessary (Van Himme & Snoeck, 2001). There are also efforts to develop
resistant material and genotypes with harder walls in their pods (ICCO, 2018).
Vascular streak dieback is a disease caused by the fungus known as Oncobasidium theobroma.
The fungus colonises the xylem, spreads internally to other branches or the trunk, usually
causing the tree to die (ICCO, 2018).
In the Americas witches' broom diseases and frosty pod rot, both caused by basidiomycetes, are
important causes of production losses (Petithuguenin, 1998; van Vliet & Giller, 2017).
Witches’ broom disease is caused by the fungus Crinipellis perniciousa. In the presence of free
moisture (rain and dew) and high relative humidity, basidiospores germinate and penetrate
young meristematic tissues in vegetative and floral buds through stomata, epidermis, or
trichomes. The colonized tissues undergo several physiological and hormonal changes leading
to swelling and the formation of numerous succulent vegetative branches, known as brooms.
These brooms demand energy of the tree, but do not form flowers, thus reduce yield potential.
The fungus also infects pods causing necrotic lesions, uneven ripening, and various
deformations (Bowers et al., 2001). The disease had a large impact in Bahia, Brazil, where the
introduction caused a decrease of 70% during a period of 10 years (ICCO, 2018).
Frosty pod rot (or Moniliophthora pod rot), caused by Moniliophthora roreri, destroys the
cocoa pods. A dense layer of white spores appears on the outside of the pod, one to three months
after infection. To mitigate the fungus, the infected pods should be removed, and in some cases
fungicides are recommended. In addition, some genotypes are less susceptible to the frosty pod
22
rot. Next to genetic improvements, also biocontrol treatments can reduce pod loss (Bowers et
al., 2001; ICCO, 2018).
2.2.6 Crop requirements
2.2.6.1 Climatic requirements for cocoa cultivation
According to the FAO (1966) no conclusions about soil and nutrient requirements can be made,
without considering the climatic conditions (Smythe, 1966).
Climate represents a complex inter-relationship between temperature, rainfall (quantity and
distribution), humidity, cloud cover and wind speed. For the cultivation of cocoa temperature
is the main limiting factor. The optimum annual mean temperature is 25,5°C with a daily range
of 9°C. Also seasonal fluctuations should be small. 15°C is set the monthly mean minimum and
30°C the monthly mean maximum (Sys et al., 1993). Low temperatures inhibit flower formation
and reduce growth (Smythe, 1966). The limits of the altitude at which cocoa can grow is
determined by these temperatures (Wood & Lass, 1985). In general a hot and moist climate will
favour the growth of cocoa (Wood & Lass, 1985).
Cocoa is a very sensitive crop when it comes to water shortage or to excess of water in the soil.
This is closely related with the nature of the soil and the temperature. Evenly distributed rainfall
throughout the year appears to be desirable. According to Sys et al. (1993) an annual total
precipitation between 1600 – 2500 mm, well distributed and no marked dry season is an ideal
situation. In some areas the harvest is, more than by any other ecological factor, regulated by
rainfall. This might be because moisture availability has a direct effect on flower and fruit
formation. In most cocoa growing areas annual precipitation lies between 1250 and 2800 mm.
Below 1250 mm, irrigation is needed (Landon, 2013). When annual rainfall exceeds 2500 mm,
the burden of diseases might become higher, particularly Phytophthora pod rot. High rainfall
can also cause less fertile soils because of heavy leaching (Wood & Lass, 1985). Sometimes
there is a negative correlation between yield an rainfall 2-3 months before harvest, probably
because of increased fungal diseases. It should be stressed that the pattern of the annual rainfall
is more important than the total rainfall (Wood & Lass, 1985).
Atmospheric humidity and wind speed both control the rate of evapotranspiration which has an
influence on soil moisture. High relative humidity seems favourable although not necessary if
soil moisture is adequate. On the other hand, when soil moisture is adequate, very high humidity
can be undesirable since it favours the development of various diseases. According to Sys et al.
(1993), the relative air humidity should be around 80% without significant fluctuations. Higher
relative air humidity may enhance diseases. Having a shallow root system which offers weak
anchorage, cocoa is very susceptible to mechanical damage by wind. Also the leaves are quickly
23
harmed by high wind speeds (Smythe, 1966). This is confirmed by Sys et al. (1993) and Landon
(2013), who state that high wind velocities may causes excessive damage.
Climate also has an influence on the methods used for drying the beans. In West Africa for
example they can harvest and dry the beans during dry season, using the sun to dry the beans.
The advantage here is that this method needs no facilities, making it possible for small-holders
to prepare cocoa of good quality. In other countries, where the crop is harvested during wet
season, artificial dryers are needed (Wood & Lass, 1985).
To evaluate climate (also for landscape and soil, see further) for cocoa cultivation, different
levels of suitability are assigned per variable. The guidelines most used are those of FAO
(1976). They are divided into Order, Class, Subclass and Unit. There are two different Orders
(S for suitable and N for not suitable). For the semi detailed maps (scale 1:25.000-1:50.000) the
S order is divided into the classes (S1) Highly Suitable, (S2) Moderately Suitable and (S3)
Marginally Suitable. For less detailed maps the classes are (S) Suitable, (CS) Conditionally
Suitable and (N) Unsuitable.
In Table 1 and Table 2 the most important climatic variables to grow cocoa are listed and
divided into classes of suitability according to respectively Sys et al. (1993) and Ritung et al.
(2007). For most variables, these two sources correspond, for some others the differences are
small.
Table 1: Climatic requirements for cocoa (Sys et al., 1993)
Climatic characteristics
Climatic class, degree of limitation and rating
0 S1
1 S1
2 S2
3 S3
4 N1 | N2
Annual precipitation (mm)
1800 – 2000 1600 – 1800 2000 – 2500
1400 – 1600 2500 – 3500
1200 – 1400 3500 – 4400
- <1200 - >4400
Length of dry season (months)
0 – 1 1 – 2 2 – 3 3 – 4 - >4
Mean annual temp. (°C)
25 – 28 23 – 25 28 – 29
22-23 29 – 30
21 – 22 -
- <21 - >30
Mean annual max. temp. (°C)
< 28 28 – 30 >30 - - -
Mean annual min. temp. (°C)
>20 15 - 20 13 - 15 10 - 13 - <10
Relative humidity dryest month (%)
45 – 60 40 – 45 60 – 65
35 – 40 65 - 75
30 – 35 75 – 85
<30 - >85 -
24
Table 2: Climatic requirements for cocoa (Ritung et al., 2007)
Climate characteristics
Land suitability class
S1 S2 S3 N
Annual average temp. (°C)
25 – 28 20 – 25 28 – 32
- 32 – 35
<20 >35
Average annual rainfall (mm)
1500 – 2000 - 2500 – 3000
1250 – 1500 3000 – 4000
<1250 >4000
Dry months (month)
1 – 2 2 – 3 3 – 4 >4
Relative humidity (%)
40 – 65 65 – 75 35 – 40
75 – 85 30 – 35
>85 <30
Fasina et al. (2007) used mostly the same data in their climatic requirements, except for rainfall.
They state that 2500 to 3500 mm is highly suitable and 3500 to 4500 mm is still moderately
suitable.
2.2.6.2 Soil and landscape requirements for cocoa cultivation
The major soil and landscape characteristics important for cocoa cultivation are summarized in
Table 3.
Table 3: Soil and nutrient requirements for growth of cocoa (Sys et al., 1993; Ritung et al.,
2007), when values were different data from Ayorinde et al. (2014) were added
Characteristics Land suitability class
S1 S2 S3 N
Rooting conditions (physical soil characteristics)
Texture C<60s, C>60,s
Co, SiCL, CL,
SiL, L, SC
SCL, C<60v C>60v, LfS, SL Cm, SiCm, LS,
LcS, fS, S, cS
Coarse
materials (%)
<15 15 – 35 35 – 55 >55
Soil depth (cm) >100 75 – 100 50 – 75 <50
Wetness soil characteristics
Flooding F0 F1 F2+
Drainage Good, GW >
100 cm
Moderate Imperfect Poor and very
poor
Nutrient retention (soil fertility characteristics)
CEC-clay
(cmol/kg)
>16 ≤16 (-) < 16(+) -
CEC-clay
(cmol/kg)
>24 20 – 24 16 – 20 12 – 16 <12
Base saturation
(%)
>35 20 – 35 <20
25
Base saturation
(%)
>50 45 – 50 40 – 45 35 – 40 <35
Sum of basic
cations
(cmol(+)/kg
soil)
>4 4 – 2.8 2.8 – 1.6 <1.6
pHH2O 6 – 7 5.5 – 6
7 – 7.6
<5.5
>7.6
Organic C (%) >1.5 0.8 – 1.5 <0.8
Nitrogen >0.18 0.15-0.18 0.1-0.15 0.05-0.1 <0.05
Salinity (ds/m) <1.1 1.1 – 1.8 1.8 – 2.2 >2.2
Land (topography)
Slope (%) <8 8 – 16 16 – 30 >30
Slope (%) <4 4 – 8 8 – 16 16 - 20 >20
Surface
stoniness (%)
<5 5 – 15 15 – 40 >40
F0 : not flooded ; F1 : flooding during some time of the year; F2 : more frequent flooding (1 in 2 years)
Textures according to USDA-triangle; C: clay, SC: sandy clay, SCL: sandy clay loam, CL: clay loam, SiC: silty
clay, SiCL: silty clay loam, L: loam, SL: sandy loam, LS: loamy sand, S: sand (s = structured, m = massif,
o = Oxisol structure; v = vertisol structure, f = fine, c = coarse).
S1: suitable; S2: moderately suitable; S3: marginally suitable; N: non suitable.
In general the same values are used in the research of Fasina et al. (2007) as in Ritung et al.
(2007). They are both based on the data of Sys et al. (1993). When it comes to soil depth
however, they differ. Fasina et al. (2007) state that a soil depth of 150-200 cm, or even >200 cm
is highly suitable. In the landscape and soil requirements of Sys et al. (1993), both the classes
S1 and N are divided into two subclasses. Consequently, there is a better indication of the
extremely good conditions and the extremely bad conditions.
Values used for the requirements in the research of Ayorinde et al. (2014) are slightly different
from those of Sys et al. (1993). Where the difference was remarkable, the values used in
Ayorinde et al. (2014) were added to Table 3, indicated in italic.
A short description of the major soil and landscape characteristics for cocoa is given below.
Physical fertility soil characteristics
Soil texture
The soils best suited for cocoa are mixtures of sand, silt and clay. Very sandy and very clayey
soils are not suitable, optimum soil textures are loamy and sandy (loamy) clayey soils (Sys et
al., 1993). In the latter soils, the finer particles are often aggregated with Fe-oxides or organic
matter to form particles about the size of coarse sand. These soils possess both desirable
characteristics of good drainage and aeration associated with coarse sand, and large moisture
capacity associated with clay soils (Smythe, 1966). Organic matter content (important soil
26
fertility characteristic) is thus an important factor, next to the clay content. Clayey soils are
likely to have larger quantities of nutrients as they have a greater ability to retain them.
Soil depth and gravel content
The rooting system of a cocoa tree consists of a dense network of superficial feeding roots and
one taproot that reaches deep into the soil. 80% of the roots are concentrated in the upper 15
cm of the soil (Sys et al., 1993). The tap root can penetrate to a depth of 1,5 m or even more. If
physical obstructions occur, like stones, petroplinthite, bedrock, saprolite, …, it will follow a
tortuous path (Wood & Lass, 1985). Excessive quantities of stones and gravel can cause
bifurcation of the tap root, which causes the tap root to be less developed (Figure 12). The
quantity of gravel and stones that causes a less developed root system depends on different
factors such as texture and the size of the gravel. At lower depths, if the quantity exceeds 40%
gravel of the total volume of the horizon, there is a noticeable risk of impeded root penetration.
In the top soil however (25 – 30 cm), an appreciable quantity of gravel is undesirable, as in this
soil layer the bulk of the feeding roots are developed. Besides that, the more coarse particles
there are, the more volume they occupy, volume that in this case cannot store water or nutrients.
So a higher gravel content will thus lower the chemical soil fertility, even if there is no influence
on root penetration (Smythe, 1966).
According to Smythe (1966) a root penetrable soil depth of 150 cm is a general rule. Ritung et
al. (2007) on the other hand states that a soil depth of 1 m is sufficient. Other factors should be
taken into account when checking if soils are of sufficient depth. For example in dryer areas
deeper soils are desirable, especially when soils are rather sandy (Smythe, 1966).
27
Figure 12: Different types of root distribution depending on the nature of the subsoil and height of
water table
Wetness soil characteristics
Soil drainage and aeration
Adequate soil aeration is absolutely essential for satisfactory growth according to Sys et al.
(1993). Furthermore, when annual precipitation exceeds 3750 mm, soils need to be very
permeable. Ideal cocoa soils consist of aggregated sand, silt and clay with a total pore space of
about 66% to the soil volume (Landon, 2013). Non-capillary pore space should be more than
10% (Sys et al., 1993). Aeration in the soil is important for root respiration as well as promoting
a supply of moisture for absorption of nutrients. Cocoa trees can withstand flooding for short
periods, but are sensitive to waterlogging because it prevents root respiration (Sys et al., 1993;
Wood & Lass, 1985). So poor drainage conditions (permanently high water table at shallow
depth) have a negative impact. Exchange of gases in the soil is reduced if the pores are
(partially) filled with water. This creates anaerobic conditions which stop the roots from
growing. This situation is not uncommon in heavy clay soils. In these circumstances cocoa
develops an especially dense mat of fibrous rootlets in the surface horizons, which makes the
28
tree very sensitive to changes in the soil moisture regime and the tree dies rapidly from drought
(Smythe, 1966).
At the same time, cocoa is very susceptible to shortages of soil moisture. Water deficiency
indirectly effects growth rates of cocoa, by stomatal closure (Smythe, 1966).
The main external factors which affect soil drainage and aeration are intensity and distribution
of rainfall and the topographical position of the site. In the areas where rainfall is very heavy
throughout the year, sufficient root aeration is the most vital aspect. Under these rainfall
conditions organic matter is especially important. In sandy soils organic matter is a storehouse
for moisture and plant nutrients. In finer textured soil it serves to improve aeration (Smythe,
1966).
Chemical fertility soil characteristics
Since the majority of the feeding roots of cocoa (80%) are located in the surface soil layers, an
analysis of the top 15 cm is likely to be particularly informative. Nevertheless, when selecting
cocoa soils, the chemical properties of deeper horizons should not be ignored (Smythe, 1966).
pH
The tolerated pH range is 5,5 to 7,5 with an ideal pH of 6,5. Within 1 m of the surface no layer
should have a pH above 8 or below 4. Alkaline soils often induce deficiencies of micro nutrients
like iron, manganese, zinc. High pH for example causes malformation of the leaves due to lack
of available Fe, Cu and Zn (Landon, 2013). Very acid soils on the other hand can cause
phytotoxic concentrations of these micro nutrients. Cocoa might be more tolerant to acid soils
than many other tropical crops. According to Smythe (1966) the best pH-H2O range to grow
cocoa is from 6 to 7,4.
C/N and organic matter (OM)
Ideally, the organic matter content in the top 15 cm should be > 1.75% organic carbon (OC) (or
3% OM) (Landon, 2013). According to Sys et al. (1993), optimum values for OC should be >
1.5%, ideally > 2.4%. C/N ratio is an important value to distinguish good from bad soils for
cocoa, when examining agricultural land. The lower limit of the C/N ratio in the upper 15 cm
soil layer is 9. When organic matter content is too low and C/N drops below 10, there might be
enough N available but the storage of nutrient bases will rapidly diminish. Yet, when underlying
soil layers are high in nutrients, a C/N below 9 should not be problematic. A ratio above 14 is
also not desirable. These high ratios often occur in areas of high rainfall and acid soils.
However, this ratio has no valuable meaning when examining soils of recently cleared forest
(Smythe, 1966).
29
Geus (1973) states that the yield is positively correlated with the C/N ratio and the organic
matter content of the top 15 cm at the soil surface. The organic matter content in the top 15 cm
should ideally be ≥ 3% (Landon, 2013), similar to the values proposed by Sys et al. (1993).
Sum of basic cations, K, Mg, Ca, Na and Base Saturation
To achieve high productivity, cocoa requires a soil abundant in nutrients (Wessel, 1971). The
importance of several other soil characteristics, such as pH and organic matter, is largely due
to their influence on the availability of nutrients. When it comes to basic cations, the ratio of
the monovalent (K + Na) to divalent (Ca + Mg) is an important factor, having influence on plant
growth and development. The ratio should not exceed 1:50 (Sys et al., 1993). Optimal
CaO/MgO and (CaO+MgO)/K2O ratios in the topsoil, first 15 cm, according to Landon (2013)
are presented in Table 4. According to IFA (2008) the optimum K/Ca/Mg ratio as % of the sum
of those three cations (expressed in cmol (+) kg-1 soil) is 8:68:24; moreover base saturation
should be at least 50-60%.
The optimum levels of exchangeable cations are (Sys et al., 1993; Landon, 2013):
Ca ≥ 8 cmol (+) kg-1 soil
Mg ≥ 2 cmol (+) kg-1 soil
K ≥ 0,24 cmol (+) kg-1 soil
The base saturation percentage should be ≥ 35% in the subsurface layers, unless the soil CEC
is exceptionally high (Landon, 2013). Sys et al. (1993) propose a base saturation value of > 50%
for highly suitable soils. However, in soils with low CEC (very highly weathered soils, sandy
soils) it is advisable to use the parameter ‘sum of basic cation’ instead of the base saturation.
The ‘sum of basic cations’ should be at least 4 cmol (+) kg-1 soil, even > 6.5 cmol (+) kg-1 soil
for highly suitable soils for cocoa cultivation.
Table 4: Rating of fertility levels for cocoa in top 15 cm of soil (Landon, 2013)
Fertility
rating
pH Tot N %
C/N P2O5* ppm
K2O** ppm
Exch. bases†
cmol (+) kg-1
CaO/
MgO
CaO+MgO
K2O
∑ bases cmol (+)
kg-1 CaO MgO K2O
High 7.5 0.35 11.5 120 275 24.0 6.0 0.55 4.0 54 50
Medium 6.5 0.20 9.5 60 175 12.0 3.0 0.35 4.0 43 15
Low 5.0 0.05 7.5 20 100 4.0 1.0 0.20 4.0 25 5
*P2O5 Truog’s method (extracting agent 0.001 M sulphuric acid buffered at pH 3 with ammonium
sulphate)
**K2O exchangeable potash
†Assuming exchange capacity around 24 cmol (+) kg-1
30
The CEC and ACEC
The CEC in the surface layer should be ≥ 12 cmol (+) kg-1 soil and the CEC in the subsoil
≥ 5 cmol (+) kg-1 soil (Landon, 2013). Sys et al. (1993) and Ritung et al. (2007) only consider
the ACEC (i.e. CECclay) indicating the weathering degree of the soil (Table 4). ACEC should
be > 24 cmol (+) kg-1 clay in the highly suitable soils. Soils with an ACEC of < 16 are
considered less suitable, particularly those with a net positive charge (ACEC of < 16(+)).
Landscape soil characteristics
Slope
Sys et al. (1993) distinguish different suitability classes for the slope depending on the land use
type. As such, the slope values are more severe for cocoa plantations with high level of
management with full mechanization. In our study, low level of management with mainly
handwork is foreseen, in this case slopes of < 8% are considered as highly suitable, while those
of > 50% are considered unsuitable. The latter hampers accessibility for weeding and
harvesting, and enhances soil erosion. Very often, effective soil depth in very sloping areas is
very limited, and many rock outcrops are present.
2.2.7 Nutrient requirements
To understand the nutrient requirements of cocoa, it is important to consider nutrient cycling.
Table 5, 6 and 7 provide some information on nutrient input and removal from cocoa systems
(Vliet & Giller, 2017; IFA, 2008). The main sources of nutrient input in cocoa production
systems are (in)organic fertilizers, rainfall deposition (e.g. in industrialized regions), and
nitrogen fixation by leguminous trees as shade trees in the plantation.
Wessel (1971) observed that combined application of K and Mg had a positive effect on the
growth and yield of cocoa. Under high light intensities, the response of cocoa to K fertilizer is
high, due to the high nutrient requirement that is induced by the high radiation flux density.
Potassium is important for translocation of carbohydrates. Large amounts of K are exported in
the harvest, especially when husks & pods are not returned to the field, since they have a high
potassium content: 86% of the K harvested is situated in the husk according to Boyer (1973)
and Wessel (1987), see also Tables 5 and 7. According to Fontes et al. (2014) and Hartemink
& Donald (2005), the variability of nutrient content is much higher in the husks than in the
beans. This is mainly the case for K. According to Boyer (1973),When it comes to exported
nutrients through harvest, the husks contain 44% of the nitrogen (N) exported, 29% of the
phosphorus (P) exported, 86% of the potassium (K) exported, 90% of the calcium (Ca)
exported, and 54% of the magnesium (Mg) exported.
Generally, nutrient removal is relatively small compared to other crops such as coconut, as
nutrients are partly recycled through litterfall (Hartemink, 2005) (Figure 13). Still nutrients are
31
lost through leaching and harvesting (Landon, 2013), see also Tables 5 and 6. The nutrients
captured in the woody parts of trees during growth are often considered as a removal from the
nutrient cycle as they are no longer available to the plants, unless trees are pruned or old trees
removed and burned. If present, shade trees may increase nutrient availability through litter
cycling and nitrogen fixation (Table 6) (Isaac et al., 2007; Ofori-Frimpong et al., 2007 cited in
van Vliet & Giller, 2017). On the other hand, while leguminous shade trees provide N, they
might compete with cocoa for nutrients and water, resulting in a lower production of cocoa
compared to non-shaded cocoa plantations.
Figure 13: Simplified nutrient cycling diagram for cocoa ecosystems (Hartemink, 2005)
32
Table 5: Nutrient inputs into and removal from cocoa systems (source: van Vliet & Giller, 2017)
33
Table 6: Concentrations and amounts of nutrients in the litter fall and standing litter of cocoa and shade trees combined (source: van Vliet & Giller, 2017)
34
Table 7: Nutrient removal in crop of 1 t ha-1 dry cacao beans (7% humidity) with 1.4 t ha-1 husks
IFA, 2008)
According to literature, the rates of application of N, P and K are very variable and may change
from country to country depending on the climate, soil type, management level, age of trees,
etc… The nutrient uptake of the crop is 25 kg ha-1 N, 4,5 kg ha-1 P, 36 kg ha-1 K for 560 kg ha- 1
of dry beans (Landon, 2013). Sys et al. (1993) state that the nutrient removal to produce 1000
kg ha-1 beans is 20-25 kg ha-1 N, 10 kg ha-1 P2O5 and 15 kg ha-1 K2O per hectare per growing
cycle. According to Sys et al. (1993) the fertilizer application per growing cycle to produce
1 ton of beans ha-1 is 35 - 60 kg ha-1 of N, 25 - 50 kg ha-1 of P2O5 and 55 - 75 kg ha-1 of K2O.
In high yielding plantations (2 or more tons of beans ha-1) or on very poor soils, literature
mentions fertilization rates of 100-150 kg ha-1 of N, 90 -150 kg ha-1 of P2O5 and 120 - 180 kg
ha-1 of K2O (van Vliet & Giller, 2017 (Table 8); IFA, 2008). Moreover, on poor acid soils as
Acrisols, Alisols and Ferralsols, IFA (2008) recommends an addition of 400-600 kg/ha rock
phosphate, 880 kg/ha ground limestone, ploughed into top 15 cm soil at pre-planting. At
planting a dose of 150-230 g plant-1 of rock phosphate per planting hole is recommended.
Rates of application of nutrients to cocoa according to various authors in Wyrley-Birch (1972,
as presented in von Uexküll & Cohen, 1980 and van Vliet & Giller, 2017) are presented in
Table 8.
35
Table 8: Rates of application of nutrients to cocoa (in van Vliet & Giller, 2017)
36
2.2.8 Shade
Cocoa originates from the Amazonian forest, where it grows as a shrub in the underwood in the
shade of bigger trees. The problems of cocoa shading are very complex. It is clear though that
shade is indispensable for young cocoa. Also, the poorer the soil and the more adverse the
climatic conditions, the more necessary shade becomes (Van Himme & Snoeck, 2001). In
general shade trees reduce stress of cocoa plantations by ameliorating adverse climatic
conditions and nutritional imbalances. On the other hand the trees may compete with cocoa for
growth resources (Beer et al., 1998).
Shade inevitably modifies climatic conditions. By reducing direct insolation and air movements
the chief effect of shade trees is the reduction of the daily soil and air temperature surrounding
the cocoa (Beer et al., 1998; Smythe, 1966). Also relative humidity within the cocoa canopy is
increased, resulting in reduced vapour pressure within the leaf and therefore reduced water loss
through transpiration (Smythe, 1966). Furthermore the buffering of the high humidity levels
and soil moisture availability are improved (Beer et al., 1998) (Schwendenmann et al., 2010).
The influence of the rainfall intercepted by the leaves of the shade trees is usually of no
significance as the total water loss because of evapotranspiration remains essentially the same
(Smythe, 1966).
Intense tropical rainfall has a big impact and these forces can cause damage to the structure of
the upper soil layer. Hence another reason for maintaining a closed canopy over a cocoa farm
is to protect the surface structure (Smythe, 1966). Furthermore shade trees protect the cocoa
trees from high wind speed and therefore from wind damage. The infection by windborne
spores of fungal diseases also reduces (Rice & Greenberg, 2000). According to Wood and Lass
(1985) this is sometimes the reason plantings without shade fail. When it comes to soil, shade
trees may contribute to the maintenance of soil fertility. The trees take up nutrients that have
been washed down into lower soils layers, returning them to the soil surface by leaf litter (Wood
& Lass, 1985).
Another advantage is the reduction in fruit abortion, resulting from soil N addition by
leguminous shade trees. Weed growth is suppressed (Rice & Greenberg, 2000) and insect
biodiversity increases resulting in improved natural control of pest populations and pollination
services (Bisseleua Daghela et al., 2013; Bos et al., 2007; Sperber et al., 2004). However
increased shade may also increase the incidence of some commercially important pests and
diseases, such as Phytophthora palmivora and Mycena citricolor (Beer et al., 1998).
Sys et al. (1993) and Landon (2013) note that shade trees are necessary, especially for young
cocoa trees. One of the main reasons for shade during the first years is to ensure the right form
of growth (Wood & Lass, 1985). Experiments in Trinidad showed that 50% shade (meaning
50% full light) for young cocoa trees gave best results. In other climates this may be different
37
in terms of energy. To compare, data on light intensity or incident energy are needed (Wood &
Lass, 1985). The protection created by shade is most needed when soil conditions are least
satisfactory. Therefore, it should only be reduced when soil conditions are entirely satisfactory.
Otherwise there is danger of a nutrient imbalance developing within the plant due to excessive
photosynthesis (Landon, 2013).
A trial that was put up by Ahenkorah et al. (1987) showed that the main yield under heavy
shade was about half that under no shade. This statement is being discussed by Vanhove et al.
(2015) who point out that only one fast growing species of shade tree is used in this trial
(Terminalia ivorensis). This tree might not be suitable as a shade tree as it competes with cocoa
for soil nutrients and light. Secondly according to Vanhove et al. (2015) the period of 20 years
is too short to see the advantages of a cocoa plantation with shade trees. Ahenkorah et al. (1987)
also conclude that the application of K and P fertilizer should be adjusted to the overhead shade
conditions.
Figure 14: Results of application of fertilizers under different degrees of shade (Ahenkorah et al.,
1987)
38
2.2.9 Agroforestry and organic cocoa
2.2.9.1 Cocoa agroforestry
Since the 1960’s input-intensive model of cocoa production has been promoted to increase
yield, in order to meet the growing demand. High yielding cocoa systems demand high external
inputs. Jagoret et al. (2014a) argue this model has reached its agronomic, socio-economic and
environmental limits. The last decades the question arises if a low-input system is not a more
appropriate production model. In order to grow cocoa sustainably, shade is needed to among
others control insect damage and premature decline of yields (Wessel & Quist-Wessel, 2015).
Thus, the concept of agroforestry arises.
Part of the reason yields are high after clearing forest and planting full sun plantation is the
good soil fertility and low pest pressure inherited from the former forest. But these properties
diminish and yield often collapses after 20 years, ‘Boom and bust cycle’. Agroforestry aims to
keep these advantages for a long period, reducing chemical inputs and designing a more
diversified and resilient model. Agroforestry systems provide ecosystem services, such as
maintaining good soil quality, habitats for wildlife, conservation of animal and plant
biodiversity and carbon storage. Jagoret et al. (2014a) state the system contributes to the general
objective of the global cocoa supply chain: producing more cocoa in the long term while
minimizing environmental impacts.
Also Vaast and Somaribba (2014) cite a range of ecosystem services such as crop productivity,
production diversification, climate adaptation, pest and disease suppression, pollination, soil
fertility, water yield and carbon sequestration; and, thereby, sustain cocoa yield. The study of
Bisseleua et al. (2013) indeed shows the importance of a diverse shade canopy in reducing
damage caused by cocoa pests. Agroforestry supports biological pest control by maintaining
natural enemies. Herzog (1994), Bisseleua et al. (2013) and Jagoret et al. (2014a) researched
the value of the non-cocoa trees in agroforestry systems, as they can offer food, fuel-wood and
medicinal products for the local market and timber for local market or export. The presence of
timber trees can reduce deforestation and increase carbon sequestration (Wessel et al., 2015).
When it comes to resilience against climate change, Abdulai et al. (2018) dispute that
agroforestry systems would be more drought resistant than full sun. In some agroforestry
systems evapotranspiration is bigger, resulting in a quicker dehydration of the soil.
Blaser et al. (2017) emphasize that agroforestry is not cost-free. The results indicate limited
benefits of agroforests on soil fertility at the scale at which cocoa is planted and managed in
West African cocoa farms. It may not be sufficient to compensate for short term costs to
production, since cocoa growth and yield decrease with increasing shade-cover. Furthermore
they state that cocoa agroforestry is a poor substitute for natural forests in terms of their C
sequestration potential. Sustainable intensification of cocoa production through improved
39
integrated management practices on existing farms, and the avoidance of further expansion of
cocoa systems into native forest ecosystems, would be a stronger mitigation alternative at the
landscape scale, than the inclusion of shade trees alone. They reckon however, that agroforesty
may influence the sustainability of cocoa production via other mechanisms, such as the ones
discussed above. Nevertheless, their study suggests that not all of the benefits commonly
ascribed to agroforests can be generally applied.
Examples of cocoa agroforest, planting densities and yields:
In Central Cameroon a traditional way of growing cocoa in agroforestry with forest and fruit
trees illustrated its sustainability by the fact that 80% of the farms are over 40 years old, without
using fertilizer. Next to the low inputs, other advantages are little degradation of soil resources
and a constant yield for a long period. The downside is the lower yield. In Cameroon this is
mainly due to high incidence of Phytophthora pod rot. A solution here would either be the use
of fungicides or intensification by planting additional trees (Wessel & Quist-Wessel, 2015).
Several other studies on agroforests have taken place in Cameroon, and other West-African
countries, each with different characteristics.
Jagoret et al. (2014a): Cameroon, 1000 cocoa trees, 70 fruit and 30 forest trees per hectare.
Yield of 900 kg ha-1 after 20 years. Lifespan of these plantations exceed 50 years.
Jagoret et al. (2014b): Central Cameroon, mean density of 1511 cocoa trees ha-1 and 180
non-cocoa trees ha-1.
Bisseleua et al. (2013): Cameroon, mean cocoa tree density was 1230 trees ha-1. On average
there were 89 shade trees ha-1 and 8 different species ha-1. Shade index was negatively
related to yield, with yield significantly higher at shade and herb cover of 50%.
Oke and Odebiyi (2007): Nigeria, average shade tree density of 23 ha-1 was found.
Eco-certification schemes, principally operating through the Sustainable Agriculture Network,
have set shade management criteria which require cocoa farmers to maintain a shade cover of
40 %. Additionally this shade should be provided by a minimum of 12 native species per ha
and with tree canopies comprising at least two strata (Figure 15).
40
Figure 15: Schematic diagram of the different vegetation layers or strata in agroforests (Bieng et al.,
2013)
In 2014 about 20% of world cocoa production was eco-certified, including 13% certified by
Rainforest Alliance (> 927 000 ha, mostly in Ivory Coast, Ghana and Indonesia). Rainforest
Alliance recently merged with UTZ certification. The goal is that certification is a financially
viable option to retain biodiversity while at the same time achieving high cocoa yield (Vaast
and Somaribba, 2014; UTZ, 2017).
2.2.9.2 Organic cocoa
Organic farming is submitted to certain rules and practices. What exact practices should be
implemented depend on the organic certificate that is worked with. Some countries have
national standards (USA, Canada, UK) and the European Union has a EU regulation. However,
there are also independent labels with their own standards, such as Bio Suisse. Most certificates
are based on the principals stated by IFOAM (International Federation of Organic Agriculture
Movements), regarding health, ecology, fairness and care. A general definition has been
formulated in the Codex Alimentarius from the FAO/WHO: “Organic agriculture is a holistic
41
production management system which promotes and enhances agro-ecosystem health,
including biodiversity, biological cycles, and soil biological activity. It emphasizes the use of
management practices in preference to the use of off-farm inputs, taking into account that
regional conditions require locally adapted systems. This is accomplished by using, where
possible, agronomic, biological, and mechanical methods, as opposed to using synthetic
materials, to fulfil any specify function within the system.” (ICCO, 2006)
What this means in practice for a cocoa plantation is shown in Table 9. Mainly crop protection
and fertilization will differ from conventional agriculture.
Table 9: Conventional and organic cocoa cultivation practices (Djokoto et al., 2016)
Many consumers perceive organic products as safer and of higher quality than conventional
ones. The organic cocoa market represents a very small share of the total cocoa market,
estimated less than 0.5% of total production. However, the demand for organic cocoa products
is growing at a very strong pace, as consumers are increasingly concerned about the safety of
their food supply along with other environmental issues. Also, the demand for chocolate with
a premium status is growing in general. In 2006, ICCO stated that future growth of the organic
cocoa market would rather depend on the supply than on the demand. The supply side faces
many challenges to meet the growing demand. At producer level, issues of supply consistency
and quality must be addressed. Some producing countries encounter problems, such as the high
costs of certification by foreign organizations and a lack of knowledge about organic trade.
Trade channels have to allow for increased volumes of organic cocoa, for instance through the
entry of bigger players in the market (ICCO, 2006). The three major consuming markets for
42
cocoa and chocolate products (EU, US and Japan) are also the most important market for
organic cocoa products. Europe is by far the biggest importer of organic cocoa beans, and has
the largest processing and manufacturing activities for certified cocoa products.
Fine & flavour and organic cocoa on Sao Tomé
The Amelonado trees imported from Brazil to Sao Tomé had initially good aromatic potential,
and the island produced cocoa of good quality. However, because purchase prices weren’t
adapted to the quality of the cocoa, less attention was paid to the aromatic quality and Sao Tomé
switched to cocoa production primarily for cocoa butter. In the most recent update in 2016,
ICCO recognized a share of 35% of the total cocoa export of Sao Tomé as classified fine and
flavour cocoa. Therefore, Sao Tomé is one of the 23 countries recognized as fine and flavour
cocoa exporters, with a quota of 50% of the national production. This is partly the reason the
archipelago keeps its status of historical production zone (ICCO, 2018).
There is one organic cocoa organization on Sao Tomé, called CECAB (Cooperativa de
Exportação de Cacao Biologico). It’s a federation of 37 smallholder farmers associations i.e.
more than 2051 families. This represents approximately 15% of the total rural population of
São Tomé & Principe. Nowadays, CECAB has become the main exporter of the archipelago
(Kaoka, 2013).
Roche and Dulcire (2007) proposed the creation of an organic cocoa market on Sao Tomé
through a producers organization like CECAB. They state that for small farmers there are
benefits in switching to organic farming. For example the maintenance of soil fertility in organic
agriculture did not seem to have an insurmountable cost in the long term: the culture was
extensive, the soils were of good quality and there were only very weak exports of biogenic
salts, the organic manure being brought by the decomposition of leaves and cut weeds in the
plots. Measurements should be taken to prevent the risks of erosion which could be the most
important cause of loss of soil fertility. Another advantage lies in the other crops produced at
the same time in the cacao-plantations (bananas, lemons, oranges, papaw, breadfruit tree, taro,
etc.). All this food is also produced under "organic" conditions that, even if it is not yet the case
currently, should be able to be marketed on a differentiated market (Roche & Dulcire, 2007).
Organic pest & disease control
Prevention and monitoring is the most important part of pest and disease control in organic
farming (FAO, 2018). By using resistant planting material, adapted to local environmental
conditions could help prevent the manifestation of diseases. Good water and nutrient
management helps the plants to be less vulnerable. Stress on the other hand encourages
pathogen infections. Other crucial measurements are conservation and promotion of natural
enemies and proper sanitary. This means removing infected plant parts and residues after
harvesting. For example, Ayenor et al, (2004) describe good practises to fight black pod in
43
cocoa by pruning, removal of infected pods (to prevent spreading of the disease) and weeding
(to reduce excessive humidity). Sometimes plant extracts can be used against pests, for example
pyrethrum, or as fungicide, for example the leaves of Carica papaya. Bordeaux mixture (copper
sulphate and lime) has also been successfully used in organic farming as both fungicide and
bacteriocide (FAO, 2018). As this mixture persists trough rains, it might be interesting to use
in this humid climate. Evans et al. (1962) showed Bordeaux mixture is effective against
Phytophthora palmivora in cocoa. Also Adejumo (2005) recommended it to control
Phytophthora pod rot. Additionally he reports that in Nigeria many plant species are used as
both local medicines and pesticides or fungicides. For example the herbal mixture “Tiwantiwa”
was developed as treatement against black pod and Olunloyo (1994) reported no significant
difference between the performance of 20% herbal extracts and Bordeaux mixture. Also
Chromolaena odorata and Piper guineense might have potential for controlling Phytophthora
pod rot according to Adejumo (2005).
2.2.10 Establishment of a cocoa plantation
According to Wood and Lass (1985) there are two general ways to establish cocoa on land
occupied by forest: clear-felling followed by planting permanent shade trees or thinning the
forest before planting cocoa. The latest is used the most. Thinning the forest is cheap and simple
and relatively quick. Also, by clear-felling al lot of nutrients, normally circulating between
forest and soil in a state of equilibrium, leave the ecosystem. This loss is minimised by thinning.
Nevertheless there are some important disadvantages. The forest trees may not provide the right
type of shade and it’s difficult to control and adjust the shade. They will probably not be
distributed uniformly, which can be a hindrance where it is desirable to move between the trees
with machinery. Also the trees are unlikely to be leguminous, while leguminous trees can offer
the advantage of nitrogen fixation. Some of the trees might be hosts to pests and diseases or in
major competition with cocoa for resources.
In West Africa the forest method is commonly used. When there is no shortage of soil moisture,
the forest is thinned leaving about 5 or more dominant trees an 35-45 intermediate trees per ha.
It is possible to plant food crops as temporary shade for cocoa seedlings. Instead of normal
felling the trees chemical poisoning technique can be used. This is a cheaper method, but care
has to be taken regarding falling branches and tree trunks (Wood & Lass, 1985). In Brazil, the
establishment of cocoa under secondary forest is recommended using the following method:
clearing all the low-growing vegetation, reduce the top shade to 25-30 medium sized trees per
ha, spaced 25-30 m. According the growing stages of cocoa further thinning is undertaken
(Wood & Lass, 1985).
44
As an example Figure 16 shows the special structure of an agroforest in Cameroon. Jagoret et
al. (2012) describe the conversion of Imperata cylindrica grasslands in Cameroon into cocoa
agroforest. Cocoa trees are intercropped with oil palm, exotic and indigenous fruit trees and
forest trees, as shown in Figure 16. About half of the non-cocoa trees were fruit trees, one
quarter oil palm and one quarter forest trees. Oil palm and cocoa trees were gradually replaced,
in order to keep a constant yield.
Figure 16: Design 20 years after the installation of cocoa agroforest on previous high density oil palm
plantation, in savannah region, Cameroon (Jagoret et al., 2012)
2.2.10.1 Suitable shade trees
Trees found suitable as shade trees for cocoa, according to Wood & Lass (1985): Terminalia
spp., Chlorophora excelsa, Albizia spp., Ficus vogeliana, Entandrophragma spp. Trees
considered unsuitable: Piptadeniastrum africanum, Pentaclethra macrophylla, Cola nitida and
other Cola spp. In general trees of the Sterculiaceae family should be cleared as they host pests
and diseases that attack cocoa trees (van Himme & Snoeck, 2001). Beer (1987) suggested a
check-list with some of the desirable characteristics for shade trees:
Compatibility with the crop (minimal competition for water, nutrients and growing
space). For example no shallow rooting system
Strong rooting systems, not susceptible to wind throw
Ability to fix nitrogen
Light crown that provides regular mottled shade pattern
Non-brittle and thornless branches and stem
Tolerance of repeated heavy pruning
Readily decomposed leaves an woody material
Small leaves to minimize rain drop coalescence and subsequent drip damage
No allelopathic properties
Valuable wood, fruit or other products
Not an alternative host for insects and pathogens which are major enemies for cocoa
45
2.2.10.2 Planting densities & yields
According to Van Himme and Snoeck (2001) optimum planting density provides the maximum
yield per unit area over a given period. Several factors influence this spacing, particularly the
plant material, tree vigour and conditions of soil, climate and shade. For example, when
environmental conditions are less optimal, denser planting is recommended to achieve this
maximum yield per area. In general planting density should be such that the canopy of the
plantation closes as quickly as possible.
Van Himme and Snoeck (2001) advise densities of 952 to 1333 seedlings per hectare with
staggered planting. A spacing of 3 x 3 m or 3.5 x 3 m is suitable for fertile soils in areas with
adequate rainfall. On poorer soils, or where the climate is relatively dry, a spacing of 3 x 2.5 m
is preferable. They state that previously higher planting densities were used, up to 1666 plants
per hectare. IFA (2008) recommends similar densities, again depending on soil fertility and
climate conditions. In some regions cocoa yields can be optimised by implementing high
planting densities (1736–2500 trees ha−1) (Badrie et al., 2015). Mooleedhar and Lauckner
(1990) demonstrated the need for determining the optimal variety × spacing design. They found
that one selected Trinitario clone was better suited to close spacing than the two other clones in
the trail. With these Trinitarios, the closest spacing (1.8 m x 1.8 m, 2990 plants/ha) resulted in
the highest yield. Tree densities in cocoa agroforest are given above, in chapter 2.2.9.1.
Smythe (1966) points out the necessity of checking the soil depth at every planting hole, and
adjusting the position to achieve the best compromise between soil depth and desirable spacing.
Current average yields in smallholder farms are small. For instance, in Ghana yields are
estimated at around 400 kg ha-1 (Aneani & Ofori-Frimpong, 2013) while theoretical potential
yields under rainfed conditions are modelled at 5000 kg ha-1 (Zuidema et al., 2005).
In Ghana for example, 25% of the annual harvest occurs in the peak month November, because
of a distinguished dry season. In Malaysia, where there is no true dry season and production is
more evenly spread throughout the year, harvest in the peak month is only 12% of the annual
crop (Wood & Lass, 1985).
2.2.11 Cadmium restrictions
Bioaccumulation of heavy metals such as cadmium has been a major concern for international
food organizations, due to the great toxicological risk to the consumer. The IARC (International
Organisation for Research on Cancer) has classified cadmium as carcinogenic to humans
(Kruszewski et al., 2017). Intake of excess Cd in contaminated food results in severe damage
in organs such as lungs and liver, which eventually leads to the onset of cancer and other deadly
46
disorders (Chavez et al., 2015). Cadmium has no metabolic function in plants but can
accumulate in the roots, shoots and edible parts such as cacao beans. Plants can tolerate Cd
concentration at low levels without expressing symptoms of toxicity. Chavez et al. (2015)
conclude in their research that, unlike other crops, cocoa accumulates most Cd in the beans,
than the shells and the least in the leaves (Chavez et al., 2015; Zarcinas et al., 2004).
In non-polluted soils, Cd concentrations range from 0.01 to 1.1 mg kg−1 with an average of 0.41
mg kg−1 (Chavez et al., 2015). Furthermore, attention should be drawn to the fact that the total
concentration may not indicate bioavailability of Cd in soils. Risk of Cd accumulation by crops
is more related to its availability due to soil, crop and environmental factors, than the total Cd
present in the soil (Roberts, 2014). Available Cd accounts only for a small part of the total Cd
in the soil. Many soil factors such as pH, organic matter content, amounts and forms of oxides
and carbonates, charge characteristics, cation exchange capacity, and clay content influence the
bioavailability, and transport of trace elements in the soil and within the agroecosystem (He et
al., 2005). Roberts (2014) states that pH is usually regarded as the most significant variable
influencing Cd uptake, increasing uptake by plants in more acid soils.
In the research of Mounicou et al. (2003) higher levels of Cd were reported in Ecuador,
Venezuela and Malaysia, as compared to Brazil, Ghana and Ivory Coast. Some of the cocoa
powders from Ecuadorian beans, that were tested, had a Cd content exceeding the limits in
some countries. High Cd soil levels are typically a problem in Latin American countries. Least
contaminated crops come from West Africa (Bertoldi et al., 2016). The source of Cd
contamination in chocolate products is ambiguous and not only derive from the soil in which
the cocoa tree grew. It could be due to the environment in which the cocoa beans undergo
various processes such as fermentation, drying and crushing, or due to the metal devices used
for grinding, mixing and storage (Kruszewski et al., 2017). Studies of Kruszewski et al. (2017)
and Mounicou et al. (2003) found that generally Cd concentration were reduced by processing.
As a result of a report presented by the European Food Safety Authority (EFSA) published in
2012 the Commission Regulation N° 1881/2006 was amended by Commission Regulation
N° 488/2014 in May 2014. The new regulation states the maximum permissible levels of
cadmium in specific cocoa and chocolate products. Restrictions of 0.1 up to 0.8 mg kg-1,
depending on the product, will apply from January 2019. To date it is the European Union’s
only restriction in terms of levels of heavy metals in cocoa and chocolate products (Kruszewski
et al., 2017; eur-lex.europa.eu).
47
3 Materials and methods
3.1 Agripalma, oil palm business on Sao Tomé
The research is conducted for Agripalma, a part of the Socfin Group. Socfin is a company
devoted to responsible agriculture in the tropics. A significant part of their activity is running
oil palm and rubber estates in Africa and Asia. Agripalma is an industrial oil palm plantation,
located in the south of the Sao Tomean island, a one hour drive from the capital (Figure 17).
Since the creation in 2009 they have planted 2210 ha of palm groves. A palm oil factory is
under construction. Additionally Agripalma is exploring the options to start cocoa plantations,
for example in the regions that are less suitable for oil palms. The rich history of cocoa in Sao
Tomé (see chapter 2.1.4) is another driver for exploring cocoa reintroduction. A cocoa nursery
has been established and a few trials have already been planted in 2016. Several areas were
indicated as ‘potential cocoa region’. One area was selected for this suitability analysis,
identified as the ‘Erika zone’ (Figure 17). Locally the region is named ‘Novo Brasil’, after the
old “roça” or (cocoa) plantation of which the ruins can still be found in the forest.
Figure 17: Map of Agripalma and the research area
48
3.2 Location of the research area
The defined area, indicated above as ‘Erika zone’ (411.44 ha), was designated to investigate
the suitability for the establishment of an agroforestry cocoa plantation. One of the reasons this
specific zone was chosen among other potential areas was because it was the easiest site to
reach. Furthermore, the accessibility is facilitated by the presence of an abandoned railway
crossing the site. The old railway served in colonial times to transport cocoa from the
plantations to the harbour. Now it was used to simplify walking through the zone, which was a
quite dense forest in most parts of the site. All observation lines started from this railway.
3.3 Field work
3.3.1 Soil observation by augering
To map the different soil types, several observation points were indicated on a topographic map.
These points were located on lines oriented east-west and with 200 m interspace. While in the
field, we tried to reach as close as possible these points, using a GPS-application (Avenza app).
The geographical coordinates of each observation point in the forest were registered with a
Garmin GPSMAP 64s. Areas that turned out very hard to reach were not observed, as they were
considered as unsuitable for cocoa because of the very steep slope (eastern part of the study
area). Other points were skipped because of lack of time. It was made sure that in all reachable
possible planting areas observations were covered.
Each observation included augering (till a limiting layer) during which soil texture, soil colour
(Munsell Soil Colour Charts, 2000), stoniness, drainage and soil depth were recorded.
Furthermore the slope was measured with a clinometer, and surface stoniness was estimated.
Photos from the environment were taken.
Those observations were performed at 76 points in the study area and at 28 of these points, a
soil sample was taken (Figure 18). These samples were mixed into 7 composite samples,
indicated as composite sample A to G. The decision on which samples should be put together
was based on the soil map by Cardoso and Garcia (1962) and the physical similarities between
the samples.
49
Figure 18: Location of the soil observation points and of the vegetation plots. Observation points were
grouped per composite sample.
3.3.2 Soil observation in soil profiles
In addition to the soil augerings, the soil was also evaluated in 5 soil profile pits. Three of them
are orientated in one line, uphill. It had been planned to dig pits at 6 different location, but the
4th location turned out impossible to reach and unsuitable for cocoa due to the steepness and
stoniness (Figure 19).
50
Figure 19: Location of profile pits
In each profile pit, the soil horizons were identified (Figure 20), photographed and several
morphological soil parameters recorded (depth, colour, texture, structure, stoniness, …). Soil
samples were taken from every horizon. Also the environmental parameters like land use, slope,
surface stoniness, … were recorded.
51
Figure 20: Soil profile, pit 6. The A, B and C horizon are indicated
3.3.3 Vegetation
In order to make an inventory and description of the current vegetation to determine its
suitability to establish an agroforestry plantation, 9 observation plots were demarcated (Figure
18). All plots were approximately 30 x 30 m, except for plot 5, which was 30 x 8 m because it
was too dangerous on the steep hill. This size was based on “field methods for vegetation
mapping” (The Nature Conservancy, 1994). Inside each plot, following characteristics were
recorded:
the number of trees with DBH > 10 cm
the DBH (diameter at breast height) of those trees
the height, estimated with clinometer
the local (vernacular) name of the tree species
canopy openness, measured with a convex densiometer
For the canopy cover 4 measurements were made per plot with the densiometer, to have an
indication on the forest density and of how much light gets through the canopy. The plots were
located near the soil profiles and in seemingly different vegetation types. The vernacular names
were translated in scientific names, using ‘Plantas úteis da flora de São Tomé e Príncipe.
Medicinais, industriais e ornamentais’ (Roseira, 1984), Figueiredo et al. (2011) and National
Biodiversity Strategy and Action Plan 2015-2020 (NBSAP II), for sometimes the scientific
names differed.
52
3.4 Laboratory analyses
3.4.1 Routine soil analyses
The soil samples from the profile horizons have been analysed at LCA (Laboratory for
Chemical Analyses) at the Ghent University. First the soil samples went through a sieve (2 mm)
to separate the stones, leaves and other big particles. Subsequently the following analyses were
performed :
pHH2O , pHKCl : suspension 1:2.5 water and 1N KCl, respectively
P content : method Bray 2
OC and total N content: total combustion method (CNS elemental analyzer, Vario Max)
CEC and exchangeable basic cations (Mg, Ca, K, Na): NH4OAc method, buffered at pH 7
soil texture: pipette method.
The composite soil samples were sent to the SoGB laboratory (Socfin) in Ivory coast. The same
analyses were performed as those on the soil profile samples, following the same methods apart
for the OC (Walkley & Black) and total N (Kjeldahl) contents.
All methods are described in detail in Van Ranst et al. (1999).
Calculated soil parameters used for the evaluation for cocoa cultivation are sum of basic cations,
base saturation and ACEC (apparent CEC, i.e. CEC of the clay fraction). The textural class
(international textural triangle), was determined with the USDA Soil Texture Calculator
(https://www.nrcs.usda.gov/wps/portal/nrcs/detail/soils/survey/?cid=nrcs142p2_054167), based on
the sand, silt and clay content.
3.4.2 Cadmium
A soil sample (depth 0-20 cm) was taken from the Matheo Sampei site, a cocoa trial from
Agripalma. A mature cocoa pod was taken from that same site.
The soil available cadmium was determined after extraction with NH4OAc-EDTA pH 4.62 and
measurement of Cd with ICP.
Cocoa beans were dried in open air at chamber temperature. Shells were peeled off. Mashed by
hand an mixed mechanically. The mashed beans were put in a drying oven for two days until
all water was removed. Afterwards the dried and beans minced mechanically until powder.
After calcination of the powder, the ash was dissolved in 7M HNO3 and total Cd content was
measured with ICP.
53
3.5 Data processing
All data were processed in Excel. ARCGIS 10.4 (ArcView) and QGIS 2.18 were used to
spatially analyse and visualise the results.
In order to check if certain parameters were related, a chi-square test has been performed on the
vegetation data.
54
4 Results and discussion
4.1 Climate suitability
4.1.1 Climatic data
The study area is located in the southern part of the island with a humid tropical climate with
annual rainfall > 3000 mm and a very short dry season, i.e. only 2 months with rainfall of < 100
mm (June and July) (Table 10). The study area lies between the two weather stations of Porto
Alegre and Dona Augusta (Figure 21). The annual precipitation at these weather stations is
respectively 3332 mm and 3689 mm with a significant drop during June, July and August. The
rainfall distribution measured at the weather station of Agripalma over the last years (2014-
2017) is shown in Figure 22.
Figure 21: Location of the weather stations Dona Augusta and Porto Alegre, and the research area in
between
55
Table 10: Pluviometric data
Month Precipitation* Precipitation Dona Augusta**
Precipitation Agripalma 2014-2017
mm mm mm
January 319.8 394 266
February 196.8 184 200
March 256.7 218 381,6
April 257.6 265 311,8
May 266.8 325 263,5
June 58.9 34 104.7
July 63.2 41 110.8
August 154,3 178 293,6
September 254.5 311 354.7
October 486.4 526 650.7
November 514.1 685 472.9
December 412.5 527 423
Mean 270.1 307.3 319.4
Minimum 58.9 34 104.7
Maximum 514.1 685 650.8
Annual 3241.6 3688 3833.3
*data collected at: latitude: 0.113° , longitude: 6.580°, elevation: 197 m
**: from weather station of Dona Augusta, north of Ribeira Peixe, latitude: 0.107° ,
longitude: 6.626°, elevation: 140 m asl (Verheye, 1986)
Figure 22: Monthly precipitation at the weather station of Agripalma (Ribeira Peixe)
Mean monthly temperatures are between 22 and 28 °C, mean minimum temperature ranges
between 20 and 23 °C (Verheye, 1986). The mean annual temperature declines 0,76 °C per 100
m altitude (Cardoso & Garcia, 1962). Relative air humidity is very high throughout the year,
0
100
200
300
400
500
600
700
JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC
mm
/m2
Average monthly precipitation (2014-2017)
56
mean value of 87% month-1. Windspeed is low, average value of 2.4 m s-1 (Table 11). Insolation
is very low throughout the year, average monthly value of 2.8 h day-1, ranging from 5.1 h day-
1 in February to 1.0 h day-1 in September. The lowest insolation is observed during the months
of August till November, being < 1.5 h day-1 during this whole period. In the north of the island
(Sao Tomé city), the average monthly insolation is +/- 4.9 h day-1.
Table 11: Climatic data, collected at latitude: 0.113°, longitude: 6.580°, elevation: 197 m
Month Mean temperature Max temperature Min temperature Relative humidity
Wind
°C °C °C % m/s
January 25.3 28.4 22.2 86.3 2.2
February 25.9 29.1 22.7 83.9 2.2
March 25.8 29.1 22.5 83.5 2.2
April 26 29.2 22.8 84 2.2
May 25.2 27.9 22.6 86.4 2
June 23.3 25.6 21 85.6 2.5
July 22.3 24.4 20.2 85.9 2.5
August 22.3 24.4 20.3 87.4 2.8
September 23.2 25.4 21 89 2.5
October 23.8 26.1 21.5 90 2.8
November 24.4 27 21.8 88.7 2.5
December 24.9 27.8 22 87.9 2.5
Mean 24.4 27.0 21.7 86.5 2.4
Minimum 22.3 24.4 20.2 83.5 2
Maximum 26 29.2 22.8 90 2.8
4.1.2 Climatic evaluation for cocoa cultivation in study area
Climatic requirements for the cultivation of cocoa have already been mentioned in the literature
review (Chapter 2), and are repeated here for practical reasons. The parameters relevant to
investigate the suitability of a climate to grow cocoa are rainfall, temperature, relative humidity
and length of the dry season. Table 12 and Table 13 show the climatic suitability per parameter,
respectively according to Sys et al. (1993) and according to Ritung et al. (2007). The conditions
at the study area are indicated in green.
Table 12: Climatic requirements for cocoa(Sys et al., 1993)
Climatic characteristics
Climatic class, degree of limitation and rating
0 S1
100 95
1 S1
95 85
2 S2
85 60
3 S3
60 40
4 N1 N2 40 25 0
Annual precipitation (mm)
1800 – 2000 1600 – 1800 2000 – 2500
1400 – 1600 2500 – 3500
1200 – 1400 3500 – 4400
- <1200 - >4400
57
Length of dry season (months)
0 – 1 1 – 2 2 – 3 3 – 4 - >4
Mean annual temp. (°C)
25 – 28 23 – 25 28 – 29
22-23 29 – 30
21 – 22 -
- <21 - >30
Mean annual max. temp. (°C)
< 28 28 – 30 >30 - - -
Mean annual min. temp. (°C)
>20 15 - 20 13 - 15 10 - 13 - <10
Relative humidity driest month (%)
45 – 60 40 – 45 60 – 65
35 – 40 65 - 75
30 – 35 75 – 85
<30 - >85 -
Table 13: Climatic requirements for cocoa (Ritung et al., 2007)
Climate characteristics
Land suitability class
S1 S2 S3 N
Annual average temp. (°C)
25 – 28 20 – 25 28 – 32
- 32 – 35
<20 >35
Average annual rainfall (mm)
1500 – 2000 - 2500 – 3000
1250 – 1500 3000 – 4000
<1250 >4000
Dry months (month)
1 – 2 2 – 3 3 – 4 >4
Relative humidity (%)
40 – 65 65 – 75 35 – 40
75 – 85 30 – 35
>85 <30
Temperature
The minimum temperature is well over the 10 °C required for cocoa. The maximum temperature
never exceeds 30 °C. The mean annual temperature is 24,4°C and the monthly mean
temperatures lie between 22,3 °C and 26 °C. When compared with the requirements of Sys et
al. (1993) all values can be assigned a S1 label. According to the annual average temperature
requirement of Ritung et al. (2007) the mean annual temperature is 0,6 °C away from a S1
classification. It can be stated that the temperature conditions are highly suitable for cocoa.
Relative humidity
In contrast to temperature, the relative humidity on the island is a lot less suitable. The mean
annual relative humidity is 86,5% with a minimum of 83,5% and a maximum of 90%.
According to the data of Verheye (1986) this is applicable to the whole island, as there is no
big difference in relative humidity between all climate stations. A relative humidity above 85%
however is considered to be unsuitable for cocoa. Strictly following the suitability requirement
for relative air humidity, the climate should be marginally suitable (S3) to unsuitable (N) for
cocoa cultivation.
58
A high atmospheric humidity can be favourable when soil moisture is inadequate. With these
high amounts of precipitation in the study however, soil moisture will be sufficient. In that case
high humidity can be undesirable since it favours the development of various diseases (Wood
& Lass, 1985). According to Sys et al. (1993) relative humidity should be 80% without
important fluctuations. This is rather contradictory to the requirements given in Table 12 that
state that a 80% relative humidity is marginally suitable (S3). They also argue that higher
humidity may enhance diseases. With an average minimum of 83,5% and maximum 90% it can
be stated that the humidity on Sao Tomé is higher than what is convenient for cocoa and diseases
are likely to be an issue. However, a hot and humid atmosphere is essential for the optimum
development of cocoa trees. In cocoa producing countries, relative air humidity is thus generally
high: often as much as 100% during the day, falling to 70-80% during the night (ICCO, 2018).
Precipitation and dry months
The precipitation data used to decide if the climate conditions are suited to grow cocoa are those
recorded by Agripalma, as the station is set up near the office, which is at a distance of about 5
km from the research site. The data are available from 2014 until 2017 and the mean monthly
rainfall is shown in Figure 22. The Agripalma data show big differences with the precipitation
at Dona Augusta, according to Verheye (1986), although Dona Augusta is only 3,5 km away
from the weather station at Agripalma. This reflects the high temporal and spatial variability of
annual rainfall even over very short distances.
Average annual rainfall at Agripalma was 3834 mm y-1. This is just within the range of a S3
classification according to Ritung et al. (2007). Also using the Sys et al. (1993) requirements,
the annual rainfall is categorised as S3, meaning marginally suitable for cocoa. Fasina et al.
(2007) on the contrary state that an annual precipitation between 3500 and 4500 mm is still
moderately suitable (S2).
There are significant correlations between rainfall and yield. Cocoa is highly susceptible to
drought. The advantage of a high precipitation rate is that this problem does not have to be
mitigated (Anim-Kwapong & Frimpong, 2005). It is sure that irrigation won’t be needed and
an inhibition of flower and fruit formation because of the insufficient moisture availability will
not appear (Landon, 2013). However, there may be an excess of precipitation water, resulting
in too much soil water increasing the risk of waterlogging. Since this prevents root respiration,
it will damage the cocoa tree. It is likely that the high amount of rainfall will have a higher
burden of fungal diseases as a consequence, particularly Phytophthora pod rot (Wood & Lass,
1985; Landon, 2013). Another problem that may occur because of the heavy rainfall concerns
the soil fertility. High rainfall leads to heavy leaching resulting in poorer soils (Wessel, 1971;
Wood & Lass, 1985). Although the soils of the northern part of Sao Tomé are very similar in
texture and parent material, the sum of basic cations, the base saturation and the ACEC is much
higher than in the soils of the study area (see further), clearly indicating the impact of rainfall
59
on leaching of nutrients and weathering. Whether or not this is inconvenient in the case of this
research area will be discussed in the section soil research.
The lack of dry months is a drawback according to Petithuguenin (1998) as wet conditions are
propitious to pod diseases such as Phytophthora sp. rot. Also, it hinders the process of natural
drying of the beans. As Sao Tomé has different climates over a short distance, a solution here
could be to transport the pods to a dryer area where the natural drying can take place. Artificial
dryers are also possible (Wood & Lass, 1985).
When it comes to amount of dry months per year at the research area and the length of the dry
season, there should be no problem for cocoa cultivation. Dry months are defined as the number
of months with less than 100 mm precipitation (Läderach et al., 2013). Over the 4 years of
rainfall measurements at Agripalma, there are 0 months per year with less than 100 mm and
therefore 0 dry months. Based on these data, annual rainfall can be awarded a S1 suitability,
according to both Sys et al. (1993) and Ritung et al. (2007). According to the climate data of
Verheye (1986) there are two dry months in Dona Augusta, being June and July. A dry season
of two months or less however, still falls under a S1 suitability. It is highly possible that when
rainfall at Agripalma is recorded over many years, the average annual rainfall of the months of
June and July will also be less than 100 mm month-1.
60
4.2 Soil and landscape suitability
4.2.1 Soil and landscape data
The physico-chemical soil data and landscape properties of the investigated soil profiles (see
Chapter 2) are presented in Table 15, the physico-chemical data of the composite samples in
Table 16. Pictures of the profiles are presented in appendix.
4.2.2 General evaluation soil profiles and composite samples
In Table 14 the soil characteristics of the five profiles are listed and for each characteristic a
suitability class is assigned. Below, the profiles will be evaluated on their chemical, physical
and landscape properties, according to the criteria listed in Table 3 (chapter 2.2.6.2, soil
requirements for cocoa).
Table 14: Suitability of soil and landscape for soil profile data
Value Suitability Value Suitability Value Suitability Value Suitability Value Suitability
Texture C-CL S1 CL S1 C S1 SL-L S2/S3 SCL-C S2 Sys et al. (1993)
Stoniness (%) 25 S2 10 S1 30 S2 15 S2 25 S2 Sys et al. (1993)
Soil depth (cm) 100-150 S1/S2 100-150 S1/S2 95 S2/S3 150 S1 100-150 S1/S2Ritung et al. (2007), Sys
et al. (1993)
Drainage/GW >150 S1 >150 S1 >100 S1 >150 S1 >150 S1 Sys et al. (1993)
ACEC (cmol/kg) 58 S1 79 S1 45 S1 186 S1 31 S1Ritung et al. (2007), Sys
et al. (1993)
ACEC
(cmol/kg)58 S1 79 S1 45 S1 186 S1 31 S1 Ayorinde et al., 2014
Base sat. (%) 14,2 S3 32,5 S2 12,1 S3 26,6 S2 16,3 S3Ritung et al. (2007), Sys
et al. (1993)
Base sat. (%) 14,2 N 32,5 N 12,1 N 26,6 N 16,3 N Ayorinde et al., 2014
∑ basic cat.
(cmol(+)/kg)4,5 S1-S2 13,5 S1 4,7 S1-S2 13,4 S1 4 S2
Ritung et al. (2007), Sys
et al. (1993)
pHH2O 5,58 S2 5,14 S3 4,8 S3 5,15 S3 5,3 S3Ritung et al. (2007), Sys
et al. (1993)
O.C. (%) 4,5 S1 6,2 S1 7,2 S1 3,2 S1 3,6 S1Ritung et al. (2007), Sys
et al. (1993)
N (%) 0,3 S1 0,6 S1 0,6 S1 0,3 S1 0,2 S1 Ayorinde et al., 2014
Slope (%) 0 S1 0 S1 45 N 0 S1 3 S1Ritung et al. (2007), Sys
et al. (1993)
Slope (%) 0 S1 0 S1 45 N 0 S1 3 S1 Ayorinde et al., 2014
Surf. Ston. (%) 50 N 20 S3 40 S3 5 S2 30 S3 Ritung et al. (2007)
Source
Rooting conditions
Nutrient retention
Landscape
Profile 1 Profile 2 Profile 3 Profile 5 Profile 6
The parameter ’base saturation’ is not a valid parameter in land evaluation for the considered
soils as they score non to marginally suitable for most soils, whereas the ‘sum of basic cations’
is optimal.
61
Table 15: Soil and landscape data of soil profiles
Horizon Depth Stoniness Soil depth CEC Ca K Mg Na ∑ basic cat Base sat, Ca/Mg/K ACEC pH OC N Avail P Slope surf. ston.
cm % cm % cmol/kg cl H2O mg/kg % %
Profile 1 25,0 100-150 35,1 3,0 0,3 1,0 0,2 4,6 13 0,1 49,4 5,5 4,49 0,34 15 3,8 flat 50
A 0-10 C 15 34,1 3,44 0,47 1,64 0,28 5,83 17 62/29/8 0,15 24 5,3 6,89 0,57 12 3,04
B 10-70 CL 35 36,1 2,62 0,13 0,36 0,19 3,30 9 84/12/4 0,11 74 5,7 2,09 0,12 18 4,5
C 70-100 L 35 26,1 3,16 0,18 0,76 0,19 4,29 16 77/19/4 0,10 76 5,8 1,77 0,13 14 15,2
Profile 2 7,5 100-150 47,1 10,1 0,5 2,8 0,3 13,7 35 0,1 69,5 4,8 6,18 0,64 10 4,6 flat 20
A 0-16 CL 10 48,1 9,21 0,52 2,37 0,27 12,36 32 76/20/4 0,07 65 4,7 6,76 0,71 10 4,82
AB 16-23 CL - 43,1 13,71 0,26 4,73 0,24 18,94 44 73/25/1 0,03 86 5,2 3,87 0,38 10 3,49
B 23-60 C 5 43,1 6,76 0,36 5,10 0,19 12,40 29 55/42/3 0,05 74 5,3 1,95 0,16 13 0,99
C 60-110 C 50 42,1 6,21 0,83 3,19 0,16 10,39 25 61/31/8 0,10 90 5,4 1,46 0,12 12 1,72
Profile 3 32,5 95 51,1 4,0 0,5 1,9 0,3 6,7 13 0,1 42,7 4,3 7,16 0,58 12 3,4 45 40
A 0-20 C 40 51,1 3,98 0,50 1,94 0,27 6,69 13 62/30/8 0,13 43 4,3 7,16 0,58 12 3,42
B 20-70 C 25 35,1 3,00 0,18 0,59 0,21 3,98 11 79/16/5 0,11 45 4,9 2,10 0,16 13 2,93
C 70-95 C 35 30,1 2,74 0,16 0,36 0,27 3,53 12 84/11/5 0,14 46 5,2 1,31 0,10 13 1,72
Profile 5 16,7 150 51,6 10,0 0,5 6,2 0,2 16,9 33 0,0 186,4 5,1 3,15 0,25 12 11,9 flat 5
A 0-10 SL 15 51,1 11,36 0,92 9,41 0,28 21,97 43 52/43/4 0,06 198 5,0 3,70 0,29 13 10,60
AB 10-25 L 15 52,1 8,66 0,16 2,95 0,15 11,91 23 74/25/1 0,03 175 5,2 2,60 0,21 12 13,10
B 25-85 L 20 51,1 6,51 0,21 5,29 0,24 12,23 24 54/44/2 0,04 298 5,1 1,15 0,08 14 80,00
C 85-115 SL - 44,1 4,64 0,40 2,09 0,16 7,29 17 65/29/6 0,08 455 5,3 0,48 0,03 17 182,00
Profile 6 27,5 100-150 24,6 3,2 0,3 0,8 0,2 4,4 18 0,1 27,7 5,3 3,57 0,25 15 4,2 3% 30
A 0-10 SCL 30 29,1 3,68 0,40 1,33 0,31 5,72 20 68/25/7 0,14 30 5,1 5,50 0,39 14 6,22
B 10-70 C 25 20,1 2,68 0,10 0,22 0,18 3,17 16 89/7/3 0,10 25 5,5 1,65 0,10 16 2,24
C 70-115 C 50 23,1 2,62 0,12 0,24 0,16 3,13 14 88/8/4 0,10 38 5,3 1,53 0,12 12 6,65
%
C/N Textural
class cmol(+)/kg soil
(K+Na) /
(Ca+Mg)
Chemical data for land evaluation for top 25 cm (weighed average) in bold
Table 16: Soil data of composite samples
Bloc Stoniness CEC Ca K Mg Na ∑ basic cat Base sat, Ca/Mg Ca/K Mg/K ACEC pH OC N Total P Avail P
0-20 cm % % cmol/kg cl H2O
A F4 L 14 41,5 7,10 0,40 4,94 0,23 12,67 31 1,44 17,76 12,35 129 4,9 4,06 0,35 12 2266 23,9 0,7
B F4 shallow/old forest L 9 21,0 3,62 0,06 1,75 0,09 5,52 26 2,07 58,57 28,35 39 5,6 3,49 0,30 11 2579 22,4 0,5
C F4 shallow CL 9 27,0 2,63 0,25 2,23 0,17 5,27 20 1,18 10,65 9,02 35 4,9 3,96 0,30 13 1854 13,3 0,7
D L1+F4+F1+F4+F1 CL 8 26,2 1,89 0,16 2,86 0,34 5,24 20 0,66 11,92 18,08 50 5,1 2,49 0,19 13 2477 21,4 0,3
E L1+F4+F1+F4+F1 CL 23 32,9 2,10 0,23 2,05 0,16 4,54 14 1,03 9,26 9,02 75 4,9 2,60 0,20 13 2376 16,1 0,4
F L1+F4+F1+F4+F1 SCL 20 19,8 0,79 0,08 0,74 0,18 1,79 9 1,06 9,31 8,80 27 4,8 3,56 0,27 13 2500 13,2 0,5
G L1+F4+F1+F4+F1 CL 23 24,6 1,53 0,12 1,78 0,16 3,59 15 0,86 13,18 15,26 39 4,9 2,88 0,21 14 2350 14,3 0,4
mg/kg%
Ntot /
P2O5tot
Soil type by Cardoso
and Garcia (1962)
Textural
class
C/N
cmol(+)/kg soil
C: clay (blocky structure), CL: clay loam; L: loam; SL: sandy loam; SCL: sandy clay loam
62
4.2.2.1 Chemical soil characteristics
ACEC
When inspecting the chemical analysis of the profiles (Table 15) and the composite samples
(Table 16), it stands out that the ACEC is generally high, > 24 cmol(+) kg-1 clay, which is
evaluated as S1. ACEC indicates the weathering stage of the soil, the higher the value the higher
the amount of high activity clays (HAC), i.e. 2:1 clay minerals with a high - charge and specific
surface area thus capable of retaining a lot of basic cations, depending on the soil pH (see
further). On the contrary, Cardoso and Garcia (1962) indicate high amounts of kaolinite in the
ferralitic soils occurring in the study area according to their soil map. As the landscape in the
study area is strongly dissected, most probably a rather import rejuvenation of the soils has
occurred due to admixture of younger materials eroded and/or deposited on the lower slopes.
Based on the data of the soil profiles, most of the soils with a B-horizon in the study area can
therefore be classified as Alisols (base saturation < 50%, ACEC > 24 cmol(+) kg-1 clay) or
Dystric Cambisols. Also Acrisols might occur. Shallow soils are classified as Lithosols.
Base saturation and sum of cations
The base saturation rate in both the topsoils of the profiles and in the composite samples,
conversely, is very low, << 50%, coinciding with the low soil pH. Smyth (1966) stated that
base saturation should not be less than 30 to 40% within 50 cm from the surface and that lower
levels may indicate nutritional problems. IFA (2008) even states base saturation should at least
be 50 to 60%. Attention should be drawn to the fact that this is applicable in soils rich in
kaolinitic clays or low activity clays (LAC). These soils have a low ACEC as they mainly
contain these low activity clays. The soils investigated in our study however, hold more high
activity clays, as evidenced by the higher ACEC of > 24 cmol(+) kg-1 clay . The reason that
base saturation is low, is either because the presence of basic cations is low or because the CEC
is high. When looking at the sum of the basic cations it can be stated that the value of this sum
is rather high. Therefore, it is recommended to evaluate the sum of cations instead of the base
saturation, as the base saturation below 40% underestimates the suitability for cocoa (S3 to N)
while the sum of basic cations is rated as suitable S1 for cocoa cultivation. In the suitability
tables established by e.g. Ritung et al. (2007), only the base saturation is mentioned in the
suitability table, while in the table of Sys et al. (1993) also the sum of basic cations of the topsoil
(0-25 cm) for the evaluation of the chemical soil suitability. Here base saturation is still given
because this characteristic can be inferred from the soil classification names when universal
classifications are used. The sum of basic cations is, however, decreasing with soil depth, so
erosion of the topsoil should be prevented.
63
According to Sys et al. (1993) the soils with the highest suitability for cocoa should have a sum
of basic cations > 6.5 cmol(+) kg-1 soil. However, the authors (as also confirmed by Landon,
2013), state that the optimum levels of exchangeable basic cations are:
Ca ≥ 8 cmol(+) kg-1 soil
Mg ≥ 2 cmol(+) kg-1 soil
K ≥ 0.24 cmol(+) kg-1 soil
Profile 1, 3 and 6 have a Ca concentration between 2.5 and 4 cmol(+) kg-1 soil, much less than
the minimum value of 8 cmol(+) kg-1 soil. Profile 2 and 5 have a Ca concentration, closer to or
above the minimum. Profile 1, 3 and 6 have a Mg concentration between 0.20 and 2 cmol(+)
kg-1 soil, all beneath the minimum value of 2 cmol(+) kg-1 soil. The Mg concentrations of profile
2 and 5 are all above cmol(+) kg-1 soil. K concentrations are mostly above the 0.24 cmol(+) kg-
1 soil minimum in the topsoil, except for the lower horizons of profile 1,3 and 6, where it is
about 0.15 cmol(+) kg-1 soil. The highest concentration can be found in the A horizon of profile
5. There is 0,92 cmol(+) kg-1 soil K, which is almost four times the minimum. From the
comparison with the optimum levels of the exchangeable cations it can be deducted that there
are too little divalent cations (Mg and Ca), especially in profile 1,3 and 6. The latter soils should
thus better be rated as S2 rather than S1 for sum of cations according the table of Sys et al.
(1993). Similarly, the composite samples follow the same trend. Here sample A has the highest
content of basic cations, whereas the other samples have lower values, particularly sample F
with a som of basic cations as low as 1.79 cmol(+) kg-1 soil. Sample F is also the soil with the
lowest ACEC (27 cmol(+) kg-1 clay), reflecting high amounts of kaolinite.
In addition to the sum of the cations, it is also important to look at the ratio. Jadin (1976)
mentions an optimal Ca/Mg/K ratio of 68/24/8 for cocoa. This is confirmed by IFA (2008). The
average Ca/Mg/K ratios of the five profiles are shown below. Generally K is lower than the
optimum value in the Ca/Mg/K ratio.
Table 17: cation ratios of the profiles
Profile Ca Mg K
Profile 1 72% 22% 6%
Profile 2 67% 29% 4%
Profile 3 72% 22% 6%
Profile 5 59% 38% 3%
Profile 6 79% 16% 5%
Table 18: cation ratios of composite samples
Composite sample Ca Mg K
A 57% 40% 3%
B 67% 32% 1%
C 52% 44% 5%
D 38% 58% 3%
E 48% 47% 5%
F 49% 46% 5%
G 45% 52% 3%
Profile 5 has relatively high % of Mg, compared to Ca. However, this is more striking in the
composite samples, where (except for samples A and B), the amounts of Mg equal or even
exceed the amounts of Ca, (the latter in composite samples D and G) (see Ca/Mg ratios in Table
64
17 and Table 18 above). Correction of the nutrient balance will be required. The relatively high
amounts of Mg relative to Ca is related to the mineralogy of the parent material, as basalt
contains high amounts of Mg-bearing minerals as pyroxenes and amphiboles.
According to Landon (1984), the ratio of monovalent (K + Na) to divalent (Ca + Mg) cations
should not exceed 1:50 (or 0,02). For all five of the profiles however, this number was exceeded.
Profile 1 having the highest ratio of 0,117 and profile 5 having the lowest ratio of 0,051.
Wood (1985) stated that the determination of individual nutrients is of limited value when
discussing the nutritional status of the soil for cocoa. This is because most nutrients exist in a
variety of forms in the soil, not all of them available to the plant. Furthermore there can be
interaction and antagonisms between the nutrients (Landon, 1984).
Soil pH
The optimum pH for cocoa production lies between 6 and 7 (Ayorinde et al., 2014; Ritung et
al., 2007; Sys et al., 1993), or between 6 and 7.5 according to Wood (1985) with an optimum
of 6.5. The pH-water of the profile samples (Table 15) and composite samples (Table 16) was
overall lower than this optimum. It can be stated that these soils are relatively acid and most
profile and all composite samples, except for profile 1, are rated as S3 or N (for those with pH
of topsoil < 5). When soils are too acid for cocoa, phytotoxic concentrations of micronutrients
(Al, Mn) can occur. The major nutrients (basic cations) on the other hand, become less
available. Generally however, cocoa is more resistant to acid soils than many other tropical
crops (Landon, 2013). Geus (1973) has recorded cocoa-producing areas in DR Congo where
pH values were as low as 4.5, suggesting that the requirements for pH in the tables of Sys et al.
(1993) and Ritung et al. (2007) are too severe. Wood (1985) argues that cocoa can be cultivated
on acid soils, as long as it provides enough nutrients. In acid soils aluminium toxicity is a major
problem and according to van Vliet and Giller (2017) a limiting factor for plant production.
Due to aluminium toxicity liming might need to be very high. Liming will also increase the Ca
content of the soil. Overliming, however, can cause additional problems such as zinc deficiency.
OC
Organic matter in the soil is of uppermost importance for the increase in retention and
availability of nutrients (basic cations) and supply of nutrients as N, P and S. The importance
of organic matter is thus largely due to their influence on the availability of nutrients (van Vliet
& Giller, 2017). All investigated soils (profile topsoils and composite samples) have high
amounts of OC, above the optimum values (some soils have OC contents > 6%) stated in the
requirement tables of Sys et al. (1993) and Ritung et al. (2007). They are thus all rated as S1.
As organic matter contents are high, so are the total N contents as the C/N ratio are situated
between 11 and 14 in most soils (see Tables 15 and 16).
65
Phosphorous
Although P is not mentioned in the tables of Sys et al. (1993) and Ritung et al. (2007), it is an
important soil nutrient. Unfortunately, data on available P in literature is often confusing as this
parameter can be measured by different methods (Olson, Bray,…). In our study, available P has
been measured with the Bray2 method. According to IFA (2008), suitability levels based on the
Bray2 extraction method are the following:
Low (L) < 6 mg kg-1
Average (A) 7-15 mg kg-1
High (H) >15 mg kg-1.
Considering the profile data (analysed at UGent), all soils, except for profile 5, have low
amounts of available P, < 6 mg kg-1. On the contrary, although measured by the same method
at SoGB, the contents in P of the composite samples are much higher, i.e. > 13 mg kg-1.
The total P was also measured for the composite samples and is situated between 1850 and
2500 mg kg-1. CIRAD (Snoeck et al., 2016) investigated the response of N- and P-fertilizers in
f( ) the N/total P2O5 as shown in Figure 23. This graph shows the effect of P-fertilizers in
relation to the optimum equilibrium of the N/total P2O5 ratio for various cocoa fields in Ghana
having different contents in N and P. If the soil is below the required optimum for cocoa as
shown in the graph, an addition of phosphate enables an increase in production. On the contrary,
if the soil is already provided with P, phosphate addition may have a negative effect on the
production and an addition of N should have been applied. If the relationship in the graph below
is also valid for the soils of the study area, the total P2O5 % is high (0.42-0.59 %), the N content
is ranging between 0.19 and 0.35. This would indicate that an N fertilization is required.
Figure 23: N/P2O5 ratio of some cocoa plantations in Ghana in comparison with the optimum
(Source: Snoeck et al., 2016)
66
4.2.2.2 Physical soil characteristics
Physical factors that influence the cocoa cultivation are soil depth, drainage, stoniness and soil
texture.
From the data, it can be concluded that overall the texture of the soils are very suited (S1) to
grow cocoa, since most of them are mixtures of clay and loam. As visible in the soil textural
triangle (Figure 24) the majority of the soils are clay loam, or close. A few are more sandy and
a few real clay. The latter soil was found in profile 3, located on a steep slope in the eastern part
of the area. Differences in soil texture are related to a variation in parent material or erosion.
The latter is particularly the case for the sandy loamy texture observed in profile 5: the topsoil
is more sandy probably due to erosion of the loamy particles, whereas the sandy loam in the C
horizon might be the result of sand size particles of weathered rock. Profile 5 could be rated as
S3, due to the sandy loam topsoil texture. However, the AB and B horizons are loamy, which
is assigned an S1 suitability. This puts the “less suitable class” in perspective and makes the
profile 5 soil still a good soil for cocoa cultivation. The texture of the composite samples is
similar to the topsoil texture of most soil profiles, as also represented in Figure 24.
Figure 24: Texture of the soil profiles (red for topsoil, green for subsoil) and composite samples (blue)
In general the soils in the research area are not very deep. However, cocoa can grow in rather
shallow soil, when water supply is sufficient. As discussed in chapter 4.1 rainfall ensures plenty
of water. On how deep the soil exactly should be, literature varies. The most used source for
land evaluation, Sys et al. (1993), states that a soil should be deeper than 150 cm to be assigned
a S1 suitability. A depth between 150 and 100 cm is still moderately suitable and even if soil
67
depth is less than one meter, cocoa cultivation is possible, however marginally suitable. In most
profiles, it was possible to dig deeper than 1 m, before too much stones occurred. Therefore
most profiles are assigned a S2 rating for soil depth. A second source used in this evaluation
(Ritung et al., 2007) notes different numbers for soil depth, giving > 100 cm already an S1
suitability and soils between 100 cm and 75 cm deep are still moderately suitable. Soils not
deeper than 50 cm are unsuitable according to this later source. Suitabilities according to both
sources are indicated in Table 14. The evaluation of the soil depth of the augerings was based
on values of Ritung et al. (2007).
The spatial distribution of the soil depth as evaluated from the augerings is shown in Figure 25.
The limitation for rooting was evaluated from the presence of too many stones preventing
deeper augering. It appears on this map that most of the soils in the flatter areas, present in the
western part of the study area, have a deeper depth. The very steep areas in the eastern part had
a limited accessibility, and were characterized by many stones at the soil surface as well as rock
outcrops.
Not once groundwater accumulated at the bottom of a pit, although it had been raining almost
every day. As a consequence it can be stated that drainage in the area is very good (S1) and
should not cause problems for cocoa growth. There was also no risk for flooding.
68
Figure 25: Spatial distribution of soil depth observed in the augerings
The evaluation of the soil also comprises the average soil stoniness till the effective soil depth
(Sys et al., 1997) and the surface stoniness (Ritung et al., 2007). The latter is important for
mechanized farming, less for low level (non-mechanized) of management. When evaluating
surface soil stoniness according to the criteria of Ritung et al. (2007), most soils are non (N) or
marginally (S3) suitable. As we consider a low level of mechanized management, those ratings
are not valid, and most soils (apart from profile 1) should be rated as S2 or S3 if we consider a
value of 30 vol% as limit between the two suitability classes for this level of management.
A high amount of stones at the soil surface does not always indicate a high soil stoniness below.
This is particularly the case for profiles 2, 3 and 6 where the vol% of gravel is clearly lower
than the surface stoniness. When considering the weighted average over the whole profile
giving a decreasing weight from the top towards the bottom of the profile, all profiles are rated
S2, except for profile 2 with a weighted average of < 15 vol%, thus rated as S1. Based on the
weighted gravel content in profiles 1 and 5, those soils are rated as S2.
69
The gravel content has also been estimated during soil prospection. The spatial distribution of
the soil stoniness is presented in Figure 26. The soil stoniness appears to be lower in the soils
in the least sloping areas. As already mentioned above, the eastern part of the study area with
steep slopes is very stony, already at the soil surface.
Figure 26: Spatial distribution of soil stoniness observed in the augerings
The gravel and stones present at the soil surface or in the soil are weathered or unweathered
rock fragments, generally basalt rock, as shown in Figure 27. Weathering of rock continuously
provides nutrients as Ca and Mg to the soil.
70
Figure 27: Weathered basalt fragments present in soil
4.2.2.3 Landscape characteristics
An important parameter to evaluate when choosing a suitable area for the cocoa plantation is
slope, most certainly in this strongly dissected research area, with strong variation in
topography as evidenced in Figure 29 showing the contour lines. Most profiles were dug in a
rather flat area, only pit 3 was located on a steep slope (Figure 28). The reason why it was
decided not to dig pit 4 was the very steep slope and difficult accessibility of the location.
Certainly, this area (eastern part of the study area) is too steep for cocoa cultivation, suitability
class N according to Sys et al. (1993).
Figure 28: Landscape position of profile 3: steep slope and rather high surface stoniness
The slope classes as determined at each observation point during the soil survey are shown in
Figure 29. The classes are those defined in the suitability table of Sys et al. (1993) for low
level of management of cocoa plantation.
71
Figure 29: Slope classes measured at observation points during survey
4.3 Fertilization in organic cocoa plantation
4.3.1 Introduction
As mentioned in the literature review (Chapter 2.2.7), nutrient demands for optimum cocoa
growth and yield are high. In high yielding plantations (2 or more tons of beans ha-1) or on very
poor soils, literature mentions fertilization rates of 100-150 kg ha-1 of N, 90 -150 kg ha-1 of
P2O5 and 120 - 180 kg ha-1 of K2O (van Vliet & Giller, 2017; IFA, 2008; Uexküll & Cohen,
1980). Without added fertilizer, the nutrient balance in cocoa systems is negative for all
nutrients considered when assuming a yield of 1000 kg ha-1 y-1 (van Vliet & Giller, 2017). The
study of Bai et al. (2017) indicates that even in cocoa agroforestry plantations fertilization and
better nutrient management are required. Cocoa established from virgin forest on fertile soils
may not require fertilizers for many years (Appiah et al., 2000). When no fertilizers are applied,
cocoa production will deplete the soil of nutrients (Gockowski et al., 2013), therefore the
addition of nutrients is needed to sustain productivity. Gradually, the benefit of planting on
virgin forest rather than replanting is lost as soil fertility declines (Ruf and Schroth, 2004 in van
72
Vliet & Giller, 2017; Ahenkorah et al., 1974; Hartemink & Donald, 2005; Ofori-Frimpong et
al., 2007).
Nutrient reserves will be more rapidly depleted with increased production, for instance when
agronomic practices are improved and/or shading is reduced (Appiah et al., 2000). According
to Ahenkorah et al. (1987), who established a famous fertilizer and shade trial during the 1970’s
and 80’s, P and K fertilization should be adjusted to the overhead shade conditions. They
showed that the positive impact of fertilizer application is bigger in absence of shade. In contrast
to potassium, little phosphorus is exported through harvest. Ahenkorah et al. (1987) found in
their research that P application in shaded plots resulted in continued increase of soil available
P and positive yield responses. They recommend large rates of P application especially under
shaded conditions. The authors also observed that yield increases through K fertilizers are
bigger in unshaded cocoa. Under heavy shade, P fertilized plots yielded more than K fertilized
plots. Therefore, large K application is especially economical in absence of shade.
In the study of Jadin (1988) soils of cocoa plantations in the northern part of Sao Tomé were
investigated for their fertility. Remarkable was the fact that available P was higher than in other
west African countries and above the optimum level. The Ca/Mg/K ratio’s had to be corrected
with fertilizer is most cases. pH in these soils was generally higher than in our research, as
leaching of soil nutrients is higher related to the very high rainfall in our study area, resulting
in more acid soil conditions. Those soils need liming to increase soil pH-water above 5.5.
Liming will also adjust the Ca/Mg imbalances (high Mg relative to Ca contents, related to the
weathering of the basaltic parent rock, rich in Mg-containing minerals), when CaCO3 is used
as liming material.
In non-organic cocoa cultivation, nutrients are supplied by mineral fertilizes, generally as: urea,
DAP, MAP for N (and P); SP, TSP, rock phosphate for P, muriate of potash or sulphate of
potash for K, ground limestone or dolomite for liming (also supplies Ca and Mg). Rock
phosphate is generally applied on acid soils, liming is needed only in exceptional circumstances
where exchangeable Al > 3 cmol(+) kg-1 soil (IFA, 2008). Since soils in the research area are
rather acid, Al becomes more available and its toxicity might become a limiting factor for plant
production (van Vliet & Giller, 2017). The major nutrients become less available in acid soils
(Landon, 2013). Thus liming is advisable up to a pH 5.5 to 6, whereby a S2 suitability would
be assigned for cocoa production according to Sys et al. (1993).
Kamprath (1970) suggested that lime recommendations should be based on the amount of
exchangeable Al in the topsoil and that lime rates be calculated by multiplying the cmol(+) of Al
by 1.5. The result is the cmol(+) of calcium needed to be applied as lime (1 cmol(+) exch Al
1.5 cmol(+) Ca 1.65 tons/ha CaCO3-equivalent). Lime rates calculated by this method
neutralize 85 to 90 percent of the exchangeable Al in soils containing 2 to 7 percent organic
matter. The reason for 1.5 as a factor is the need to neutralize the hydrogen ions released by
73
organic matter or Fe and Al hydroxides as the pH increases. In soils with higher organic matter,
the factor has to be raised to 2 or 3.
There are a range of liming materials that are available for agriculture use, listed in Table 19. The
amount of liming material required to rectify soil acidity depends on the neutralizing value of the
liming material and the pH buffering capacity of the soil.
Table 19: Liming materials
Type Composition CCE
Calcite CaCO3 100
Dolomite CaMg(CO3)2 105-110
Burnt (quick) lime CaO 150-180
Slaked lime Ca(OH)2 130-140
Slag CaSiO3 75-85
Basic silicate rocks Variable, e.g. basalt rock 15-35
Wood ashes Variable 60-80
CCE: Calcium Carbonate Equivalent (f( ) sufficient fineness and purity of material)
Question is: can organic farming, which relies primarily on the cycling of organic matter to
maintain soil fertility, supply the required nutrients (particularly K), without the use chemically
produced fertilizers? In an organic cocoa plantation, besides the focus on organic fertilizers,
also chemically untreated rock dusts (agrominerals, see van Straaten, 2002 and 2007) can be
used. Use of chemically produced fertilizers should be limited.
4.3.2 Organic materials
Organic fertilization may comprise, amongst others, the returning of the cocoa pod husks to
the soil, mainly as a source of K and to lesser extent of P and N (Boyer, 1973; Fontes et al.,
2014; Thong & Ng, 1978), as commonly practiced in many Latin American cocoa plantations
(Santana & Cabala-Rosand, 1982). However, there is a risk of infection with black pod disease
when returning the husks to the plantation (Adejumo, 2005; Boyer, 1973). Attention should
thus be paid to the appearance of black pod disease. The problem might increase as the disease
is spread through oospores on dead materials, such as husks of previously infected pods. To
prevent this infection, cocoa husks can be burned or composted prior to application although
this is not common practice. Because N is lost during burning, composting is preferable. Kongor
et al. (2018) suggests cocoa pod husk ash and wood ash as organic liming materials, also
increasing soil P, K, Ca and Mg content.
The use of organic fertilizers and the inclusion of N2 fixing trees, also serving as shade tree,
can greatly contribute to nutrient availability in organic cocoa production (Agbeniyi et al.,
2011). The use of alternative organic amendments must be feasible and profitable, as they have
74
the advantage over standard NPK or other chemical fertilizers of adding other nutrients such as
Ca, Mg, and micronutrients. They also assist in maintaining soil organic matter content, source
of N and other nutrients as P and S. Moreover, soil organic matter increases the CEC and
improves soil structure and water holding capacity. Waste materials, other than those of cocoa
mentioned above, produced close to the cocoa production site may offer opportunities
(Wichmann, 1992). Those organic materials present in Sao Tomé might be:
Coffee husks, rich in organic matter, and chemical nutrients such as nitrogen (N) and
potassium (K) (Table 20), often composted with addition of rock phosphate and manure
(Nguyen et al., 2013; Biddipa et al. 1998). When composting, as coffee husks have a high
C/N ratio, the addition of manure and fruit/vegetable wastes, which are rich in easily
biodegradable nitrogen compounds, can reduce the C/N ratio and increase the rate of
degradation (Shemekite et al., 2014). Often, however, coffee husks are burned to prevent
spreading of pests and diseases, the ashes applied to the fields (Nguyen et al., 2013).
Table 20: Chemical nutrient composition of the coffee husk compost after three months composting
(Nguyen et al., 2013).
Waste products from the nearby oil palm plantations can enhance yields by the added
nutrients, mainly K through decomposition of the empty fruit bunches (EFB) or ash from
EFBs. The waste products of palm oil mills are presented in Figure 30 (Anyaoha et al.,
2018). However, these products are not likely to be available for cocoa production as they
can be applied for mulching in the same oil palm plantations that they originate from. A
study of Rabumi (1998) and Sabrina et al. (2009) revealed following chemical composition
of EFBs: 0.76 - 0.96% N, 0.13 - 0.19% P, 1.21 - 3.20% K, 0.36 - 0.60% Ca and 0.22 -0.51
% Mg. EFB ash may contain as much as 25 - 33% K (Chan et a1.1981; Anyaoha et al.,
2018). However, the bulk of nutrient content like N and organic matter are lost during this
ashing process. EFBs can also be composted, often together with other materials as POME
(Palm Oil Mill Effluent), POS (Palm Oil Sludge), manure etc. This delivers a fertile soil
amendment with high contents of nutrients, as shown in Table 21, for a compost made of
EFBs and POME in Malaysia (Baharuddin et al., 2009). Similarly, coconut husks could be
used as an additional source of nutrients, mainly K (Konduru et al., 1999), although the K-
75
contents are lower than in oil palm waste. Also waste from rubber plantations can be used
as organic fertilizer and enhance yields (Sharifuddin & Zaharah, 1991 in van Vliet & Giller,
2017).
Little research seems to have been done regarding the potential effect on cocoa yields of
animal manures. The use of chicken manure by cocoa farmers in Ivory Coast has increased
over the last decade. Farmers report significant increases in yield as a result (Ruf, 2016).
Composition of manure may vary widely depending on the diet, health, and type of animal.
The nutrients transferred to the soil further depend strongly on the handling, storage, and
application of the manure (Ahn, 1993; Rufino et al., 2007). Ruf (2016) and Ruf & Kiendré
(2017) stated that cocoa yield increases spectacularly after addition of chickens manure in
smallholder plantations in Ivory Coast. In a case study, the application of 3000 kg ha-1 of
chicken manure more than doubled the cocoa yield from 580 to 1400 kg ha-1 cocoa beans,
see also Figure 31.
Figure 30: Flow chart of fresh oil palm fruit bunch processing (solid boxes) showing points of
generation of wastes (dashed boxes) (Anyaoha et al., 2018)
76
Table 21: Properties of shredded EFB, POME and final compost (Baharuddin et al., 2009)
Figure 31: Impact of chicken manure on cocoa production (Ruf, 2016)
77
4.3.3 Agrominerals
In natural systems, nitrogen is ‘harvested’ from the air by legumes or through N-fixing
organisms and recycled in the soil. Other nutrients critical for plant growth, like P and K, Ca,
Mg, S and micronutrients are supplied by geological resources - rocks. Agrominerals are very
finely ground minerals or rocks (‘rock dust’) which might provide nutrients upon weathering.
Some agrominerals occur naturally in concentrations and forms that can be used as alternative
fertilizers or soil amendments. ‘Reactive’ sedimentary phosphate rock (PR), potash, gypsum,
dolomite, limestone, and various other minerals fall in this category. In other cases, mineral
resources do not occur in a form that is directly available to crops and must be modified
physically, chemically and biologically to become effective nutrient sources for soils and crops.
For other agromineral resources, such as ground silicate rocks, large quantities of rock material
are needed to be agronomically effective (Gillman et al., 2000, 2002; Harley and Gilkes 2000,
Van Straaten 2002, 2006, 2007). Agrominerals that weather rapidly will provide nutrients to
the crop and also increase soil pH, as evidenced in studies in Congo (Kanyankogote et al., 2005;
Kasongo et al. 2012a, 2012b, 2013).
According to Van Straaten (2002), the potential of agromineral development on São Tomé (and
Príncipe) is very limited due to the lack of suitable agrogeological resources. The mineral
industry of São Tomé and Príncipe is limited to the production of local building materials, small
clay and stone open pit operations. As already mentioned in Chapter 2.1.3, the island mainly
consists of basalts (see Figure 4) (Caldeira & Munha, 2002; Caldeira et al., 2003). The
geological map of the study area is presented in Figure 32. Basalt can be seen as a slow-release
fertilizer as it may take more than 9 months for basalt to react effectively in soils. After 27
months, still not all basalt had dissolved in the field (Shamshuddin et al., 2011). Basalts are
often used in organic farming as source of Ca and Mg, and for their impact on soil pH. In cocoa
farming, Shamshuddin et al. (2011) used peat, chicken manure, and basalt mainly to reduce Al
toxicity and the additional effects of Mg and Ca as nutrients. Chemical analysis of some basaltic
rocks by Caldeira and Munha (2002) reveal following amounts of major nutrients: +/- 10 %
CaO, 9-10% MgO and +/- 1.5% K2O. Also phonolitic and trachytic rocks are present in Sao
Tomé, particularly in the southeastern part of the island. Phonolites are more resistant against
weathering than basalt and occur as chimneys in the landscape (Figure 32 and 33). As they are
rich in nepheline and potash feldspars, the K2O content is around 5 – 6 %, thus much higher
than in basalts. If quarries on those rocks exist close by the study area, finely ground quarry
waste could be used as a source of K, Mg and Ca.
It will surely be interesting to investigate the possibility of using agrominerals for cocoa
farming, mainly as additional source of Ca, Mg and K. Lithosols (shallow soils rich in gravel
or close to parent rock), as described and characterized by Cardoso & Garcia (1962) sometimes
78
contain very high contents of K, up to 2.5 cmol(+) kg-1 soil (often higher than the Mg content)
evidencing the presence of K-rich parent rock.
Rock phosphate and limestone & dolomite are also agrominerals which can be applied as
phosphate fertilizer or liming materials respectively. However, those materials are not present
in Sao Tomé. K-salts can also be used as agrominerals.
Figure 32: Geological map of the study area and surroundings (extract of Geological map of Sao Tome,
sheet 5, on scale 1:25000)
79
Figure 33: Phonolite chimney in Sao Tomé (Cão Grande)
80
4.4 Evaluation of the vegetation
As the purpose of this study is associated with the establishment of a cocoa agroforestry
plantation, it was necessary to have a broad view on the current vegetation in the research area
and to verify in what way it could be useful as shading in a future plantation. Using the
parameters, measured in the 9 investigated vegetation plots (Figure 34), multiple conclusions
can be drawn concerning several agroforestry topics.
Figure 34: Location of the investigated vegetation plots
81
Forest without remains of crops or with abandoned crop
Cocoa trees
Coffee
Isolated trees
Coconut palms
(Oil) palms
Figure 35: Legend of the vegetation of the map (1958) used as background for the map in Figure 3434
4.4.1 Tree density and canopy openness
In order to have an idea of the density of the forest and the amount of light incidence, the canopy
cover (or canopy openness) was measured using a densiometer (Table 22). In the literature the
degree of shade is usually not quantified in a way that is comparable across studies. The shading
density is often expressed as percent of full daylight transmitted. However, as radiation intensity
at “full daylight” varies between regions, so will the optimal percentages of daylight for cocoa.
Also, different variables such as soil moisture, which may influence optimal degrees of shade
as well, are not taken into account. This has as consequence that there is no unambiguous
optimal shade percentage for this specific situation. Murray (1975) recommends not more than
75% of full daylight. Wood & Lass (1985) suggest 50% for young cocoa. In the study of
Bisseleua et al. (2013) a shade index is used, based on eight parameters, including shade cover,
tree density and number of tree species. They advise a 0.5 shade index. It should be noted that,
as there is a lot of rainfall, the sky is covered with clouds most of the time. This means less full
sunlight, and might have as a consequence that less shade is needed in this area than in very
sunny areas.
In the current forest, on average 7% to 10% of the canopy is not covered. The least covered
point measured had 16% canopy openness. The darkest points in the plots had often 6% canopy
openness. This means too much shade for an optimum compromise between yield and
ecosystem services in an agroforestry system. As a consequence quite a lot of trees will have to
be felt for the establishment of a cocoa agroforestry plantation. In order to decide which trees
should remain, the biodiversity and tree height (see below) should be taken into account. Also,
the desirable characteristics of shade trees according to Beer (1987), listed in chapter 2.2.11,
should be considered.
82
Table 22: The canopy openness, tree density and amount of big trees per plot
plot n°
Mean canopy
openness (%)
Lightest
point
Darkest
point
Tree density
(trees/m2)
Amount of trees bigger
than 50 cm DBH
1 10.3% 16.12 6.24 0.077 2
2 7.2% 8.32 5.72 0.047 5
3 7.7% 9.88 5.98 0.068 2
4 6.9% 9.36 5.72 0.040 1
5 7.5% 9.10 5.98 0.075 1
6 9.3% 11.18 8.06 0.043 4
7 8.0% 10.40 6.50 0.051 3
8 10.1% 13.26 8.06 0.039 0
9 10.5% 13.26 6.76 0.067 2
As discussed earlier, 3 major parameters used to describe the vegetation are canopy openness,
tree density and tree stem diameter at breast height (DBH) (Table 22). To check if these
parameters are interrelated, a visual inspection in two by two scatter plots was made (Figure
36). These charts show no relationship between the variables and also linear regression fails to
explain any of the variance (R2 near 0). A Chi-square test for independence confirms that there
is no relation between variables in their distribution over the plots. Whether the Yates correction
for small data sets is applied or not, the null hypothesis is firmly accepted. This means that in
dense forest (large amount of trees per m2) the canopy is not necessarily closer. Nor can it be
said that more big trees lead to a closer canopy.
Figure 36: Scatter plots of tree density, canopy openness and amount of big trees (DBH > 50 cm)
y = -21,723x + 4,0951R² = 0,0396
0
1
2
3
4
5
6
5,0% 6,0% 7,0% 8,0% 9,0% 10,0% 11,0%
# b
ig t
ree
s
Canopy openness
y = 0,2151x + 0,0377R² = 0,0408
0,000
0,020
0,040
0,060
0,080
0,100
5,0% 6,0% 7,0% 8,0% 9,0% 10,0% 11,0%
Tre
e d
en
sity
Canopy openness
y = -0,0014x + 0,0594R² = 0,0207
0,000
0,020
0,040
0,060
0,080
0,100
0,05 1,05 2,05 3,05 4,05 5,05 6,05
Tre
e d
en
sity
# big trees
83
Figure 37: Picture of the canopy in plot 6, plot 1, plot 3
4.4.2 Biodiversity
Diversity is an important factor to take into account when establishing a cocoa agroforest. In
the first place because one of the goals of an organic agroforestry plantation is the conservation
of biodiversity (Schroth & Harvey, 2007; Vaast & Somarriba, 2014). Secondly there is
functional biodiversity. For example biological control of pests and diseases through
maintaining populations of natural enemies using shade trees. Also using non-host tree species
as a barrier to prevent the spread of pests and diseases (Vaast & Somarriba, 2014). Bisselau et
al. (2013) and Sperber et al. (2004) found that greater shade tree diversity supported more
natural enemies. Table 23 shows the different parameters that give information about the
biodiversity in the current vegetation.
Table 23: Biodiversity parameters for the nine plots (30 m x 30 m)
Plot n° # trees % palm #different
species
Remarks #trees with
known use
1 69 38% 9 1
2 42 52% 9 6
3 61 34% 10 1
4 36 39% 10 3
5 18 22% 6 plot only 8x30m 0
6 39 26% 11 1
7 46 15% 12 1
8 35 37% 10 one fifth of the plot covered by
a big fallen tree (Fruteira)
5
9 60 43% 8 12
A total of 406 trees of 22 different species have been measured in the 9 vegetation plots, with
a total surface of 7440 m2. This amount seems low, compared to the research of Sonwa et al.
(2007) in agroforests in Cameroon, where 0.75 ha contains at least 50 different species (Figure
38). In Table 24 the 22 vernacular names of the trees can be found with their scientific name
and their use, if mentioned, in the paper of Jaroget et al. (2014b).
84
Figure 38: Cumulative curve of plant species associated with cocoa plantations in southern Cameroon
(Sonwa et al., 2007)
A remarkable finding is the amount of palm trees in the current forest. In most plots more than
25% of the trees were palms, young or old. They seemed to be all oil palms. No coconut palm
was registered in the plots. This could be a residue of old oil palm plantations that were located
in this area. Oddly the vegetation map of 1958 (Figure 34) shows no oil palms, only coffee and
cocoa. Oil palms can be used in agroforestry (Wood & Lass, 1985), but the ones currently in
the forest are not highly productive, and should maybe be replaced. Although the map of 1958
(Figure 34) indicates that there was coffee and cocoa plantation before in this area, strangely
no cocoa trees nor coffee has been encountered during the study. Furthermore, it can be noted
that no leguminous trees were encountered in the vegetation plots. As discussed earlier these
trees are interesting in agroforest system, for their N-fixation. Outside the plots some pulses
were found, indicating that there might be some legumes present in the forest.
Plot 1 has the highest amount of trees, but there are mainly palms and Salla Salla (Figure 39).
One tree is a Jaquinteiru (Treculia africana), named by Jaroget et al. (2014b) as useful for cocoa
shading (Table 24).
85
Figure 39: Salla salla in plot 1(upper left) and shrub layer plot 7, plot 2 and plot 6
Plot 5 is the least diverse one. This could be due to the smaller size of the plot, but also the
slope might play a role here. The plot was located close to profile 3, and slopes reached 40% to
60%. Less trees might be able to develop on such steep slopes, causing a less diverse
composition of the vegetation. There was also a very high percentage of surface stoniness (80%)
and no shrub layer.
Plot 6 is a rather diverse plot, containing a Amoureira (Chlorophora excelsea) that is good for
soil fertility and shading in a cocoa plantation, and also has marketable products (Jagoret et al.,
2014b; Wood & Lass, 1985).
11 of the 60 trees in plot 9 are Jaquinteiru (Treculia africana), explaining why there are so
many useful trees. There is also a Muindru (Bridelia sp.) that can be used locally for its wood
and non-wood products.
86
4.4.3 Tree height (and strata)
According to van Himme & Snoeck (2001) cocoa prefers a discontinuous, fairly heterogeneous
cover at various levels and with gaps. This means shade trees of different heights. Figure 40
visualises the different heights of the trees in the current forest. It shows that most trees are
between 9 and 26 meters in height. The tallest trees were estimated 40 m and located in plots
1, 4 and 6. In general plots 3, 4, 5 and 8 have less tall trees. Keeping in mind the statement of
Himme & Snoeck (2001) about the heterogeneous cover, it would be interesting to preserve
some trees in this upper stratum.
Figure 40: Individual height of the 406 trees measured in 9 vegetation plots.
In order to have a clearer view on the difference in height between the plots, the estimated
heights were categorised in three strata: [0-10], [11-25] and [26-40] meters in height (Figure
41). The value ‘unknown’ was assigned if the tree was dead or because it was impossible to see
the height through the canopy. The bottom stratum [0-10] contains mainly palms. In most plots
less than five trees are taller than 26 m.
0
10
20
30
40
50
0 50 100 150 200 250 300 350 400
He
igh
t (m
)
Tree number
Height of all indiviual trees1 2 3 4 5 6 7 8 9
87
Figure 41: Trees per plot divided in three strata ([0-10], [11-25] and [26-40]).
Steffan-Dewenter et al. (2007) conclude that low-shade agroforestry provides the best available
compromise between economic forces and ecological needs.
0
5
10
15
20
25
30
35
0-1
0
11
-25
26
-40
0-1
0
11
-25
26
-40
un
kno
wn
0-1
0
11
-25
26
-40
un
kno
wn
0-1
0
11
-25
26
-40
un
kno
wn
0-1
0
11
-25
un
kno
wn
0-1
0
11
-25
26
-40
0-1
0
11
-25
26
-40
0-1
0
11
-25
un
kno
wn
0-1
0
11
-25
26
-40
un
kno
wn
plot1 plot2 plot3 plot4 plot5 plot6 plot7 plot8 plot9
Am
ou
nt
of
tree
s
Height (m) divided in three stata
Amount of trees per stratum per plot
88
Table 24: Vernacular and scientific names of the trees found in the nine vegetation plots, and their use
Vernacular
name given
by Amancio
Scientific name
Use according to Jagoret et al. (2014) Source: Lopes
Roseira, 1984
Source: National
Biodiversity Strategy
and Action Plan
2015-2020 (NBSAP
II)
Source: Figueiredo
et al., 2011
Muindro Bridelia sp. Bridelia micrantha Aidia quintasii Wood, non-wood and medicinal products for local consumption
Cashueiru Anacardium
ocidentale
Anacardium
occidentale
Anacardium
occidentale
Not mentioned
Viro (pretu) Cleistanthus
libericus
Cleistanthus spp. Cleistanthus libericus Not mentioned
Safuzeiru Pachylobus edulis Dacryodes eludis Dacryodes eludis Wood, non-wood and medicinal products for local consumption
and marketable products. Also good for shading and soil
fertility
Pau branco Tetrorchidium
didymostemom
unknown Discoglypremna
caloneura /
Tetrorchidium
didymostemon
Local medicinal consumption
Palmeira Elaeis guineensis Elaeis guineensis Elaeis guineensis Wood, non-wood and medicinal products for local consumption
and marketable products. Also good for shading.
Pau lischia unknown unknown Ficus exasperata Wood, non-wood and medicinal products for local consumption
and marketable products. Also good for shading and soil
fertility
Azetuna Sideroxilon
densiflorum
Manikara obovata Manilkare obovata / Synsepalum revolutum
Pau ferro unknown Margaritaria
discoidea
Margaritaria
discoidea
Not mentioned
Amoureira Chlorophora
excelsea
Milicia excelsa Milicia excelsa Wood, non-wood and medicinal products for local consumption
and marketable products. Also good for shading and soil
fertility
89
Grigo Morinda lucida unknown Morinda lucida Wood and non-wood marketable products and cocoa tree
shading
Nicolau Trema orientalis Pauridantha
floribunda
Pauridiantha
floribunda
Not mentioned
Pau preto unknown unknown Polyalthia oliveri /
Heisteria parvifolia
Not mentioned
Pau agua unknown unknown Psychotria venosa Not mentioned
Viro branco unknown Scytopetalum
kamerunianum
Scytopetalum
klaineanum
Not mentioned
Pau vermilio Siauditia
microcarpas
Staudtia pterocarpa Staudtia pterocarpa Not mentioned
Jaquinteiru
(Izaquente)
Treculia africana unknown Treculia africana Cocoa tree shading
Fruteira Artocarpus altilis Artocarpus altilis Unknown Wood and non-wood products for local consumption. Also
good for shading and soil fertility
Pau bischcu Funtumia
africana
unknown Unknown Not mentioned
Brubuwé unknown unknown Unknown Not mentioned
Salla salla unknown unknown Unknown Not mentioned
Marapiau Zanthoxylon
rubescens
Zanthoxylon gilletii Zanthoxylon gilletii Not mentioned
90
4.5 Cadmium restrictions
The analysis on cadmium in the soil and the beans of the Matheo Sampei site of Agripalma
gave the results listed below.
Soil sample:
Available cadmium 0.0142 mg.kg-1
Bean sample:
Total cadmium 0.091 mg.kg-1
It is remarkable that there is less available Cd in the soil than total Cd found in the beans. This
is probably because the tree accumulates the absorbed Cd in certain parts of the plant, mostly
in the beans and shells (Chavez et al., 2015).
The Cd content in these beans lays close to the mean Cd content in beans in West-Africa, that
was tested by Bertoli et al. (2016). This amounts 0,092 mg.kg-1. This is the lowest content of
all the tested regions. The highest Cd content was found in South America with a mean content
of 1,388 mg.kg-1. As Sao Tomé is closest to West-Africa, it is likely to have comparable
amounts and no problems with an excess of cadmium.
The limits in the EU regulations on cadmium in chocolate products are from 0.1 mg.kg-1 to 0.8
mg.kg-1, depending on the product. The total cadmium concentration in the examined beans
however, lay under the strictest limit of 0.1 mg.kg-1. The concentration will even lower, when
these beans are processed and mixed with other product, like sugar. Thus, regulation limits for
cadmium will probably not be exceeded when these Agripalma cocoa beans are used in
chocolate products.
However, attention should still be paid when applying fertilizers. They should be screened for
Cd content. In particular, phosphate fertilizers can contain high Cd levels due to the presence
of cadmium in the phosphate rock, if they are manufactured from this source (Lugon-Moulin et
al., 2006). Levels vary widely between countries where the phosphate rock originates from, as
shown in Table 25.
91
Table 25: Cadmium contents (mg.kg-1) of sedimentary and igneous phosphate rocks (Van
Kauwenbergh, 2001)
92
4.6 Overall evaluation of the potential suitability for
cocoa agroforestry
This chapter is called “potential suitability” since it is based on physical characteristics. The
chemical characteristics, such as pH and sum of basic cations, can more easily be remedied
through for example liming and fertilizer. Soil depth, stoniness and slope on the contrary are
hard to change. Therefore we analysed the suitability of the area based on the latter
characteristics, assuming the potential situation where chemical conditions are not a limitation.
Texture and drainage are also physical characteristics that cannot be changed, but since they
were generally good to optimal in het whole area, they were not taken into account. Climate is
discounted since this is considered the same for the whole area. The temperature is nearly
perfect and shortage of water is never an issue. Only excessive humidity and rainfall might
cause problems from the climatic point of view, mainly by enhancing diseases and leaching soil
nutrients.
As indicated, the soil depth, the stoniness and the slope are evaluated and each assigned a
suitability (S1 to N) per observation point. The suitability of this point in question was achieved
by assigning the worst of the three suitability classes. For example, when soil depth was S3,
stoniness S1 and slope S2, the general suitability of the point is S3. When the point in question
was assigned N (not suitable), it was indicated what was the most limiting factor, causing this
point not to be suitable for cocoa.
In Figure 44 a map of the research zone is shown, with the suitability class per observation
point. Furthermore, areas have been indicated that include the observation points with a
suitability S1 to S3. In other words, this shows the potentially suitable areas for cocoa
plantation. All these potential areas together cover an area of 150.49 ha, which is over one third
of the total area of the study zone (411.44 ha). As mentioned before, the eastern part of the area
was usually too steep for observations. In general it can be stated that this part is unsuitable due
to the slope and it was not included in the potentially suitable areas.
The north-western part of the research zone contains a high number of highly suitable to
marginally suitable points. This area is also the flattest part and offers opportunities for a cocoa
plantation. There are however also unsuitable points included and impediments, observed
during the field work, such as large boulders and several hills covered with a large amount of
stones. Furthermore several streams cross the area, sometimes small, but also one too deep to
cross by foot was encountered. This potential area is the largest, with 121.43 ha.
93
Figure 42: Examples of streams crossing suitable areas
In the surroundings of ‘Brion’, a ruin from the abandoned colonial plantation, most observations
points are considered unsuitable for cocoa. The ruin lies on the top of the hill with rather steep
slopes, causing the environment to be unsuitable. More to the west and also to the south the
area becomes more suitable, however, stoniness and a river would make planting more difficult.
Surface stoniness is a general problem, that needs extra emphasis. As shown in Figure 43 it
could sometimes be very severe. A suitable approach for this issue should be considered before
planting.
Figure 43: Example of severe surface stoniness and boulders (right)
94
Figure 44: Suggestion of potentially suitable areas based on the suitability classes per observation
point. When a point is not suitable, the reason is indicated.
95
5 Conclusions and recommendations
According to Wood and Lass (1985) temperature is most of all the limiting factor and has large
influence on yield. In this study the parameter temperature is near perfect, having a S1
suitability. Having an annual precipitation of > 3500 mm and zero (to maximum 2) dry months,
the likelihood of water shortage is very low. From a climatic point of view, the limiting factors
in this case will be the high relative humidity and the high amount of rainfall, giving an overall
marginal climatic suitability. They both may enhance diseases, particularly Phytophthora sp.
rot. Planting resistant varieties, adapted to the local environment (very humid) seems to be the
best approach to counter this problem. Moreover, high rainfall will induce high leaching of
nutrients and a stronger weathering of the soil, resulting in a lower chemical fertility. Wind
speeds are rather low so there will probably be no wind damage. In order to determine optimal
shade cover, radiation data would be interesting in further research.
The evaluation of soil profiles and topsoil composite samples revealed that soils in the research
area, derived from basaltic parent material, are mainly Alisols, for soils with B horizon, and
Lithosols, when soil depth is shallow. For chemical characteristics it can be said that ACEC is
high in general (very suitably) and base saturation low (not suitable). As this low percentage of
base saturation underestimates the suitability, it is better to look at the sum of cations and their
ratio. This sum of cations proves to be suitable, except the rather low concentration of divalent
cations (Mg and Ca). The pH of the soils is relatively low, only marginally suitable. This doesn’t
have to be a problem, since cocoa is quite resistant against acid soils and pH can be increased
by liming. Yet, attention should be paid to Aluminium toxicity. Organic matter content is
currently optimal. The first physical characteristic, texture, is very suitable for cocoa. Also
drainage is optimal. In general soils are rather shallow, and limited soil depth is often the reason
for unsuitability. However, because of the climate, water supply is sufficient for cocoa to grow
on these shallow soils. Another often limiting factor was the high level of stoniness. It was
remarkable that sometimes surface stoniness was higher than the stoniness in the soil. Finally,
the Cd concentration in the soils is not high and is unlikely to cause any problems with
regulations in further processing of the cocoa.
Several conclusions can be drawn from the parameters measured in the nine vegetation
plots. The canopy openness in the current forest is 7% to 10%, which is probably too low
for economically interesting cocoa plantations. To determine the suitable percentage of
shade and the corresponding percentage of canopy openness/cover, further research is
needed. Which and how many trees should be felt depends on this optimal shade and
furthermore biodiversity, useful characteristics of the trees and tree heights should be taken
into account. Most trees are found in the 11 to 25 m stratum. It is interesting to keep trees
in the 26 to 40 m stratum to have a heterogeneous canopy. Biodiversity, important for
96
conservation and for functional biodiversity, is not very high in the current forest. 22
different species were documented, some of them described as useful by several sources.
Furthermore, the high number of oil palms and the lack of coffee and cocoa trees was
remarkable. Even more because a vegetation map of 1958 shows former coffee and cocoa
plantations in this area.
The overall evaluation shows that the research site contains several areas that are
potentially suitable for a cocoa agroforestry plantation, covering a total area of 150.49 ha.
This means that physical characteristics are sufficient and chemical shortcomings can be
corrected. However, ways to deal with obstacles such as streams, boulders and surface
stoniness should be contemplated.
For the establishment of an organic cocoa agroforestry plantation the conditions of the
labels and certificates should be taken into account. However ‘organic’ implies tha t weed,
pest & disease control and soil fertility management should be performed without using
synthetic materials. Several sources are available on the island for organic fertility
management. Leguminous trees and the return of cocoa pod husk inside the plantation are
recommended. Furthermore, other organic materials, such as coffee husks, empty fruit
bunches, coconut husks and animal manures, might be available on the island. Also
agrominerals can be used for fertility management, although currently there are little
sources on the island. It would however be interesting to investigate the use of agrominerals
as additional Ca, Mg and K source, since plenty of basaltic and phonolitic rocks are present
on the Sao Tomean island. For disease control it is important to evaluate the major threats
for this specific case, starting with, but not limited to, phytophthora sp. This in order to
plant resistant varieties, also adapted to the local environment. As organic fungicide
Bordeaux mixture is often mentioned, as well as plant extracts of multiple plants. To check
the effectiveness, costs and local possibilities of these plant extract fungicides further
research is advisable, since fungal diseases, such as phytophthora sp., are likely to be a
major issue in this humid climate.
Similar methods can be applied in other regions on the island to research the suitability for
cocoa cultivation. Since climate differs over short distances on Sao Tomé, it might be
interesting to check radiation, rainfall and humidity locally. However a renewed study of
complete climate data would not be crucial. Important physical parameters to look at are
slope, stoniness and soil depth, since these were the most limiting factors in this study. A
chemical soil survey should be performed yet again. In case of agroforestry and with regard
to certain certificates, another vegetation survey is desirable for these new regions.
97
6 Bibliography
Abdulai, I., Vaast, P., Hoffmann, M. P., Asare, R., Jassogne, L., Van Asten, P., & Graefe, S.
(2018). Cocoa agroforestry is less resilient to sub‐ optimal and extreme climate than
cocoa in full sun. Global Change Biology, 24(1), 273-286.
Adejumo, T.O. (2005). Crop protection strategies for major diseases of cocoa, coffee and
cashew in Nigeria. Afr. J. Biotechnol. 4, 143–150.
Agbeniyi, S., Oluyole, K. & Ogunlade, M. (2011). Impact of cocoa pod husk fertilizer on cocoa
production in Nigeria. World J. Agric. Sci. 7, 113–116.
Aguilar, P. (1997). La cacaoculture à São Tomé et Principe. CIRAD Plantations, recherche,
développement, Cacaoculture, 165-174.
Ahenkorah, Y., Halm, B. J., Appiah, M. R., Akrofi, G. S., & Yirenkyi, J. E. K. (1987). Twenty
years' results from a shade and fertilizer trial on amazon cocoa (Theobroma cacao) in
Ghana. Expl Agric., 23, 31-39.
Ahn, P. (1993). Tropical Soils and Fertiliser Use. Longman Scientific & Technical, England.
Aneani, F., & Ofori-Frimpong, K. (2013). An analysis of yield gap and some factors of cocoa
(Theobroma cacao) yields in Ghana. Sustainable Agriculture Research, 2(4), 117.
Anim-Kwapong, G. & Frimpong, E. (2005). Vulnerability of agriculture to climate change-
impact of climate change on cocoa production. Accra, Ghana.
Anyaoha, K. E., Sakrabani, R.,Patchigolla, K., M.Mouazen, A. M. (2018). Critical evaluation
of oil palm fresh fruit bunch solid wastes as soil amendments: Prospects and challenges.
Resources, Conservation and Recycling 136: 399-409.
Appiah, M., Ofori-Frimpong, K., Afrifa, A. (2000). Evaluation of fertilizer application on some
peasant cocoa farms in Ghana. Ghana J. Agric. Sci. 33, 183–190.
Ayenor, G. K., Röling, N. G., Padi, B., Van Huis, A., Obeng-Ofori, D., & Atengdem, P. B.
(2004). Converging farmers' and scientists' perspectives on researchable constraints on
organic cocoa production in Ghana: results of a diagnostic study. NJAS-Wageningen
Journal of Life Sciences, 52(3-4), 261-284.
Ayorinde, K., Lawal, R. M. & Muibi, K. (2014). Land Suitability Assessment for Cocoa
Cultivation in Ife Central Local Government Area, Osun State. International Journal of
Scientific Engineering and Research (IJSER), 3(4), 2347-3878.
Badrie, N., Bekele, F., Sikora, E. & Sikora, M. (2015). Cocoa agronomy, quality, nutritional,
and health aspects. Critical reviews in Food Science and nutrition, 55(5), 620-659.
Baharuddin, A.S., Wakisaka, M., Shirai, Y., Abd-Aziz, S., Abdul Rahman, N.A. & Hassan,
M.A. (2009). Co-Composting of Empty Fruit Bunches and Partially Treated Palm Oil
Mill Effluents in Pilot Scale. International Journal of Agricultural Research, 4: 69-78.
Bai, S. H., Trueman, S. J., Nevenimo, T., Hannet, G., Bapiwai, P., Poienou, M., & Wallace, H.
M. (2017). Effects of shade-tree species and spacing on soil and leaf nutrient
concentrations in cocoa plantations at 8 years after establishment. Agriculture,
ecosystems & environment, 246: 134-143.
Becker, K. (2014). Sao Tome & Principe: Bradt Travel Guides.
Beer, J. (1987). Advantages, disadvantages and desirable characteristics of shade trees for
coffee, cacao and tea. Agroforestry systems, 5(1), 3-13.
98
Beer, J., Muschler, R., Kass, D. & Somarriba, E. (1998). Shade management in coffee and cacao
plantations. In: Nair P.K.R., Latt C.R. (eds) Directions in Tropical Agroforestry
Research. Forestry Sciences, vol 53. Springer, Dordrecht
Bertoldi, D., Barbero, A., Camin, F., Caligiani, A. & Larcher, R. (2016). Multielemental
fingerprinting and geographic traceability of Theobroma cacao beans and cocoa
products. Food Control, 65, 46-53.
Bidappa, C. C. (1998). Organic manure from coffee husk: comparison of technologies for
organic manure from plantation wastes, Journal of Plantation Crop (India) 26 (2) 120-
126.
Bieng, M. A. N., Gidoin, C., Avelino, J., Cilas, C., Deheuvels, O. & Wery, J. (2013). Diversity
and spatial clustering of shade trees affect cacao yield and pathogen pressure in Costa
Rican agroforests. Basic and Applied Ecology, 14(4), 329-336.
Bisseleua Daghela, H. B., Fotio, D., Yede, Missoup, A. D. & Vidal, S. (2013). Shade Tree
Diversity, Cocoa Pest Damage, Yield Compensating Inputs and Farmers' Net Returns
in West Africa. PLOS ONE, 8(3).
Blaser, W. J., Oppong, J., Yeboah, E. & Six, J. (2017). Shade trees have limited benefits for
soil fertility in cocoa agroforests. Agriculture, Ecosystems & Environment, 243, 83-91.
Bos, M. M., Steffan-Dewenter, I. & Tscharntke, T. (2007). Shade tree management affects fruit
abortion, insect pests and pathogens of cacao. Agriculture Ecosystems & Environment,
120(2-4), 201-205.
Bowers, J. H., Bailey, B. A., Hebbar, P. K., Sanogo, S. & Lumsden, R. D. (2001). The impact
of plant diseases on world chocolate production. . Online. Plant Health Progress.
Boyer, J. (1973). Cycles de la matie`re organique et des elements mineraux dans une cacaoyère
camerounaise. Cafe Cacao The 18, 3–30.
Caldeira, R. & Munha, J. M. (2002). Petrology of ultramafic nodules from Sao Tomé Island,
Cameroon Volcanic Line (oceanic sector). J. Afr. Earth Sc. 34: 231–246.
Caldeira, R., Madeira, J., Munhá, J. M., Afonso R.S., Mata, J., Tassinari, C. C. & Nascimento,
E. (2003). Caracterização das principais unidades vulcano-estratigráficas da ilha de São
Tomé, Golfo da Guiné. Ciências da Terra (UNL), Lisboa, nº esp. V, CD-ROM, pp. A15-
A18.
Cardoso, J. C. & Garcia, J. S. (1962). Carta dos solos de Sāo Tomé e Príncipe: Junta de
Investigações do Ultramar.
Chavez, E., He, Z., Stoffella, P., Mylavarapu, R., Li, Y., Moyano, B. & Baligar, V. (2015).
Concentration of cadmium in cacao beans and its relationship with soil cadmium in
southern Ecuador. Science of the total environment, 533, 205-214.
Commission Regulation (EC) No 488/2014 of 12 May 2014 amending Regulation (EC) No
1881/2006 as regards maximum levels of cadmium in foodstuffs. Off. J. Eur. Union, L
138, 75–79.
Cuatrecasas, J. (1964). Cacao and its allies; a taxonomic revision of the genus Theobroma.
de Geus, J. G. (1973). Fertilizer Guide for the Tropics and Subtropics. Centre d’Etude de
l’Azote, Zurich.
Djokoto, J. G., Owusu, V. & Awunyo-Vitor, D. (2016). Adoption of organic agriculture:
Evidence from cocoa farming in Ghana. Cogent Food & Agriculture, 2(1), 1242181.
de Lima, R. F., Viegas, L., Sole, N., Soares, E., Dallimer, M., Atkinson, P. W. & Barlow, J.
(2014). Can Management Improve the Value of Shade Plantations for the Endemic
Species of Sao Tome Island? Biotropica, 46(2), 238-247.
99
Evans, E., Collingwood, E. F., Taylor, J. W. H., & Whitaker, E. P. (1962). Field trials with
Bordeaux mixture and various formulations of copper oxychloride for control of late
blight of potatoes and black pod of cacao. Transactions of the British Mycological
Society, 45(3), 321-334.
Eyzaguirre, P. B. (1986). The Ecology Of Swidden Agriculture And Agrarian History In Sao
Tomé. Cahiers D Etudes Africaines, 26(1-2), 113-129. doi:10.3406/cea.1986.2168
FAO (2018). Pest and disease management in organic agriculture. Retrieved from
http://teca.fao.org/read/8372.
FAOSTAT. (2017). Crops, National Production, Dataset. Retrived from
http://www.fao.org/faostat/en/#data/QC.
Fasina, A., Omotoso, S., Shittu, O. & Adenikinju, A. (2007). Properties, Classification and
Suitability Evaluation of Some Selected Cocoa Soils of South-Western Nigeria.
Figueiredo, E. Paiva, J. Stévart, T. Oliveira. F. & Smith G.F. (2011) Annotated catalogue of the
flowering plants of São Tomé and Príncipe. Bothalia 41,1: 41–82.
Fontes, A., Gama-Rodrigues, A., Gama-Rodrigues, E., Sales, M., Costa, M. & Machado, R.
(2014). Nutrient stocks in litterfall and litter in cocoa agroforests in Brazil. Plant Soil
383, 313–335.
Gillman, G. P., Buekkett D. C. & Coventry, R.J. (2000). A laboratory study of application of
basalt dust to highly weathered soils: effects ion soil cation chemistry. Austr J Soil Res
39: 799-811.
Gillman, G. P., Buekkett D. C. & Coventry, R.J. (2002). Amending highly weathered soils with
finely ground basalt rock. Applied Geochem 17: 987-1001.
Gockowski, J., Afari-Sefa, V., Sarpong, D.B., Osei-Asare, Y.B. & Agyeman, N.F. (2013).
Improving the productivity and income of Ghanaian cocoa farmers while maintaining
environmental services: what role for certification? Int. J. Agric. Sustain. 11, 331–346.
Harley, A. D. & Gilkes, R. J. (2000). Factors influencing the release of plant nutrient elements
from silicate rock powders: a geochemical overview. Nutr Cycl Agroecosyst 56: 11-36.
Hartemink, A. E. & Donald L.S. (2005). Nutrient stocks, nutrient cycling, and soil changes in
cocoa ecosystems: a review. Advances in agronomy, 86, 227-253.
He, Z. L., Yang, X. E. & Stoffella, P. J. (2005). Trace elements in agroecosystems and impacts
on the environment. Journal of trace elements in medicine and biology, 19(2-3), 125-
140.
Herzog, F. (1994). Multipurpose shade trees in coffee and cocoa plantations in Côte d'Ivoire.
Agroforestry systems, 27(3), 259-267.
ICCO (2006). A study on the market for organic cocoa. Paper presented at the Executive
Committee, One hundred and thirtieth meeting, London.
ICCO (2017). Quarterly Bulletin of Cocoa Statistics. Vol. XLIII, N° 3, Cocoa year 2016/17.
ICCO (2018). Pests & diseases. Retrieved from https://www.icco.org/about-cocoa/pest-a-
diseases.html
IFA. (2008). World fertilizer use manual, Cacao. J. Snoeck and P. Jadin, Institut de Recherches
du Café, du Cacao et Autres Plantes Stimulantes (IRCC/CIRAD), Montpellier.
Isaac, M., Timmer, V. & Quashie-Sam, S. (2007). Shade tree effects in an 8-year-old cocoa
agroforestry system: biomass and nutrient diagnosis of Theobroma cacao by vector
analysis. Nutr. Cycl. Agroecosyst. 78, 155–165.
100
IUSS Working Group WRB (2015). World Reference Base for Soil Resources 2014, update
2015 International soil classification system for naming soils and creating legends for
soil maps. World Soil Resources Reports No. 106. FAO, Rome.
Jadin, P. (1988). Fertilité et fertilisation des sols à vocation cacaoyère de Sâo Tomé. CIRAD.
Café, Cacao, Thé, 32 (2) : 111-126.
Jadin, P. & Snoeck, J. (1985). La méethode du diagnostic sol pour calculer les besoins en
engrais des cacaoyers. Cafée Cacao Thée 29, 255–266.
Jagoret, P., Deheuvels, O. & Bastide, P. (2014a). Sustainable cocoa production: Learding from
agroforestry. Perspective, ecological intensification. CIRAD, 27.
Jagoret, P., Kwesseu, J., Messie, C., Michel-Dounias, I. & Malézieux, E. (2014b). Farmers’
assessment of the use value of agrobiodiversity in complex cocoa agroforestry systems
in central Cameroon. Agroforestry systems, 88(6), 983-1000.
Kanyankogote, P., Van Ranst, E., Verdoodt, A. & Baert, G. (2005). Effet de lave
trachybasaltique broyé sur les propriétés chimiques de sols de climat tropical humide.
Etude et Gestion du Sol, Vol 12 (4) : 301-311.
Kamprath, E.J. (1970). Exchangeable aluminium as a criterion for liming leached mineral soils.
Soil Sci. Soc. Amer. Proc., 34 : 252-254.
Kaoka. (2013). Return cocoa fields, Sao Tomé & Principe. Consulted at http://uk.kaoka.fr/nos-
filieres-cacao/sao-tome/sao-tome-2/#.Wxk6kIiFM2w on 10/06/2018.
Kasongo, R. K., Van Ranst, E., Kanyankogote, P., Verdoodt, A., & Baert, G. (2012a). Réponse
du soja (Glycine max) à l'application de phosphate de Kanzi et de dolomie rose de
Kimpese sur sol sableux en RD Congo. Canadian Journal of Soil Science, 92(6) : 905-
916.
Kasongo, R. K., Kanyankogote, P., Verdoodt, A., Baert, G., Dumon, M. & Van Ranst, E.
(2012b). Effet in vitro de la roche verte de Gangila sur les propriétés physico-chimiques
des sols sableux de l'hinterland de Kinshasa (RD Congo). Canadian Journal of Soil
Science, 92(5) : 787-791.
Kasongo, R. K., Verdoodt, A., Kanyankogote, P., Baert, G. & Van Ranst, E. (2013). Response
of Italian ryegrass (Lolium multiflorum Lam.) to coffee waste application on a humid
tropical sandy soil. Soil Use and Management 29(1) : 22-29.
Konduru, S., Evans, M. R. & Stamps, R. H., (1999). Coconut Husk and Processing Effects on
Chemical and Physical Properties of Coconut Coir Dust. Hortscience 34(1): 88-90.
Kongor, J. E., Boeckx, P., Vermeir, P., Van de Walle, D., Baert, G., Afoakwa, E. O., &
Dewettinck, K. (2018). Assessment of soil fertility and quality for improved cocoa
production in six cocoa growing regions in Ghana. Agroforestry Systems, 1-13.
Kruszewski, B., Obiedziński, M. W. & Kowalska, J. (2017). Nickel, cadmium and lead levels
in raw cocoa and processed chocolate mass materials from three different
manufacturers. Journal of Food Composition and Analysis.
Läderach, P., Martinez-Valle, A., Schroth, G. & Castro, N. (2013). Predicting the future
climatic suitability for cocoa farming of the world’s leading producer countries, Ghana
and Côte d’Ivoire. Climatic Change, 119(3), 841-854.
Landon, J. R. (1984). Soil suitability for crops. In Booker Tropical Soil Manual (pp. 294-296).
New York: Longman Inc.
Lopes Roseira 1984 Plantas Utéis da Flora de S. Tomé e Principe, Medicinais e Industriais.
101
Lugon-Moulin, N., Ryan, L., Donini, P., & Rossi, L. (2006). Cadmium content of phosphate
fertilizers used for tobacco production. Agronomy for sustainable development, 26(3),
151-155.
Nguyen, A. D., Tran, T. D. & Vo Thi, P. K. (2013). Evaluation of coffee husk compost for
improving soil fertility and sustainable coffee production in rural central highland of
Vietnam. Resources and Environment 2013, 3(4): 77-82.
Mooleedhar, V. & Lauckner, B. (1990). Effects of spacing on yield in improved clones of
Theobroma cacao L. Tropical Agric.
Motamayor, J. C., Risterucci, A.-M., Lopez, P. A., Ortiz, C. F., Moreno, A. & Lanaud, C.
(2002). Cacao domestication I: the origin of the cacao cultivated by the Mayas.
Heredity, 89(5), 380.
Mounicou, S., Szpunar, J., Andrey, D., Blake, C. & Lobinski, R. (2003). Concentrations and
bioavailability of cadmium and lead in cocoa powder and related products. Food
Additives & Contaminants, 20(4), 343-352.
Murray, D. (1975). Shade and nutrition. Pages 105-124 in: Wood G.A.R. (Ed.), Cocoa.
Longman, London.
National Biodiversity Strategy and Action Plan 2015-2020 (NBSAP II)
Ofori-Frimpong, K., Asase, A., Mason, J. & Danku, L. (2007). Shaded versus unshaded cocoa:
implications on litter fall, decomposition, soil fertility and cocoa pod development. In:
Symposium on Multistrata Agroforestry Systems with Perennial Crops, CATIE
Turrialba, 2007 Costa Rica, pp. 17–21.
Oke, D. & Odebiyi, K. (2007). Traditional cocoa-based agroforestry and forest species
conservation in Ondo State, Nigeria. Agriculture, Ecosystems & Environment, 122(3),
305-311.
Olunloyo, O. A. (1994). Efficacy of “Tiwantiwa” herbal plant substance in the control of black
pod disease of cocoa-Preliminary Results. Ann. Rept. Cocoa Res. Inst. Nigr, 21.
Orlandi, F. (2011). Cadeia de valores do cacau em São Tomé e Príncipe. MSc thesis
Universidade de Lisboa, Lisboa: ISA.
Pape, D. & Rebelo de Andrade, R. (2013) As roças de São Tomé e Príncipe. Lisboa: Tinta da
China.
Petithuguenin, P. (1998). Les conditions naturelles de production du cacao en Côte d'Ivoire, au
Ghana et en Indonésie. Plantations, recherche, développement, 5(6), 393-411.
Rabumi, W. (1998). Chemical composition of oil palm empty fruit bunch and its decomposition
in tile field. MSc thesis Universiti Putra Malaysia.
Rice, R. A. & Greenberg, R. (2000). Cacao cultivation and the conservation of biological
diversity. Ambio, 29(3), 167-173.
Ritung, S., Wahyunto, A. F. & Hidayat, H. (2007). Land Suitability Evaluation with a case map
of Aceh Barat District. Indonesian Soil Research Institute and World Agroforestry
Centre, Bogor, Indonesia
Roberts, T. L. (2014). Cadmium and phosphorous fertilizers: the issues and the
science. Procedia Engineering, 83, 52-59.
Roche, G. & Dulcire, M. (2007). Setting up an organic cocoa sector in Sao Tomé. In : AIEA2
International Conference “Knowledge, Sustainability and Bio-Resources in the further
Development of Agri-food Systems”, within the XLV SOBER Congress in Brazil ,
Londrina, Brazil, July 22nd-27th 2007, 11 p.
102
Roseira (1984) Plantas úteis da flora de São Tomé e Príncipe. Medicinais, industriais e
ornamentais. Lissabon
Ruf, F. (2016). Mineral and Organic fertilization stories in Côte d’Ivoire. Re-internalization of
deforestation-led externalized costs. Conference: ICCO. World Cocoa Conference.
‘Building bridges between producers and consumers’At: Punta Cana, Dominican
Republic
Ruf, F. & Schroth, G. (2004). Chocolate forests and monocultures: a historical review of cocoa
growing and its conflicting role in tropical deforestation and forest conservation. In:
Schroth, G., Fonseca, G., Harvey, C., Gascon, C., Vasconcelos, H., Izac, A.-M. (Eds.),
Agroforestry and Biodiversity Conservation in Tropical Landscapes. Island Press,
Washington, 104–134.
Ruf, F. & Kiendré, J. (2016). L’innovation « fiente de poulet » en Côte d’Ivoire. 2. Révolution
agro-écologique : 1500 kg/ha en Côte d’Ivoire? CIRAD, UMR Innovation.
Ruf, F. & Kiendré, J. (2017). Innovations agro-écologiques villageoises. Impact de la fiente de
poulet dans les cacaoyères de Côte d’Ivoire. Report ‘Projet CITAD «Innovations
Fumures organiques» et résilience au changement climatique dans les cacaoyères de
Côte d’Ivoire». CEDEAO-AFD.
Rufino, M.C., Tittonell, P., Van Wijk, M.T., Castellanos-Navarrete, A., Delve, R.J., De Ridder,
N. & Giller, K.E. (2007). Manure as a key resource within smallholder farming systems:
analysing farm-scale nutrient cycling efficiencies with the NUANCES framework.
Livest. Sci. 112: 273–287.
Sabrina, D.T., Hanafi, M.M., Mahmud, T.M.M. & Nor Azwady, A.A. (2009).
Vermicomposting of oil palm empty fruit bunch and its potential in supplying of
Nutrients for crop growth. Compost Sci. Util. 17 (1), 61–67.
Santana, M. & Cabala-Rosand, P. (1982). Dynamics of nitrogen in a shaded cacao plantation.
Plant Soil 67, 271–281.
Schroth, G. & Harvey, C.A. (2007). Biodiversity conservation in cocoa production landscapes:
an overview. Biodiversity and Conservation, 2007, 16.8: 2237-2244.
Schwendenmann, L., Veldkamp, E., Moser, G., Holscher, D., Kohler, M., Clough, Y. & van
Straaten, O. (2010). Effects of an experimental drought on the functioning of a cacao
agroforestry system, Sulawesi, Indonesia. Global Change Biology, 16(5), 1515-1530.
Shamshuddin, J., Anda, M., Fauziah, C.I. & Omar, S.R.S. (2011). Growth of cocoa planted on
highly weathered soil as affected by application of basalt and/or compost. Commun.
Soil Sci. Plant Anal. 42: 2751–2766.
Sharifuddin, H. & Zaharah, A. (1991). Utilization of organic wastes and natural systems in
Malaysian agriculture. In: First International Conference on Kyusel Nature Farming,
1991 Washington, pp. 71–78.
Shemekite, F., Gómez-Brandón, M., Franke-Whittle, I. H., Praehauser, B., Insam, H. & l
Assefa, F. (2014) Coffee husk composting: An investigation of the process using
molecular and non-molecular tools. Waste Management 34(3) : 642-652.
Smythe, A. J. (1966). The selection of soils for cocoa. FAO, Rome.
Snoeck D., Koko, L., Joffre, J., Bastide, P. & Jagoret, P. (2016). Cacao nutrition and
fertilization. In: Lichtfouse E (ed) Sustainable agriculture reviews, 19th edn. Springer
International Publishing, Switzerland
103
Sonwa, D. J., Nkongmeneck, B. A., Weise, S. F., Tchatat, M., Adesina, A. A., & Janssens, M.
J. (2007). Diversity of plants in cocoa agroforests in the humid forest zone of Southern
Cameroon. Biodiversity and Conservation, 16(8), 2385-2400.
Sperber, C. F., Nakayama, K., Valverde, M. J. & Neves, F. d. S. (2004). Tree species richness
and density affect parasitoid diversity in cacao agroforestry. Basic and Applied Ecology,
5(3), 241-251.
Sys, C., Van Ranst, E., Debaveye, J. & Beernaert, F. (1993). Land evaluation, Part III.
Agricultural Publications Nr. 7, General Administration for Development Cooperation,
Brussels, Belgium.
The Nature Conservancy (1994) Field methods for vegetation mapping. California, USA
Thong, K., Ng, W. (1978). Growth and nutrients composition of monocrop cocoa plants on
inland Malaysian soils. In: International Conference on Cocoa and Coconuts, 1978
Kuala Lumpur.
Urquhart, D. (1955). Cocoa. Longmans, Green and Co, London.
UTZ (2015). Core code of conduct for group and multi-group certification. UTZ Standard and
Certification Department, Amsterdam (https://utz.org/wp-
content/uploads/2015/12/EN_UTZ_Core-Code-Group_v1.1_2015.pdf).
UTZ. (2017, 2017). UTZ and Rainforest Alliance joining forces for a better future. Consulted
at www.utz.org on 14/06/2018.
Vaast, P. & Somarriba, E. (2014). Trade-offs between crop intensification and ecosystem
services: the role of agroforestry in cocoa cultivation. Agroforestry systems, 88(6), 947-
956.
Van Himme, M., & Snoeck, J. (2001). Beverage and stimulant crops: Cacao. In R. H.
Raemaekers (Ed.), Crop Production in Tropical Africa (pp. 889-921). Brussels,
Belgium: DGIC.
Van Kauwenbergh, S. J. (2001). Cadmium in phosphate rock and fertilizers. In Fertilizer
Institute 2001 World Fertilizer Conference, Chicago, Illinois (p. 25).
Van Ranst, E., Verloo, M., Demeyer, A., & Pauwels, J. M. (1999). Manual for the soil chemistry
and fertility laboratory: analytical methods for soils and plants equipment, and
management of consumables. University of Ghent, Ghent, Belgium
van Vliet, J. A. & Giller, K. E. (2017). Chapter five - Mineral nutrition of cocoa: A Review. In
D. L. Sparks (Ed.), Advances in agronomy (Vol. 141, pp. 185-270): Academic Press.
Vanegtern, B., Rogers, M. & Nelson, S. (2015). Black pod rot of cacao caused by Phytophthora
palmivora. Plant Dis, 108, 1-5.
Vanhove, W., Vanhoudt, N. & Van Damme, P. (2015). Effect of shade tree planting and soil
management on rehabilitation success of a 22-year-old degraded cocoa (Theobroma
cacao L.) plantation. Agriculture, Ecosystems and Environment 219 (2016) 14–25.
van Straaten, P. (2002). Rocks for crops: Agrominerals of sub-Saharan Africa. University of
Guelph, Guelph, Ontario Canada.
van Straaten, P. (2006). Farming with rocks and minerals: challenges and opportunities. An.
Acad. Bras. Ciênc. 78(4).
van Straaten, P. (2007). Agrogeology: The use of rocks for crops. Enviroquest Ltd. Canada.
Verheye, W. (1986). Etudes des faisabilités de la zone de Agua Ize, Ribeira Peixe et Porto
Alegre. Rijksuniversiteit Gent, Belgium.
104
von Uexküll, H. & Cohen, A. (1980). Potassium requirements of some tropical tree crops (oil
palm, coconut palm, rubber, coffee, cocoa). Potassium Requirements of Crops,
International Potash Institute, Bern, Switzerland.
Wessel, M. (1971). Fertilizer requirements of cacao (Theobroma cacao L.) in South-Western
Nigeria. Koninklijk Instituut voor de Tropen.
Wessel, M. & Quist-Wessel, P. F. (2015). Cocoa production in West Africa, a review and
analysis of recent developments. NJAS-Wageningen Journal of Life Sciences, 74, 1-7.
Wichmann, W. (1992). IFA World Fertilizer Use Manual. In: International Fertilizer
Association, Paris
Wood, G. A. R. & Lass, R. A. (1985). Cocoa. USA: Longman Inc.
Wyrley-Birch, E. (1972). Manuring of cocoa. In R.L. Wastie, D.A. Earp (Eds.), Cocoa and
Coconuts in Malaysia, Inc. Soc. of Planters, Kuala Lumpur (1972).
Zarcinas, B. A., Ishak, C. F., McLaughlin, M. J., & Cozens, G. (2004). Heavy metals in soils
and crops in Southeast Asia. Environmental geochemistry and health, 26(4), 343-357.
105
7 Appendix
Pictures of the individual soil profile pits:
Profile 1
Large stones at the surface
106
Profile 2
Profile 3
107
Steep slope and severe stoniness, both at the surface and in the soil
Profile 5
108
Spots of rust appear at the bottom of the pit, probably a sign of weathering stones
Profile 6
Weathering basalitic rock
