CHAPTER 1

7

WHY SOIL GENESIS?

1.1. What is soil genesis?

Soil formation or soil genesis refers to changes of soil properties with time in onedirection: the content of one component or mineral in a certain horizon decreases orincreases‚ sedimentary layering disappears‚ etc. Mostly‚ such changes are slow and canbe seen only after decades to millennia. So‚ most soil properties that change during soilformation are relatively stable. Sometimes‚ however‚ effects of soil formation can beseen within weeks or months. Examples are the quick drop in pH when sulphidesoxidise to sulfuric acid upon exposure to air and the formation of gley mottles when asoil becomes very wet. Most rapid processes are cyclic‚ however‚ and are not consideredpart of soil formation

Soils may be moist or dry‚ and warm or cold‚ depending on weather and season.Seasonal variations in weather also drive biological processes‚ which in turn change soilproperties. Examples of such biological processes are plant growth and uptake of waterand nutrients‚ supply of fresh plant litter‚ and decomposition of plant litter by micro-organisms and soil fauna. These cause temporal variations in soil pH‚ in contents ofcertain fractions of soil organic matter (e.g.‚ microbial biomass)‚ and of soluble andadsorbed nutrients. Most of such soil properties change in a cyclic way: they arereversible on an annual or seasonal basis‚ and do not constitute a unidirectional changein soil properties. Therefore they are not considered part of soil genesis.

Question: 1.1. Which of the following soil properties may vary strongly within a year‚and which can change strongly only over much longer times (decades to millennia)?(a) soil temperature in °C‚ (b) cation exchange capacity (CEC)‚ (c) dissolved salts‚ (d)clay mineralogy‚ (e) soil water retention characteristics‚ (f) soil organic matter (SOM)content.

The rapid‚ cyclic processes are part of the complex set of processes that cause theunidirectional changes typical of soil formation. E.g.‚ seasonal snow melt causes strongpercolation by water. Over centuries to millennia‚ this causes a marked decrease ofweatherable minerals by leaching. Part B treats many of such short-term physical‚chemical and biological processes that are important in soil genesis.

8

SOIL FORMING PROCESSES

The properties of any soil are‚ at least theoretically‚ determined by five SOILFORMING FACTORS (V.V.Dokuchaev‚ 1898 cited by H. Jenny‚ 1980):

parent material‚ topography‚ climate‚ biota‚ and time.

Any particular combination of these factors will give rise to a certain SOILFORMING PROCESS‚ a set of physical‚ chemical and biological processes thatcreate a particular soil. The factors hydrology and human influence have been

added later.

If we could fully characterise and quantify each factor and describe all relevantprocesses in a simulation model‚ we could exactly predict the resulting soil profile.

As you will come to realise in this course‚ reality is too complex for that!

It will be clear that a soil is not a static object that can be described once and for all‚ buta natural entity with a time dimension. A soil comprises living and non-livingcomponents. It can be considered as part of an ecosystem. Therefore‚ a soil should notnormally be studied in isolation: the interactions with the rest of the ecosystem to whichit belongs should be taken into account. Many publications about soil formation dealwith one or a few aspects only‚ for example‚ soil chemistry or soil mineralogy. Theserefer to a subsystem of the soil‚ which itself is a subsystem of the ecosystem.

1.2. Why study soil genesis?

The study of soil genesis brings order to the overwhelming variety of soils that areobserved in the world‚ and links the field of soil science to other scientific disciplines.A basic understanding of the main soil forming factors and soil forming processes (seebox above)‚ helps to order soil information. This can be very useful during soil surveyor when setting up a system of soil classification. Also when you study plant-soilinteractions or investigate consequences of large-scale human perturbations (climatechange‚ acid rain‚ salinization or alkalinisation due to improper irrigation and drainage)a good general understanding of soil genesis is indispensable. Last but not least‚ thestudy of soil formation is the way to satisfy your curiosity about the many different andwondrous phenomena that can be observed in soil profiles all over the world.

Question 1.2. The Soil Forming Factor (state factor) approach is often used to studythe effect of one factor‚ by seeking sequences of soils where one factor varies‚ and theothers remain constant. a) Give an example of a chronosequence (soil age varies)‚ aclimosequence (climate varies) and a toposequence (elevation varies). b) The statefactor approach assumes that (i) the factors are independent‚ and (ii) state factorsinfluence the soil‚ but not vice versa. Criticise these assumptions.

1.3. How to study soil genesis?

WHAT HAPPENED?

When you try to explain the morphology and underlying physical‚ chemical andmineralogical properties of a certain soil profile‚ you have to distinguish two kinds ofquestions.

First: “WHAT physical‚ chemical/mineralogical and biological properties of a soilprofile are due to soil formation? ”‚ or: “In what respect does the soil differ from itsparent material?”

HOW DID IT HAPPEN?

Second: “HOW did the soil form?”‚ or: “Which physical‚ chemical and biologicalprocesses have formed the soil?”

A problem with the “WHAT?” question is that we first have to distinguish betweenproperties caused by variations in parent material (geogenesis)‚ and the effects of soilformation upon a given parent material by the action of soil forming factors(pedogenesis). A related problem is that we can rarely be sure of the nature of the parentmaterial of a soil profile‚ and of variations of parent material with depth. Is a clayeysurface soil over a sandy subsoil the result of weathering of more sandy parent material‚or did it result from sedimentation of finer material? Furthermore‚ geogenesis andpedogenesis may alternate‚ which sometimes blurs the distinction between "parentmaterial" and "soil". Ways to test assumptions about parent material will be discussedin Chapter 5.

Question 1.3. Give examples of parent materials or landscapes where the distinctionbetween geogenesis and pedogenesis is difficult‚ and where it is relatively easy.

Often‚ “HOW? ” questions are difficult too‚ because most processes cannot be observeddirectly: they take place under the soil surface‚ and most processes are so slow that theireffects are not noticeable within the few years normally available for research. A largepart of this book is devoted to tricks to get around these problems. But as with allsciences dealing with the past‚ it is fundamentally impossible to be really certain aboutan answer‚ because we have not been there to observe what happened.

WHAT has happened can often be inferred in the field from morphological differencesbetween the C-horizon and the overlying soil horizons. Samples from different soilhorizons can be studied further in the laboratory‚ e.g. microscopically‚ or by chemical‚physical and mineralogical methods. Specific analyses to identify certain features of soilformation will be presented in later chapters. With knowledge about the referencesituation (= the unchanged parent material)‚ one can identify and sometimes quantifythe changes that have taken place in the soil.

9

10

HOW soil formation takes place‚ can be extrapolated from processes observed overshorter time scales in the field or in the laboratory: mineral weathering‚ ion exchange‚oxidation-reduction reactions‚ peptisation and coagulation of colloids‚ transport ofsolutes or suspended solids‚ nutrient uptake by plants‚ decomposition of organic matter‚burrowing activities of soil organisms‚ etc. Soil formation can also be simulated inartificial soil columns in the laboratory‚ or by computer simulations of one or moreprocesses acting on a hypothetical soil. To test such models one can use two kinds ofdata. First: data on the relatively stable properties‚ such as texture and mineralogy.Second: data from repeated measurements (monitoring) of seasonally variable‚ dynamicproperties such as soil moisture content‚ the composition of the soil solution‚ or the soilgas phase‚ etc. The relationships between the different approaches to study soilprocesses are shown diagrammatically in Figure 1.1.

Question 1.4. A silt-textured soil profile in a dry region has a water table within 1 mof the soil surface and has a white crust on the soil surface. You hypothesise that thewhite layer is a crust of easily soluble salt (your answer to: WHAT?), that has formed byevaporation of slightly saline ground water that has risen by capillary action to the soil

11

surface (your answer to: HOW?). Discuss how you would test your hypothesis. UseFigure 1.1 and refer to A1‚ and B2-4.

Tools to recognise WHAT happened‚ and explanations of HOW it happened are givenin Part 3. Examples of reconstructions of HOW and WHAT in complex situations‚ inwhich we use the iterations indicated in Figure 1.1‚ are given in Chapter 15.

1.4. Answers

Question 1.1Soil temperatures undergo daily and annual cycles. Clay mineralogy can change onlyvery slowly. Soil organic matter (SOM) content varies very slightly (by a few % of totalSOM) on a seasonal basis. Differences in SOM content between soils result from soilformation. The same is true for CEC and water retention‚ which depend mainly onSOM‚ clay content‚ and clay mineralogy. Dissolved salts vary seasonally in most soils.Very high salt contents as in saline soils‚ however‚ are the result of

Question 1.2

soil formation.

a) Chronosequence: beach ridges or river terraces of different age; climosequence:continental-scale transects in loess landscapes; toposequence: any elevation gradientwhere parent material stays constant. b) (i) The state factors are at best only fairlyindependent: climate varies with elevation; biota vary strongly with climate. (ii) Soilproperties influenced by biota may strongly feed back to the vegetation. Such feedbacksare even used by some plants (e.g. peat moss‚ Sphagnum) to outcompete other plants(VanBreemen‚ 1995).

Question 1.3Geogenesis and pedogenesis can be distinguished easily in soils on Quaternary loess‚and in residual soils derived from underlying igneous or metamorphic rock.Difficult cases: soils in layered volcanic ash‚ in active floodplains‚ and on very old‚extended land surfaces that have undergone repeated cycles of erosion andsedimentation.

Question 1.4A1. Analyse the white crust and the groundwater (does the crust material dissolve inwater? Has a water extract of the crust a composition similar to that of the groundwater?).

B2. Calculate the capillary rise permitted by the texture of the soil‚ and compare that tothe depth of the ground water below the surface.

12

B3. Does the thickness of the crust or the concentration of water-extractable salts in thesurface layer increase with time?

B4. Fill a tube (say, 1 m long) in the lab with the soil from the field, create a watertable at the bottom and supply slightly saline water to make up for evaporation losses.Does a salt crust develop?

1.5. References

Jenny, H., 1980. The Soil Resource. Springer Verlag, 377 pp.

Van Breemen, N., 1995. How Sphagnum bogs down other plants. Trends in Ecologyand Evolution, 10:270-275.