The Role of Green Manures in Rainfed Farming Systems in the Humid Tropics

-The Role of Green Manures in Rainfed Farming Systems in the Humid Tropics

Jan van der Heide and Kurniatun Hairiah

Cover crops have been used for a I Leguminous green manure crops Nitrogen supply to food crops long time by small farmers in the (Anon., 1979) can be grown tropics in their crop rotations, mostly successfully if their growing period or In terms of nitrogen supply, green as a cheap source of biologically plant geometry does not negatively manures may contribute 30 - 60 kg N fixed nitrogen, for the recycling of affect the performance or the labour per ha annually (Greenland, 1985) or leached nutrients, for protection requirement of the assoc~atedfood even over 100 kg N per ha (Bouldin, against erosion, for the b"i1d-u~and c~oP(~).Therefore, twining cover crops 1988) to the following crop. A substantial fraction of N in leguminous maintenance of soil organic matter are usually grown in relay cro~~'"g w~tha food crop, if rainfall in the green manures mineralizes rapidly. and for the suppression of weeds' following months will allow the crop to The quantity and timing of N supply by these benefits have produce sufficient biomass, However, decomposing residue should be only recently begun to receive in drier conditions the establishment of adapted as well as possible to the attention by scientists or the legume as a companion crop to the demands of the succeeding food crop, development agencies, after a long (cereal) food crop may be required taking into account the reutilization of period of neglect. (Aggarwal and Garrity, 1989). residues from other crops in the rotation. The residual long-term effect The use of cover crops is particularly Since many green manures cannot be of only one green manure crop is relevant in the humid tropics, as high used for human consumption, other relatively small. The cumulative effects rainfall generally has depleted the soil benefits should become obvious to the of continued use of such crops are of nutrients, especially nitrogen, which farmer after one or two cropping important, not only in terms of N-supply leaches easily. Levels of soil-organic cycles. Grain legumes, although but also in regard to soil organic matter matter aregenerally low, and there is a providing immediate economic benefit, and soil productivity. rapid and persisting weed growth, tend to accumulate nutrients in the which is one of the main reasons why grain which is harvested, so that their These observations may be illustrated farmers have to abandon their plots effect on the performance of the by experimental results from the (Nye and Greenland, 1960). Under following crop is usually low (Van der high-rainfall zone of Nigeria (Van der these conditions, sustainable Heide et al., 1985). Nitrogen-fixing Heide et al.. 1985) where the residual semi-permanent agriculture has to not 1 cover crops can be an excellent way to effects of 4 years of continuous only compensate for the nutrients I supply a substantial quantity of cropping and annual fertilizer removed by the harvesting of products, nitrogen and recycled nutrients to the applications of 0, 45, 90, 135 and 190 kg but also recycle the nutrients that have annual crop rotation, by returning the N per ha on soil productivity were leached to deeper layers in the soil total biomass produced to the soil, just assessed, using the following annual profile and suppress weed growth. Last before planting one, or a combination crop rotations: but not least, the prevailing socio- of food crops in the following season. - Intercropping maize (first season) economic conditions require that these Leguminous forage crops to provide I and cassava (whole year) measures be cheap, locally feasible feed for cattle can have a double - Monocropped maize (first season) and not labour-intensive. function. followed by cowpea (second season)

MAIZE/CASSAVA MAIZE/COVER CROP

ELOMASS-N (KG N/HA)

300 7-

I *): STRONG WEED EMERGENCE

YEAR + FERTILIZER-N KG/HA 'YEAR + FERTIL\ZERN KG/HA

RETURNED*l80 KG/HAU HARVESTED 180 KG/A RETURNED + 180 N/HAO HARVESTED + 180 N/HA

RETURNED + 0 NtHA HARVESTED 180 N/HA 81 RETURNED 4 o YG~AB HARVESTED . o KGIHA

Figure 1. Nitrogen in plant residues harvested and returned to the soil in two crop rotations in S.E. Nigeria.

ILElA - JULl 1989 Vol 5, No. 2 - Malze (first season) followed by areen manure Stylosanthes 1 guianensis (second season, 1982) and Mucuna utilis (second season 1983- 1985) - lntercropping maize (first season). cassava and tree type Pigeonpea MAIZE GRAIN YIELD KG/HA (Cajanus cajan) (whole year).

Considerable differences among these cropping systems exist in the ration of N removed from the field and N returned through plant residues (Van der Heide, 1988) (fig. 1). As a result, significant differences in residual effects occur, as measured in the 5th cropping year in unfertilized monocropped maize used as test crop (fig. 2). This experiment demonstrated that incorporation of a leguminous A --- green manure in the cropping system I iI/ Ill led to a better soil productivity than the other cropping systems, whereas no significant residual effect from previous N-fertilizer applications was observed. Similar observations have been reported from the humid lowlands of Mexico (Gliessman. 1982).

Soil organic matter FERTILIZER APPL 1981-1985 KG bI/H4

Generally, soils in the humid tropics are highly weathered and leached, with Figure 2. Residual effects of 5 years continuous growing of 4 cropping systems. most of the available nutrients present and applications of 5 levels of N-fertilizer in S.E. Nigeria. in the organic topsoil (Sanchez. 1976). Because organic matter supplies most of the cation exchange capacity, the rapid decrease in organic matter content that usually occurs when primary forest is converted to not be limited to a cropping cycle of 1 Weed Control agricultural land, results in a marked one year. reduction in nutrient hold~ngcapacity of ,,There is no doubt that the increasing the soil (Von Uexkull, 1986). An Recycling of nutrients effort of keeping the land free of weeds adequate level of organic matter in the as the cropping period proceeds is topsoil is therefore vital to the Deep-rooted crops in a rotation system often the primary reason for a patch of sustainability of cropping systems may help to recover nutrients leached land being abandoned" (Nye and where no or only a limited fallow period to the subsoil. How deep roots have to Greenland, 1960). Particularly if land IS included. go in order to contribute to the resources are becoming scarce, and recycling of nutrients depends on the fallow periods have to be shortened, Slowly decomposing residues may amount and d~stributionof the rainfall, persistent weeds such as lmperata spp. have an important effect on the organic the buffer capaclty of the soil for the proliferate, ultimately resulting in the matter content of the soil (Bouldin, nutrient in question (biological or establishment of anthropogenic 1988), but this effect will occur only chemical) and the duration of the grasslands that are extremely diff~cult after a period of several years (Van process of leaching (Van Noordwijk, to recover for agricultural purposes Faassen and Smilde, 1985). 1989). Under high rainfall conditions, (Srivastava, 1986). especially at the start of the rainy It is remarkable that the potential of As an example, in an experiment in season, permanent deep root systems, cover crops to control weeds has Indonesia we found that the estimated as some trees have, are required for hardly been investigated. However, on N-input (biomass multiplied by recycling. Most food crops have a several occasions where shifting N-content) to the sol\ was higher for shallow root system due to the adverse cultivators have been obliged to Crotalaria juncea than for Mucuna utilis soil conditions and their root system shorten or abandon the fallow period, (198 and 71 kg ha respectively), but may develop too slow to Intercept the they have resorted to the use of cover maize production was higher after mineralization flush which occurs when crops to suppress weed emergence, Mucuna, as Mucuna residue the soil is rewetted. As far as we know, such as Mucuna (Stizolobium) decomposes more quickly than the leguminous cover crops deeringiana (Bort.) In Northern Crotalaria. The extra N-uptake by Centrosema, Pueraria and Crotalaria Colombia (Van Eijk-Bos, 1986), Yam maize (uptake following cover crop can develop deep root systems on acid beans (Pachyrhizus erosus) in S.E. minus uptake by control) was 121 and soils in the humid tropics. The fast Asia, or Mucuna sloanei (Anon.. 1979) 147 kglha respectively (Hairiah and Van growing Mucuna is shallow-rooted and and Mucuna utilis (Akobundu, 1983) in Noordwijk, 1989). The extra N-uptake by is not drought-tolerant on acid soils. W. Africa. maize following Mucuna was even The deep-rooted cover crops however, higher than the estimated N-input, usually establish themselves relatively A rapid establishment is crucial to the possibly due to decomposition of dead slowly, and are therefore unable to seed suppressing potential of a cover Mucuna leaves during growth of this control weeds effectively (Hairiah and crop. Akobundu and Poku (1984 and cover crop. In a similar experiment, Van Noordwijk, 1987). 1985) observed that Mucuna utilis was which is still going on, we anticipate Normally shallow-rooted crops can able to completely cover plots infested that Crotalaria will have a stronger develop deep roots by following old with lmperata cylindrica (density before impact on soil organic matter build-up 1 root channels left in the soil by crops or treatment 100 standsIm2) rn a period of in the long run, as compared to trees. In deeper layers of the so11they 19 weeks, and continued to suppress Mucuna. The evaluation of the effect of can recover nutrients from leachates, the weed throughout the year. This is the incorporation of a green manure which are mostly transported through due to the vigorous growth in the first into a crop rotation system should take the same channels (Van Noordwijk, weeks after planting, as demonstrated a prolonged period into account, and 1989). by observations of light interception by

ILElA - JULl 1989 Vol 5, No. 2 6 cover crops in Nigeria (figure 3, light tnterceptlon Hairiah and Van Noordwijk, 1986). by crop(%)

When introducing a cover crop in an existing crop rotation system, a good potential for weed suppression is the key to farmers acceptance, as beneficial effects from weed control may be observed right after establishment of the cover crop, while effects of improved nutrient supply may occur only in the longer run.

Kretschmer (1989) discussed the growth habit, suitability as fodder, tolerance to soil acidity and water requirements in the growing season of Figure 3. Light interception by six cover crops comparing light intensities inside 50 tropical legumes. Root and outside plots. characteristics are known for only a few species. Within each species, considerable variation in important rlce farming, pp. 151-164, Proc. Symp - Srivastava, P.B.L., 1986. Shifting characteristics occurs. The task of 25-29 May 1987, lnternatlonal Rice Cultivation, Problems and Alternatives. Food collecting and comparing tropical Research Institute, Los Banos, and Agricultural Organization of the United germplasm has only recently been Philippines. Nations. GCP/RAS/lO6/JPN Field taken seriously. - Gliessman, S.R., 1982. Nitrogen Document 10, Bangkok, Thailand. distribution in several traditional agro- - Van Eyk-Bos, C.G.. 1986. Recuperation Jan van der Heide, lnstitute for Soil ecosystems in the humid lowlands of de tierras invadidas por el lmperata Fertility, P.O. Box 30003, 9750 RA south-eastern Mexico. Plant and Soil 67: contracta Haren, the Netherlands. 105-117. (H.B.K.) Hitchc, por medio de la siembra - Greenland. D.J., 1985. Nitrogen and food Kurniatun Hairiah, Brawijaya de Leguminosas, especialmente el production in the tropics: Contributions Mucuna deeringlana (Bort.) Small en University, Malang, Indones~a. from fertilizer nitrogen and biological Uraba, Colombia. Una tecnologla de nitrogen fixation. In: Nitrogen bajos insumos. [Reclamation of lmperata m management in farming systems In humld contracta (H.B.K.) Hitchc, invaded terrains References: and subhumid tropics, pp. 9-38. B.T. Kang by sowing leguminous species, especially - Aggarwal, P.K. and D.P. Garrity, 1989. and J. van der Heide, eds. lnstitute for Mucuna deering~ana(Bort.) Small In Intercropping of legumes to contribute Soil Fertility, Haren, the Netherlands. Uraba, Columbia.] CONlF report, Bogota, nitrogen in low-input upland rice-based - Hairiah. K and M. van Noordwljk, 1986. Columbla cropping systems. In: Nutrient Root studies on a tropical ultisol in relation - Van der Heide, J.. 1988. Role of green management for food crop production in to nitrogen management. institute for Soil manure in low-input farming in the humid troplcal farmlng systems. Proc. Symp. Fertlllty. Report 7-86, 116 pp. Haren, the tropics In: Green manure in rlce farmlng. October 19-24, Malang, Indonesia, Netherlands. pp. 185-192. Proc. Symp. 25-29 May 1987. lnstitute for So11Fertility, Haren, the - Hairiah, K. and M. van Noordwijk. 1987. lnternational Rlce Research Institute. Los Netherlands Cover crops on acid soils, experiences In Banos, Phil~ppines. - Akobundu. 1.0.. 1983 No-tillage weed the humid tropics. ILElA 3 (1): 9. - Van Noordwijk, M., 1989. Rooting control in the tropics. In: No-tillage crop - Halriah, K. and M. van Noordwijk, 1989. aspects of farming systems in the humid production In the tropics. Proc. Symp. Root distribution of leguminous cover crops tropics in relation to nutrients. In: Nutrlent Aug. 6-7 Monrovia, pp.32-44. lnternational in the humid tropics and effects on a management for food crop production in Plant Protection Centre, Corvallis. USA. subsequent maize crop. In: Nutrient tropical farming systems. Proc. Symp. - Akobundu, LO., and J.A. Poku, 1984. management for food crop production in Malang, 19-24 Oct. 1987. lnstltute for Soil Control of lmperata cylindrica. In: trop~calfarming systems. Proc. Symp Fertility, Haren, the Netherlands. lnternational lnstltute of Tropical Malang, 19-24 Oct. 1987. lnstitute for Soil - Von Uexkull, H.R.. 1986. Efficient Agriculture, Annual Report for 1983, pp. Fertility, Haren, the Netherlands. fertilizer use in acid upland soils of the 175-176. lbadan, Nigerla. - Kretschmer Jr., A.E.. 1989. Tropical humid tropics. Food and Agricultural - Akobundu, l O., and J.A. Poku, 1985. forage legume development, diversity, and Organlzatlon of the United Natlons, Control of lmperata cylindrica. In: methodology for determining persistence Fertilizer and Plant Nutr~tionbulletin no. lnternational lnstitute of Tropical pp. 117-138. In: Persistence of forage 10. Rome, Italy. Agriculture, Annual Report for 1984. pp. legumes, ASA, CSSA, SSSA, Madlson. Further reading: 174. lbadan, Nigeria. Wiscons~n,USA. Green Manuring with Fast-Growing Shrub - Anon., 1979. Tropical Legumes: - Nve, P.H. and D.J. Greenland, 1960. The Fallow in the Tropical Highland of Rwanda. Resources for the Future. U.S. Natlonal soil under Shifting Cultivation. Technical Kurt Raquet. In: Ecofarming Practices for Academy of Sciences. Washington D.C . Communication No. 51, Commonwealth Tropical Smallholdings, Research and USA. Bureau of Soils, Harpenden, England. Development inTechnical Cooperation. - Bouldin, D.R., 1988. Effect of green - Sanchez, P.A.. 1976. Properties and Johannes Kotschi (ed). Worklng Papers for manure on soil organic matter content and management of soils in the tropics. Wiley, Rural Development Nr. 14. 1989. GTZ, P.O. nitrogen availability. In: Green manure in New York, USA Box 5180, D-6236 Eschborn 1, R.F. Germany. The .,Projet Agro-Pastoral'' of Nyablsindu, Rwanda (altitude: 1600-1850 m, rainfall: 950-1400 mm) has positlve results with Mucuna utilis in s~ngle-seasonfallow Promising results were also obtained in multiple-season fallow with a mixture of shrubs. Cajanus cajan, Crotalaria lachnophora, Tephrosia vogelii, and the soil coverers Desmodium intortum and Desmodfum uncinatum which were occasionally supplemented by the shrubs Crotalaria pallida andlor Sesbania macrantha. With regard to the sod- desinfecting effects of some compositae, notably Tagetes, deliberately tolerated weed growth in the first year is highly effective with the rnult~ple-seasonfallow. In the same publication Conservation of soil fertility by peasant farmers in Atlantic Yam bean, interesting Province, Benin Anne Floquet, Besides multi-purpose crop farmers' innovations in soil fertility conservation, successful project experiences wlth Cajanus cajan as green manure are described.

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