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MULCHING

Effect of Mulching in Temperate Re-gions in Japan

Yield Increases and Early Harvest

There are many reports confirming thestimulation of growth and consequent yield increasesby the use of plastic mulches. Their effects areparticularly noticeable in early spring. For example,Fig. 2 shows groundnut yields with and withoutmulching at various experimental stations in theKyushu district in Japan (Iguchi 1977). Although theexperiments were carried out under different cli-matic conditions, soils and years, mulching wasalways found to be effective in producing a yieldincrease. The practice of mulching also shortenedthe growth period. Mulching thus stabilized produc-tion by eliminating various constraints, and madepossible more intensive multiple cropping by ad-vancing the harvest (Fig. 3) (Iguchi 1977).

Reasons for Yield Increases with Mulching

Increase in Soil Temperature

Polyethylene mulch raises the soil tempera-ture. This effect derives mostly from the suppression

INTRODUCTION

Protected vegetable cultivation started inJapan at the beginning of the 17th century. The firstrecords describe the use of greasy paper for forcingcultivation in Shizuoka, so that early vegetablescould be shipped to Tokyo (Edo).

However, it was after the commercial pro-duction of plastic films in 1951 that protective meth-ods of vegetable cultivation became widespread inJapan. At first, various materials were used as coversfor forcing cultivation during the cold season. Later,they came into use as mulches, rain shelters and rowcovers throughout the year.

With regard to the use of plastic materialsfor vegetable production, there are currently fivetypes of protective cultivation carried out in Japan(Fig. 1). The area under protective vegetable culti-vation using these methods reached 160,163 ha in1987, comprising 26% of the total area under veg-etable cultivation (617, 600 ha in 1987). The use ofplastic materials for vegetable cultivation, for e.g.mulches, row covers and rain shelters, seems also tobe applicable to vegetable production in the tropicsand subtropics. The effect and feasibility of mulch-ing and row covers are discussed in the paper, withregard to their effect in stabilizing vegetable produc-tion in both temperate and tropical climates.

THE EFFECT OF MULCHING AND ROW COVERS ON VEGETABLE PRODUCTION

Toshio HanadaChugoku Agr. Exp. Stn.Ueno 200, Ayabe city,Kyoto Pref. 623, Japan

ABSTRACT

Plastic mulches have various beneficial effects on crop product in temperate regions, in-cluding an increase in soil temperature; the conservation of soil moisture, texture and fertility;and the control of weeds, pests and diseases. However, mulching with fresh leaves gave betteryields than plastic films in summer in the subtropics, since plastic mulching produced a markedincrease in soil temperature.

Row covers, using plastic nets and non-woven fabrics, also increased the yield of veg-etables, especially that of leafy vegetables, in the tropics and the subtropics. The yield increasewas the combined result of shading, suppression of soil temperature increase, conservation of soilmoisture, and protection from wind and pests.

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Fig. 1. Five types of protected vegetable cultivation using plastic materials, andthe extent of their use in Japan

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Fig. 2. Yields of groundnut with and without mulching at various locations in Kyushu district, JapanResults from the years 1971 (a), 1972 (b) and 1973 (c)

Source: Iguchi 1977

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of latent heat loss through evaporation. The extentof the increase in soil temperature depends on thecolor of the film and the intensity of solar radiation.Soil temperatures with and without mulching weremonitored in a field of carrots sown on April 27 (Fig.4). The penetration of sunlight and the increase insoil temperature was most marked when transparentfilm was used, while black film was not as effectivein raising soil temperatures. The difference in soiltemperature between mulched and bare soil in earlyMay reached 7°C with transparent film and 5°C withblack film. However, this increase in soil tempera-ture diminished later, as the canopy of carrot leavesbegan to cover the mulch.

Conserving the Soil Moisture and Texture

The soil moisture content of bare soil in thecarrot fields fluctuated as rainfall occurred. How-ever, it was quite stable in soil under mulch. The soilmoisture was always higher under black mulch thanunder transparent mulch (Fig. 5).

Although some rainfall penetrates the holesin the mulch and soaks into the soil, the moisturecontent of soil under mulch becomes lower in thelong term, as there is not much supplementary wateraccumulation under mulch after covering. The soil

temperature and soil moisture content monitored insoybean fields with and without mulch are shown inFig. 6 (Onuma et al. 1971). It is apparent that theconservation of soil moisture under mulches lastsonly during the early stage of growth.

Rainfall onto bare soil leads to the destruc-tion of the soil structure. The distribution of onionroots in soil with and without mulch is shown in Fig.7. The texture of the soil under the mulch remainedporous, which contributed to root growth expan-sion.

Conserving Soil Fertility

Mulching prevents the leaching of fertil-izer, because it acts as a physical barrier to rainfall.Fluctuations in the nitrate and ammonium content inthe soil of carrot fields with and without mulch areshown in Fig. 8. Compound fertilizers of N-P-K:220-210-200 kg/ha were applied as a basal dressing.The amount of nitrate was highest in the soil undertransparent film, followed by black film, and waslowest in bare soil. The amount of ammoniumshowed the same trend, except that there was littledifference between soil under black film mulch andbare soil.

The movement of water is directed up-

Fig. 3. Advance of growth stages and avoidance of constraints by mulching in Kyushu district,Japan. A: Southernmost lowlands, B: Southern lowlands, C: Central lowlands and D:Uplands.

Source: Iguchi 1977

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Fig. 4. Fluctuations in soil temperature (5cm below surface) in carrot fields withand without mulch

Fig. 5. Fluctuations in soil moisture in carrot field with and without mulchSource: Takahashi and Chiba 1971

Source: Takahashi and Chiba 1971

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Fig. 6. Fluctuations in soil temperature and soil moisture in soybean fields withand without mulch(Measurement was carried out at a depth of 10 cm at 10 a.m.)

Fig. 7. Distribution of roots of onion grown with and without mulchSource: Kawaguchi 1974

Source: Onuma et al. 1971

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wards in soil under plastic mulch, which results in theaccumulation of salts on the soil surface beneath thefilm. The electric conductivity of the soil wasmarkedly higher under mulch. The content of ex-changeable cations was also higher under transpar-ent mulch during the middle stage of growth, whileboth transparent and black mulch gave a highercation content at the final growth stage (Table 1).However, the pH value was lower by 0.2-0.5 in soilunder mulch compared to bare soil (pH 6.1), whichindicates that anions such as nitrate, sulfate andchloride also remain in the soil without being leachedout. The amount of nutrients (N, P, K, Ca, Mg)absorbed by the plant from the soil was 1.4-1.5 timeshigher in soil under mulch than in bare soil.

Weed Control

Black polyethylene film gives effective weedcontrol by cutting down solar radiation by more than90%, resulting in etiolated growth and the eventualdeath of weeds under the film. Transparent film, onthe other hand, has no effect on weed control unlessthe film is coated with weedicides.

Green plastic film was innovated as a mulchto combine the effects of black film in terms of weedcontrol, and transparent film in terms of increasedsoil temperature. Although the increase of soiltemperature under green film was almost compa-rable to that under transparent film, the effect ofgreen film on weed control was inconsistent. Greenfilm was found to be quite effective in controllingweeds such as Portulaca oleracea L., but did notcontrol the growth of Cyperus difformis L. orDigitaria adscendens Henr. (Table 2). A wavelength of around 520 nm has a selective effect ininhibiting the germination and growth of plants (Fig.9). The effect on weed control of green film, whichpermits a wave length of 500-600 nm to pass through,is rather dependent on the species of weed con-cerned.

Control of Pests and Diseases

Silver film was developed as a mulch tosuppress the increase in soil temperature and tocontrol pests and diseases. The silver color acts asa repellent to aphids which transmit viruses. Thenumber of aphids and the occurrence of virus symp-toms in melons grown with and without silver mulchis shown in Fig. 10. At an early stage of growth, therepelling effect of the silver color on aphids wasobserved even when it covered only 10% of the soilsurface. Later this effect disappeared, as plant leaves

began to cover the mulch. Silver film was also foundto be effective in reducing attacks by Aulacophoraspp. on melons.

With regard to diseases, there is a reportthat sclerotinia rot of lettuce was reduced by mulch-ing, due to the prevention of direct contact betweenthe leaves and the soil (Nishitani 1979).

Effect of Mulching in Summer in theSubtropics

Experiments were carried out at theOkinawa branch of the Tropical Agricultural Re-search Center (TARC) to investigate the effect ofmulching during summer in the subtropics.

Soil Temperature under Mulches

Diurnal fluctuations in soil temperature ata depth of 0 cm (surface), -10 cm and -30 cm undervarious mulching materials during summer are shownin Fig. 11. Plastic films (transparent, green, blackand silver), white non-woven fabrics and freshly cutNapier glass (Pennisetum purpureum Schum.) wereused for the field tests, and compared to the control.The surface temperature of the bare soil rose to56°C. Green, transparent and black films increasedsoil temperature more than the control, to a level of70°C under black film. On the other hand, a slightdecrease in soil temperature was observed underwhite fabrics and a significant decrease under silverfilm, while freshly cut Napier grass suppressed anysoil temperature increase so completely that thetemperature remained almost constant throughoutthe day. Soil temperatures became lower withgreater soil depth (-10 cm and -30 cm), but the sametrends were still observed in the effect of the variousmulching materials on increased soil temperatures,regardless of soil depth.

Reasons for Increased Soil Temperature underPlastic Mulch

In view of this extreme increase in soiltemperature under plastic mulch, especially withblack polyethylene film, differences in surface tem-perature in relation to the color of the covering wereanalyzed, using colored steel plates (black, green,silver and white). The black plate absorbed mostsolar heat, followed by green and silver. The in-crease in temperature was least pronounced whenthe white plate was used (Fig. 12).

Evaporation from the surface of mulchingmaterials of various colors followed the same pattern

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Fig. 8. Fluctuations in nitrate and ammonium content in soil with and without mulchSource: Takahashi and Chiba 1971

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Table 1. Electric conductivity and exchangeable cations in soil of carrot fields with andwithout mulch

in bare soil. The increase of surface temperaturesoon caused the black film to shrink and tear, and thetransparent film also tore during the experiment.(No data on yields were obtained).

The mulch of freshly cut Napier grass gavethe greatest yield and highest harvesting percentage.The yield of cabbages grown under silver plasticmulch was slightly higher than the control: howeverwhite fabrics resulted in a lower yield than thecontrol (Table 3).

Similar effects of plastic mulches on veg-etable growth in a hot climate have been reportedfrom Burkina Faso, in West Africa.

as the temperature increases under colored plates(Fig. 13).

The soil moisture contents were lower inbare soil and under white fabric mulch (water perme-able), than those under plastic film mulches. Amongthe plastic film mulches, silver film and black filmwere most effective in maintaining soil moisture,while transparent film was least effective (Fig. 14).

Growth and Yield of Cabbage with Mulching

Cabbages were grown under variousmulches (black, transparent and silver polyethylenefilms, white fabrics and freshly cut Napier grass) and

Table 2. No. and dry weight of weeds under plastic mulch, compared to the control

Source: Takahashi and Chiba 1971

Source: Takahashi and Chiba 1971

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Fig. 10. Number of aphids and occurrence of virus symptoms on melons with andwithout silver plastic mulch

Source: Wakayama Prefectural Agricultural Station, 1973

Black polyethylene mulch yielded 3.3 timeshigher than soil without mulch in eggplant, and 2.3times in tomato, when grown during the relativelycool season (sown in September and harvested inJanuary. However, in a trial carried out during the hotseason, the use of plastic mulch had an adverse effecton vegetables and decreased their yields signifi-cantly (Djigma et al. 1986).

The extreme increase in soil temperaturesunder plastic mulch seems to indicate that it should

not be used in climates where there is high solarradiation.

PLASTIC ROW COVERS

In hot climates, the use of plastic film mulchesdoes not seen to be justified. Only mulching withfresh leaves gave higher productivity than the con-trol. Another approach to modify the micro-climateto enhance plant productivity by plastic

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Fig. 11. Soil temperatures at a depth of 0 cm, 10 cm and 30 cm under various mulchingmaterials during summer in Okinawa

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Fig. 12. Increase in temperature of colored steel plates

Fig. 13. Surface evaporation from different mulching materials

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Table 3. Growth and yield of cabbage with and without mulching

Fig. 14. Soil moisture content under different mulching materials

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material is the use of row covers.Okinawa’s farmers began using plastic nets

as row covers for leafy vegetables during the late1950’s. This practice proved quite effective forvegetable production in a hot climate. The area underrow covers has now increased to 4,225 ha in Japan.

Row Covers

Materials Used for Row Covers

Row covers used in vegetable productionhave different purposes in temperate and tropicalregions. In temperate regions, they are used mainlyduring the cold season to conserve warmth, stimu-late germination and early growth, protect plantsfrom frost injury, and improve the quality of thecrops. Other beneficial effects, such as maintainingsoil texture, and protecting crops from the attacks ofbirds and pests, can also be expected. For suchpurposes, especially to minimize heat loss duringcold weather, materials such as polyethylene film, ornon-woven fabric layer sheets of polyester orpolyvinylalcohol, are used.

In the tropics, however, materials used asrow covers need to have adequate permeability to airand moisture, to prevent the accumulation of exces-sive heat inside the covers.

The most popular materials used in summerin Okinawa are woven polyester wind-break nets,cheese-cloth, and non-woven fabrics ofpolyvinylalcohol. Various colors such as transpar-ent, white, silver, blue and black are available. InOkinawa, most farmers prefer to use blue row covers.

Types of Row Cover

Originally, row covers were laid directly onthe beds, but recently covers resting on low tunnelhoops or low pipe-framed houses are becoming morepopular. Covers resting directly on the plants meansthat there is little space between the plants and thecovering material, and provide a less favorable envi-ronment for plant growth than covers stretched overa frame.

Effect of Row Covers in the Tropicsand the Subtropics

Yield Increases in the Subtropics (Okinawa)

An experiment was carried out at TARC inOkinawa over the summer. Chinese cabbage of thenon-heading type (var. Santousai) was cultivatedunder row covers resting on a tunnel-shaped frame-work. Eleven kinds of perforated plastic materialswere tested, together with the control (bare soil). Theshading intensity given by the covers ranged from 0to 88%. Compared with untreated control, yieldswere increased by covers with 5-40% shading inten-sity. The highest yield was seen in the treatmentusing black cheesecloth (36% shading intensity),which gave a yield 1.8 times higher than the control(Table 4).

Yield Increases in the Tropics (Malaysia)

Six kinds of vegetables (water convolvulus,radish, cucumber, tomato, French bean and sweetcorn)were grown in Malaysia under covers resting on lowwooden frame houses over a two-year period. Theshading intensity under the covers ranged from 0 to98%.

Plant height and leaf weight were the great-est under heavy shading (37-87% shading intensity),but the root weight of radish decreased as shadingintensity increased. As for fruit vegetables, maxi-mum yields for cucumber and French bean wereobtained under medium shading (20-37% shadingintensity), while maximum yields of tomato andsweetcorn were obtained when no covers were used(Fig. 15). The reason why the yields of tomato andsweetcorn declined under row covers may be thephotosynthetic characteristic of these crops, both ofwhich have a high saturation point for light intensity.

Quality Improvement

The quality of the product, particularly thatof the leafy vegetables, improved under row coversin both Okinawa and Malaysia. The texture of theChinese cabbage leaves became tender, and theleaves were pale green in color, as is preferred byconsumers. The number of larvae and of holes in theleaves was significantly reduced by the use of rowcovers.

Results of experiments both in the subtrop-ics and the tropics indicated that row covers ofplastic net or cheesecloth are an effective measure invegetable production, especially of leafy vegetables,in a hot climate.

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Fig. 11. Soil temperatures at a depth of 0 cm, 10 cm and 30 cm under various mulchingmaterials during summer in Okinawa

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material is the use of row covers.Okinawa’s farmers began using plastic nets

as row covers for leafy vegetables during the late1950’s. This practice proved quite effective forvegetable production in a hot climate. The area underrow covers has now increased to 4,225 ha in Japan.

Row Covers

Materials Used for Row Covers

Row covers used in vegetable productionhave different purposes in temperate and tropicalregions. In temperate regions, they are used mainlyduring the cold season to conserve warmth, stimu-late germination and early growth, protect plantsfrom frost injury, and improve the quality of thecrops. Other beneficial effects, such as maintainingsoil texture, and protecting crops from the attacks ofbirds and pests, can also be expected. For suchpurposes, especially to minimize heat loss duringcold weather, materials such as polyethylene film, ornon-woven fabric layer sheets of polyester orpolyvinylalcohol, are used.

In the tropics, however, materials used asrow covers need to have adequate permeability to airand moisture, to prevent the accumulation of exces-sive heat inside the covers.

The most popular materials used in summerin Okinawa are woven polyester wind-break nets,cheese-cloth, and non-woven fabrics ofpolyvinylalcohol. Various colors such as transpar-ent, white, silver, blue and black are available. InOkinawa, most farmers prefer to use blue row covers.

Types of Row Cover

Originally, row covers were laid directly onthe beds, but recently covers resting on low tunnelhoops or low pipe-framed houses are becoming morepopular. Covers resting directly on the plants meansthat there is little space between the plants and thecovering material, and provide a less favorable envi-ronment for plant growth than covers stretched overa frame.

Effect of Row Covers in the Tropicsand the Subtropics

Yield Increases in the Subtropics (Okinawa)

An experiment was carried out at TARC inOkinawa over the summer. Chinese cabbage of thenon-heading type (var. Santousai) was cultivatedunder row covers resting on a tunnel-shaped frame-work. Eleven kinds of perforated plastic materialswere tested, together with the control (bare soil). Theshading intensity given by the covers ranged from 0to 88%. Compared with untreated control, yieldswere increased by covers with 5-40% shading inten-sity. The highest yield was seen in the treatmentusing black cheesecloth (36% shading intensity),which gave a yield 1.8 times higher than the control(Table 4).

Yield Increases in the Tropics (Malaysia)

Six kinds of vegetables (water convolvulus,radish, cucumber, tomato, French bean and sweetcorn)were grown in Malaysia under covers resting on lowwooden frame houses over a two-year period. Theshading intensity under the covers ranged from 0 to98%.

Plant height and leaf weight were the great-est under heavy shading (37-87% shading intensity),but the root weight of radish decreased as shadingintensity increased. As for fruit vegetables, maxi-mum yields for cucumber and French bean wereobtained under medium shading (20-37% shadingintensity), while maximum yields of tomato andsweetcorn were obtained when no covers were used(Fig. 15). The reason why the yields of tomato andsweetcorn declined under row covers may be thephotosynthetic characteristic of these crops, both ofwhich have a high saturation point for light intensity.

Quality Improvement

The quality of the product, particularly thatof the leafy vegetables, improved under row coversin both Okinawa and Malaysia. The texture of theChinese cabbage leaves became tender, and theleaves were pale green in color, as is preferred byconsumers. The number of larvae and of holes in theleaves was significantly reduced by the use of rowcovers.

Results of experiments both in the subtrop-ics and the tropics indicated that row covers ofplastic net or cheesecloth are an effective measure invegetable production, especially of leafy vegetables,in a hot climate.

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Table 4. Environment and yield of Chinese cabbage under various types of row cover

Fig. 15. Yield of vegetables in relation to shading intensity in the tropics (Malaysia)Source: Hanada 1987

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Reason for Yield Increases of Leafy Vege-tables Grown under Row Covers in a Hot Climate

Both the yield and the quality of leafyvegetables are improved by the use of row covers. Toinvestigate their effects in more detail, several experi-ments were carried out using leafy vegetables, mainlypak-choi (Chinese mustard, Brassica rapa L.).

Effect of Shading

Covering the plants with plastic net orcheesecloth reduced the radiation and preventedscorching or wilting of leaves caused by markedtemperature increases within the leaf tissue fromstrong sunlight.

The Relative Growth Rate (RGR) of 16 leafyvegetables under covers with 0-90% shading inten-sity was investigated. All leafy vegetables exceptedible amaranth (Amaranthus tricolor L.) and pak-choi showed an increase in RGR under 20-40% shad-ing intensity. Cabbage and Chinese cabbage showedbetter growth than the control, even under 60%shading intensity (Fig. 16). Shaded environmentswith 30-40% shading intensity seemed to be suitablefor most of the leafy vegetables.

Air Temperature

Air temperature under white cheesecloth(30% shading intensity) increased by 1°C when thematerial was stretched over a framework, and by 3°Cwhen it was used as a direct cover, at midday com-pared with the control. The use of less porouscovering materials further raised the inside air tem-perature. At night, there was little temperature differ-ence between treatments with and without covers(Fig. 17).

The ideal air temperature for the growth ofpak-choi was found to be 25°C, with a temperaturedifference of 5-10°C between the daytime and thenight. Since the ambient air temperatures in Okinawaat midday in summer exceed 30°C, it is important notto increase air temperatures under the covers.

Soil Temperature

An increase in soil temperature at middaywas suppressed by the use of covers. Soil tempera-tures at a depth of 5 cm were lower by as much as 6°Cunder covers with a shading intensity of more than67%, compared to the control. On the other hand,

non-woven fabrics with a low shading intensity (5%)gave higher soil temperatures than the control (Table4).

The growth of pak-choi decreased as thesoil temperature increased to more than 25°C (Fig.18). As soil temperatures reach almost 50°C on thesurface and 40°C at a depth of 10 cm in summer inOkinawa, row covers are effective in producing anunderground environment more suitable for rootgrowth.

Soil Moisture

Soil was kept moister for a longer periodwhen row covers were used. Direct covers were moreeffective in retaining soil moisture than covers sup-ported by a framework, although the soil beneaththem was apt to become waterlogged after rain (Fig.19).

The ideal soil moisture content for thegrowth of pak-choi was found to be pF 1.5 (Fig. 20).As the soil in summer is liable to dry out after a shorttime, row covers also provide a protective environ-ment for plants against water stress.

Protection from Wind

Wind velocity decreased to as little as 1/5under direct coers, and by 1/2 under covers over aframework, compared to the control. The growth rateof pak-choi was highest at a wind velocity of 0.4-1.4m/sec (Table 5). As wind usually blows at avelocity of 3-4m/sec in Okinawa, the effect of rowcovers in improving yields by protecting crops fromwind is important.

Protection from Pests

Row covers made of plastic net or cheese-cloth act as a physical barrier against attacks bypests. In experiments in Malaysia, cheesecloth witha mesh size of 1.4 mm gave good control of attacks bypests.

In the case of cruciferous vegetables, cov-ering materials with a larger mesh size of 4.0 mmprovide a sufficient barrier against armyworm andcabbage worm, although a mesh of 2.0 mm is neededto protect crops from diamondback moth. Materialswith a mesh of 1.0 mm are needed to protect veg-etables from yellow-striped flea beetles.

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Fig. 16. Relationship between shading intensity and relative growth rate (RGR) on16 kinds of vegetable

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Fig. 17. Diurnal fluctuations in air temperature in the open field, with andwithout row covers

CONCLUSION

Mulching with appropriate materials has anumber of effects: it increases the soil temperature,conserves soil moisture, texture and fertility; andcontrols weeds, pests and diseases. Various kindsof mulching material are available for vegetable pro-duction in temperate regions, depending on theirpurpose. However, mulching with plastic film causedan extreme increase in soil temperature during sum-mer in the subtropics. Organic matter such as freshleaves, fresh grass or straw are better mulchingmaterials than plastic in a hot climate.

The effect of row covers, another way of

stabilizing vegetable production, was also investi-gated in the tropics and the subtropics. Coveringcrops with plastic net or non-woven fabrics, espe-cially when these were supported by a framework,increased the yield of vegetables, especially of leafyvegetables, in both subtropical and tropical areas.These yield increases were found to be the combinedresults of shading, suppression of increases in soiltemperature, conservation of soil moisture, and pro-tection from wind and pests.

The use of plastic nets as row covers andmulching with freshly cut grass seem to be highlypromising techniques of vegetable production in thetropics.

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Fig. 9. Germination and survival percentages of weeds under radiation ofdifferent wave lengths

Source: Inada 1971

Fig. 18. Growth of Pak­choi under different soil temperatures

Fig. 19. Fluctuations in soil moisture (pF value) with and without row covers

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Table 5. Effect of wind velocity on the growth of pak­choi

Hanada, T. 1990. Leafy vegetable productionwith row covers in the subtropics in Ja-pan. Greenhouse and Horticulture 69:18-24. (In Japanese).

Iguchi, T. 1977. Cultivation of groundnutwith plastic film mulch in Kyushu. Veg-etables and Field Crops Technology Se-ries VI. Agricultural Statistics Associa-tion, Tokyo, pp. 238-239. (In Japanese).

Fig. 20. Growth of Pak­choi under different soil moistures

REFERENCES

Djigma, A. and D. Diemkouma. 1986. Plasticmulch in dry tropical zones. Trials onvegetable crops in Burkina Faso.Plasticulture 69, 1: 19-24.

Hanada, T. 1987. Promotion of VegetableProduction in the Tropics. A researchreport on vegetables under MARDI. 86 pp.

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Inada, K. 1971. Principle of green film formulching and it’s effect on use. Agricul-ture and Horticulture 46, 1: 17-22. (InJapanese).

Kawaguchi, T. 1974. Early Harvest of Onionby Polyethylene Mulching. Vegetablesand Field Crops Technology Series II,Agricultural Technology Association, To-kyo, pp. 86-87. (In Japanese).

Onuma, H., K. Okada and J. Onuma. 1976.Establishment of the High Yielding Soy-bean Production by the Use of SheetMulch. Progress Reports on MulchingCultivation IV, pp. 222-223 (In Japanese).

Nishitani, K. 1979. Adaptability of silverpolyethylene mulching for vegetable pro-duction in the open fields. Agriculture

and Horticulture 54, 5: 657-662. (InJapanese).

Takahashi, Y., and A. Chiba. 1971. Polyethyl-ene film mulching on carrot cultivation.Agriculture and Horticulture 46, 12:1725-1730. (In Japanese).

Takeuchi, E. 1974. The Use of Green Mulchfor Crop Production. Vegetables andField Crops Technology Series II, Agricul-tural Technology Association, Tokyo, pp.98-99. (In Japanese).

Wakayama Prefectural Agricultural Station.1973. Annual Report on Pests and Dis-eases of Vegetables, pp. 19-28. (In Japa-nese).