Management of soil organic matter in the farming systems of the low land humid tropics of West Africa: a review

SOIL TECHNOLOGY

vol. 4, p. 265-279

Cremlingen 1991 1

M A N A G E M E N T OF SOIL O R G A N I C MATTER IN THE F A R M I N G SYSTEMS OF THE L O W L A N D H U M I D TROPICS OF WEST AFRICA: A REVIEW S.A. Ayanlaja & J.O. Sanwo, Ago-Iwoye Summary Soil organic matter is the key to successful and sustained productivity of soils of the tropics. This is because soil organic matter positively affects structure, aggregation, porosity, microbial activity, pore size distribution and water retention capacity of the soil. Furthermore, soil organic matter is the major nutrient storage site for the lowactivity-clay soils of the tropics and so affect nutrient retention capacity, availability and mobility of macro- and micronutrients. It increases the water use efficiency, and therefore attenuate runoff and erosion and consequently the productivity of the soil. The low land humid tropics is characterised by high temperature, high relative humidity, high rainfall intensity and high microbial activity which all encourage rapid mineralisation, depletion and erosion of organic matter leading to soil deterioration. Many cultural practices and operations encourage rapid depletion of soil organic matter while others are associated with soil organic matter build ISSN 0933-3630 (~)1991 by CATENA VERLAG, W-3302 Cremlingen-Destedt, Germany 0933-3630/91/5011851/US$ 2.00 + 0.25

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up. Practices like crop rotation, multiple cropping, mulching, alley cropping, fallowing and farm yard manuring, encourage soil organic matter accretion. However, the effectiveness of these practices in increasing soil organic matter depends on"

(a) amount and frequency of residue application; (b) the nature and C:N ratio of the mulching material, or manure; (c) rainfall amounts, intensity and distribution, soil moisture and clay contents. Land clearing with heavy machinery is associated with removal of biomass from the field, while conventional ploughing and harrowing lead to soil organic matter depletion. These practices should be discouraged or modified to reduce their negative effects on soil organic matter. Effects of green manuring and burning, on soil organic matter are questionable. Research is needed on processes and pathways of crop residue decomposition to provide clues to possible interventions aimed at management of soil organic matter. Work is also needed to improve the efficiency of agro forestry systems in

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accreting soil organic matter in soils of the low land humid tropics.

Introduction: Effects of organic matter on soil properties and plant growth Organic matter is the dead and decaying plant and animal residue while humus is the resistant, less decomposable products and includes the microbially resynthesised products (Hayes & Swift 1978). The importance of soil organic matter cannot be overemphasised as it is the key to successful and sustained crop production on soils of the tropics. Organic matter is the source of energy for soil microbes which decompose it to slimes and their by-products. These byproducts which include humic substances of low molecular weight, linear polymers, polysaccharides and polyuronides, bind individual soil particles together into micro and macro-aggregates and increase the bonding strength of existing microaggregates (Christensen 1986, Greenland 1979). Soil organic matter is gelatinous and absorbent in nature with a bulk density of 0.2 g c m 3. It acts as cementing agents for soil particles and greatly accentuate aggregation making them resistant to slaking and dispersion. It is capable of improving the structure, water holding capacity, infiltration, percolation, porosity and tilth of either heavy clay or loose sandy soils (Turchenek & Oades 1978). In South-West Nigeria, Gbadegesin & Areola (1987) correlated 20 soil properties with maize yield on 50 sites and found that 78% of the yield variations could be explained in terms of soil organic matter contents alone. It has been

reported that the available water and water retention measured at any suction were consistently higher in soils of high than of low humus contents, especially for soils low in clay contents (Ayanlaja 1987, Ayanlaja 1983, Loynet 1977). Thus, water use efficiency is improved through decrease in losses due to runoff, increased root proliferation, and better growth and yield. Decrease in runoff mitigates the destructive effects of erosion (Ayanlaja 1987). In soils of low humus contents, plants suffer from drought because of reduced root proliferation and development, and because of low water retention capacity of the soil which ensues from loss of structural porosity and compaction (Lal 1976). Plants also suffer from physiological drought even during periods of frequent rains due to low infiltration ensuing from decreases in transmission porosity leading to waterlogging and poor aeration (Alvim 1967). Soils of the tropics are inherently poor because they contain predominantly Kaolinite and sesquioxide in the clay fraction with little or no weatherable minerals in the sand and silt fractions (Moorman & Wambeke 1978). Being the next soil component in the absence of good quality clay, that can retain nutrients, humus becomes the main nutrient storage site for soil of the tropics (Agboola & Omueti 1982). This assertion is vividly supported by the facts that soil organic matter accounts for 80% of C.E.C., the availability of major and minor nutrients are associated with soil organic matter and yields of most crops are highly correlated with soil organic matter contents (Gbadegesin & Areola I987). These factors explain why nutrient and soil water shortages are prevalent in soils of low humus contents, resulting in poor

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growth, vigour and low yields (Anderson 1978). Nutrients that are tied up in the biomass of the forest vegetation are released to the soil through mineralisation of leaf fall (Cunningham 1962). However, some of these nutrients are often lost by leaching because of low soil buffer capacity attributed to predominance of low activity clays and low soil organic matter contents. Thus, soils of the tropics are quickly exhausted of exchangeable cations and acidify rapidly under intensive cultivation due to low humus contents (Ayanlaja 1990). Decomposing crop residues release important and appreciable amounts of nutrients especially S, P, N, Mo and Zn, into the soil and also influence availability of nutrients from other sources. This is evident by the high positive correlation of available Zn, Cu, Mo, P and S with soil organic matter (Adepetu & Corey 1972, Uzu et al. 1975), and the observation that incorporation of crop residues, to soil high in N resulted in increase in yield of millet due to better N utilization (Ganry 1973). Humus increases C.E.C., base absorption and nutrient retention. It also reduces, by complexation, the concentration of toxic elements like Mn 2+, A13+ and H + in acid soils (Lal & Kang 1982). Soil organic matter is so important in tropical agriculture that its levels may dictate the failure or success of crop production. The paper reviews the farming practices in the low land humid tropics of West Africa as they affect soil organic matter conservation. It highlights soil and crop management practices that have potentials of building up soil organic matter and can therefore by relied upon for sustained soil productivity in this ecosystem.

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Soil organic matter dynamics in the low land humid tropical ecosystem of West Africa The part of West Africa having the low land humid tropical ecosystem lies between latitudes 3°30 , N and 6045 ' N and longitudes 15° East and West of the Greenwich with dimensions of approximately 3500 km long and 200 km wide and an area of 0.7 million square kilometres. Topographically, this part consists of plains ranging from 150 to 450 m above see level with a belt of low coastal land area below 150 m and lower valleys of many rivers flowing into the Atlantic Ocean. The land mass is made up of crystalline igneous and metamorphic rocks of the basement complex, and the soil types are usually alfisols, ultisols and oxisols of low to medium native fertility (Sanchez 1976). The annual rainfall in the low land humid tropics is high ranging from about 1000 mm to over 2500 mm near the coast, with a bimodal pattern; and for a period of between 6-8 months in the year, precipitation is greater than evapotranspiration (Lawson 1979). There is a general high insolation with a high annual mean temperature of 25-27°C and the lowest temperature being about 18°C (Fullard 1966). Climatic factors, especially temperature, rainfall intensity and distribution, have profound effects on the rate of mineralisation, transformation and movement of nitrogen from decomposing crop residues. Alternation of wet and dry seasons has been observed to enforce a cyclic pattern on the activity of the microbial population and on the availability of mineralised N to crops. This assertion is confirmed by the fact that

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a flush of mineralization occurs when a soil is remoistened after a period of dryness. Phosphate mineralization and release of S from organic matter are very much affected by climatic factors especially soil water (Omotosho 1971, Cunningham 1962). Annual rainfall and seasonal distribution affect soil organic matter contents. In Nigeria for example, Jones (1973) reported a decrease of 0.17% per 100 mm decrease in mean annual rainfall. Kang et al. (1981), also observed large differences in soil organic matter status of soils from various ecological zones" forest 1.3_0.08%, greater than derived savannah 0.98_+0.07%, greater than guinea savannah 0.7_+0.06%. The rate of decomposition of crop residue mulch left on soil surface in the low land humid tropics is much higher than in the temperate area (Cook et at. 1979, Greenland & Kowal 1960, Blair 1979, Kang et al. 1981). This is in agreement with the Vant Hoff's temperature rule which says that the rate of decomposition increases two to three times with every 10°C increase in mean annual temperature. Consequently, the rate of organic matter decomposition is about four times faster in the low land humid tropics with mean annual temperature of 26°C compared to the temperate temperature with a mean temperature of only 9°C (Jenkinson & Ayanaba 1977). The high climatic erosivity drastically affects soil organic matter in this ecosystem. Water erosion results in preferential removal of fine clay and humus, and Lal (1981 b), reported a linear decline in soil organic matter content with accumulative erosion (E)" % Org C = 1.79 - 0.002E. r = 0.71"*

Farming practices and their influence on soil organic matter dynamics Farming system is defined as the way in which sets of resources are harmonised with sets of operations to foster agricultural production (Okigbo 1980). The appropriate farming system for an area should therefore seek to combine scrutinised farming operations with land use and crop management techniques that will enable efficient, economic and sustained production of food and fodder without soil deterioration. Discussed below are farming operations and practices common in West Africa low land humid tropical ecosystem and their effects on soil organic matter dynamics.

3.1

Land clearing

There are many methods of forest removal to facilitate the cultivation of both arable and cash crops. Land clearing methods include the slash-burn method of small holder peasant farmer, the tree poisoning and the use of heavy machinery like bulldozer, treepusher and root rake. The use of these heavy machinery is preferred by commercial or large scale farmers (Lal 1981 a). The very fragile nature of soils of the low land humid tropics often poses a series of problems when the protecting vegetation cover is removed. Owing to the high rainfall intensity, coupled with high insolation and high relative humidity, soil organic matter rapidly disappears following forest clearing due to mineralization and surface erosion (Agboola 1982). On an aifisol in the low land humid tropical ecosystem of Nigeria for example, significant declines were found

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in the level of soil organic N, C, and P after forest removal; as soil organic carbon declined from 1.55 to 1.24% within one year of cultivation (Lal 1981 a). Hoore (1961) observed that clearing and cultivation of alfisols in the low land humid tropical Ivory Coast were accompanied with transformation of a fraction of humic acids and humin to fulvic acid and compounds soluble in ethanol and bromoform. Cunningham (1963) showed that removal of forest on oxisol in Ghana resulted in decrease in soil organic C levels by between 25 and 57% for the top 0-5 cm depth and from 17-30% for the 5-15 cm depth in 3 years. Jones (1973) compared soil organic matter contents of virgin woodlands with cultivated land in the Nigerian savannah and observed that mean C contents of virgin lands and cultivated lands were 1.03 and 0.58% respectively. Ayanlaja (1983) also found soil organic matter levels of cocoa soils formed on medium grained granitic rock or basaltic rock or sericite schist to be much lower than that of the adjacent uncleared and uncultivated virgin forest lands of similar soil type. Ollagnier et al. (1978) also found soil organic matter in the cleared land to be 60% of adjacent forest in Ivory Coast. Moreover, land clearing by bulldozer and heavy machinery results in scraping of humus rich, earthworm-cast-coveredtop-soil as well as the felled vegetation (plant residues) out of the field (photo 1), into the windrows. Thus exposing the subsurface horizon which is poor in soil organic matter, for subsequent cultivation (Ollagnier et al. 1978, Lal 1981 a). Thus, mechanical land clearing with unsuitable implements and unskilled operator can cause removal of surface soil and vegetation material to windrows, leading to a rapid loss of soil organic matter conSOIL TECHNOLOGY--A cooperating Journal of CATENA

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tent from the remaining cultivated land (Lal 1981 a). The slash-burn method of forest removal has been a common practice in the low land humid tropics as the only available, affordable and quick means of removing the enormous mass of felled vegetation (Ahmad 1985). In the forested low land humid tropics, the amount of vegetation cover per hectare is large and mechanical means of copping with it are limited. The heavier browse, thicket and shrubs cannot be controlled effectively as herbicides are very expensive and not available for peasant farmers. Burning therefore becomes the only practical way to clear the land, and biomass that would have contributed to soil organic matter are rapidly oxidised. Burning is argued to be less harmfulto soil organic matter and soil fertility in the forested area than one might expect, and may even be beneficial in an indirect way, if good cropping practices are followed (Agboola 1982, Okoro 1981). Burning leads to complete oxidation of organic materials especially the litter and felled forest debris (photo 2) and most inevitable leads to loss of nutrients N, C and S through volatilization (Kayode 1981). However, the overall effect depends on the intensity, frequency and timing of burning incidence in a given ecosystem (Nwobishi 1972, Egunjobi 1971, Okoro 1981). Slight burning for example was reported by Ahmad (1985) to actually increase soil organic matter. Similarly, Moore (1960) found that 30 years of light burning made early in the dry season increased soil organic humus by 17%, but when the burning was done late in the dry season, soil humus was reduced by 12%. Lal & Cumming (1979) also observed small increases in soil organic matter af-

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. . . . . . . . . . . . .

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Photo 1" Forest clearing with bulldozer.

Photo 2: Forest burning.

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Management of Soil Organic Matter ter burning. Lal (1981 a) found the soil organic carbon contents of alfisol cleared by mechanical, slash-burn and slash alone methods to be 1.6, 1.8 and 1.7% respectively.

3.2

Mulching

Mulching implies and involves putting some materials, usually the dead stover or residue of the previous crop, on the surface of the soil to prevent splashing and crusting which ensues from direct raindrop impact on the soil surface. Mulching increases soil organic matter, conserves soil moisture, improves infiltration rates, attenuates runoff and erosion, and influences the temperature regimes. Mulching materials, on decomposition, release essential nutrients to the soil (Anderson 1978, Guisquiani et al. 1988). Some workers argue that soils of the tropics could be maintained continuously productive if adequate amount of crop residue mulch could be constantly maintained on the soil surface (Agboola & Omueti 1982). To sustain soil organic matter by mulching, frequent and large quantities of crop residue would be required. This is because 25-30% of residue from cereals and 40--50% of residue from legume can decompose in a month in the low land humid tropics (Jenkinson & Ayanaba 1977, Lal et al. 1980). However, the rate of mulch decomposition depends slightly on the amount of mulch applied. This is because the rate of decomposition is higher in cases of high mulch rate than low rates (Feller et al. 1987). Roots can also be a main source of crop residue. Maize (Zea mays), for example, can add up to one ton of residue as roots, and this decompose more readily than an equiva-

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lent amount of residue left on the surface (Lal & Kang 1982, Kayode 1981). Many types of materials of plant origin have been used as mulching materials by various workers: palm (Elaeis guineesis) oil mill slurry (Aisueni 1986), maize cob/crop residue (Okigbo 1980), papermill waste (Gabriels 1988), cocoa (Theobroma cacao L.) pod husk (Oladokun 1986), town and urban refuse and compost (Christensen 1986, Gusquiani 1988, Gabriels 1988), sugarcane (Saccharum officinarum) baggase and filter cake (Ngkee & Deville 1988, Sandhu & Bhumbla 1967) and water hyacinth (Eichhornia crassipes) (Osiyoye 1987). The quantity of the mulching material as well as its nature affect rate of decomposition and consequently the soil organic matter building up capabilities of these materials differ.

3.3

Cover cropping and green manuring

Growing leguminous cover crops has been argued to adequately provide and protect soil organic matter in soils of the low land humid tropics. An important function of cover crop has been the periodic addition of residues from dropping leaves, which are mineralised to release nutrients during cultivation period (E1Swaify et al. 1988). Cover crops also protect the soil from raindrop impact, direct insolation and from erosion. This is confirmed by the observation of Agboola & Omueti (1982) who found accelerated decline in soil organic matter of an exposed land. In cover cropping, plants are killed with herbicides so that the residue forms a slowly decomposing mat layer above the ground level. In this case, the surface soil is kept cool and so can slow down

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the rate of decline of native soil organic matter. Selection of appropriate plant species which produces large quantity of slowly decomposing woody residue, and so can ensure slow rate of organic matter decline, is essential for success of cover cropping (Agboola 1982). The practice of growing a crop and then ploughing it under while it is still in the succulent stage is green manuring. In the low land humid tropical ecosystem of West Africa, with high temperature and predominantly sandy soils, the rate of organic matter oxidation is so fast that the increase in soil organic matter due to green manuring, especially with herbaceous plants, is temporary and may not even be detectable (Lal & Kang 1982). Some cover crops that have been used for this purpose include: Mucuna beans (Mucuna utilis), Stylo (Stylosanthes gracilis), Centro (Centrosema pubescense), Melon (Cucumis melo), Pumpkin (Cucurbita maxima) and Sweet potato (Ipomea batatas). Grass fallow comprising of gamba grass (Andropogon gayanus), Guinea grass (Panicum maximum), elephant grass (Pennisetum purpureum) and grass-legume mixtures are planted as sod-forming cover crops or green manure. Reddy et al. (1986) used some tropical legumes: Pigeon pea (Cajanus cajan), Mung bean (Vigna radiata), and Indigo (Indigofera hirsuta) as green manure. 3.4

organic matter in the tropics could be maintained at satisfactory level by continuous addition of farm yard manure or compost; while Lal & Kang (1982) contented that effect of such additions even up to 20 tons/ha would last for maximum of six months under low land humid tropical conditions. Substantial increase in soil organic matter by farm yard manuring will therefore require large and continuous application for a long time before appreciable increase can be achieved. However continuous application of farm yard manure substantially improved crop yield in India through injection of essential nutrient elements and improvements in physical properties of a medium black soil (Somani & Saxena 1976). In Senegal, Feller et al. (1987) found composted millet (Setaria italica) straws to increase the >200 ~m fraction of the soil organic matter.

Cropping systems and their effects on soil organic matter dynamics Cropping system refers to the yearly spatial arrangement and sequence of crops on the land. This arrangement is harmonised with various operations and practices to bring about crop production. Discussed below are some common cropping sytems in West Africa and their effects on soil organic matter dynamics.

Farm yard and compost manuring

Farm yard manure consists of animal droppings from cow, sheep, goats, and poultry. Composts, on the other hand, include household wastes and town refuse incorporated into the soil or spread on the surface. Watson & Goldsworthy (1964), reported that soil

4.1

Continuous cultivation

This is a cropping system in which the land is repeatedly grown to a particular crop year after year. Continuous cropping leads to accelerated breakdown and depletion of soil organic matter especially under low land humid tropical en-

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vironment. In Nigeria, continuous arable farming is associated with N P K Mg and trace element efficiences, which is an indirect indication of organic matter depletion since organic mater is the major source of these nutrients (Osiname et al. 1973, Heathcote & Stockingen 1976, Vine 1953). Soil organic matter levels of alfisols and oxisols carrying 20-year old monocropped cocoa (Theobroma cacao L.) in nine locations across Nigeria and within the low land humid tropical ecosystems was 1.1 compared to 1.5% in the adjacent virgin forests of similar soil types (Ayanlaja 1983).

4.2

Alley cropping

This is the alternating arrangement of hedgerows of tree/shrub with alleys of food/arable crops. The trees are periodically pruned throughout the cropping season to prevent shading and to provide mulch and green manure. The trees in the hedgerows recycle nutrients, suppress weed, control erosion and in addition, leguminous species add fixed N to the system (photo 3) (Kang & Wilson 1987). The soil receiving leguminous pruning was reported to have higher soil organic matter, moisture and N levels with resultant higher maize yields, than soil without pruning (Kang et al. 1985). Peasant farmers have long recognised the benefits of retaining certain native trees or shrub species as fallows because of their abilities to enrich the soil with certain nutrients and probably for staking (Wilson & Kang 1980). Two tons of Sesbania rostrata pruning added to rice increased yield from 3 to 4 tons due to soil organic matter increase (Ola et al. 1988). In Nigeria, Kang & Wilson (1987) observed that by process of organic matter mineralisation, Leucaena Ieucocephala released

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246.5 kg N/ha/yr into the soil while Gliricidia sepium added 169.1 kg N/ha/yr into the soil. Other tree/shrub species suitable for alley cropping system include: Alchornea cordifolia, Cassia spectabilis, Cassia siamea, Flemingia congesta, Calliandra surinamensis, Calliandra calothymus, Acioa barterii, Gmelina arborea, Anthonata macrophyiIa, DiaIium guineense, Sesbania grandis, Sesbania sesban and Sesbania grandiflora (ICRAF 1979). Alley cropping system seems to be a viable and stable system for sustaining humus and nutrient contents of soils of the tropics, currently under shifting cultivation or related bush fallow systems. This system has been adopted successfully for food crops (Duguma et al. 1987, Budelman 1989), vegetable crops (Palada et al. 1988, Chen et al. 1989), and even tree crops - - cocoa (Alpizar et al. 1986).

4.3

Multiple cropping

This is defined as a system whereby two or more crops are grown on the same field in a year. It is a means of intensification of cropping over time and space (Roy & Brown 1983). There are many forms of multiple cropping practices in the low land humid tropical region of West Africa. The mixed intercropping involves a random arrangement of several crops in a field. The simultaneous planting of maize (Zea mays), cassava (Manihot esculenta), yam (Dioscrorea rotundata), and cocoyam (Xantosoma sagit!folium) in a field is an example. The relay intercropping involves planting a second crop after the first crop has entered the reproductive phase but prior to harvest. The row intercropping is characterized by special arrangement of crops in rows of alternating crop types

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Photo 3:Alley farming system. (Sanchez 1976). Under this system, the soil is continuously protected and residues from various harvests are added to the soil. The native soil organic matter is conserved by the provision of regular crop residues but the amount of added residues cannot ensure sufficiently high soil organic matter levels for sustained crop production (Agboola 1982).

4.4

Crop rotation and fallowing

Crop rotation refers to the yearly sequential combination of crops grown on a piece of land. Crops producing large quantities of residue with high C:N ratios favour humus build up in the soil than those producing less residue with low C:N ratios. Soils under suitable rotation have better humus content due to better growth and consequently higher biomass yields

which are subsequently returned to the soil and this is confirmed by Lal (1976), who observed significant differences in soil organic matter contents in soil under different cropping sequences and combination, as soil organic C levels were higher under maize than under cowpea and soyabean. After cultivating a piece of land for a couple of years, the land is abandoned for some years to regain its lost fertility. This is referred to as fallowing. Different types of crops with differing root systems, exploit nutrients from different soil horizons. Nutrient recycling under the new plant species quickly uplifts the nutrient status of the previously exhausted land. Soil organic matter is usually higher under fallow than under cultivation. The level of organic matter depends on the length of cultivation/fallow period, since the annual increase in humus during the

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natural fallowing phase depends in annual amount contributed by litter and root residue minus losses through mineralisation and leaching (Nye 1958). Soil organic matter of fallow land is usually found to be more than that of cultivated soil even when the residue is returned to the soil at harvest (Aina 1979), and a short cultivation period followed by a long natural fallow period is known to be a sound way of maintaining soil organic matter (IITA 1979). The biomass produced by grasses are appreciably greater than natural regrowth. Grasses are, therefore, more efficient in soil fertility restoration and soil organic matter accretion. Compared with weed or natural fallow, soil organic C content was significantly higher under Cyanodon than under Stylosanthes. The percentage increase ranged between 31% for psophocarpus to 3.0% for weed fallow. High soil organic C build-up was also observed for soils planted to Panicum maximum, Setaria sp. and Melinis sp. than natural bush regrowth (Agboola 1982, Feller et al. 1987).

4.5

Tillage practices and soil organic matter

In the tropics, there are many tillage implements and practices for seedbed preparation, weed control, fertilizer and herbicide/insecticide application. Tillage practices have significant effects on soil organic matter. Frequently plouglaed lands have lower soil organic matter contents than less frequently ploughed or untilled land, because ploughing encourages rapid oxidation as a result of increase in aeration and biotic activity in ploughed land. Consequently frequently ploughed land is more prone to erosion and loss of soil organic matter and fine SOIL TECFINOLOGY--A cooperating Journal of CATENA

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soil fractions (Aina 1979). In ploughed lands, subsoil may contain more organic matter because of soil inversion and burying of crop residue during mouldboard ploughing. In notillage system, earthworm activity is substantially high and root activity is confined to surface horizons and the roots contribute to increasing soil organic matter status in this layer. Consequently, Lal (1981 b) found mechanical tillage to decrease soil organic matter, as untilled soil had 50% more humus than tilled soil. Climatic conditions, especially rainfall amount and distribution dictate the effect of tillage on soil organic matter levels. In the forest region of Nigeria, for example, no-tillage system with crop residue mulch has been shown to be better suited for maintenance of favourable soil organic matter levels. In this region, soil under no-tillage maintained higher soil organic matter contents than where residue was ploughed in. The rate of soil organic matter decline was 0.129% per year in no-tilled lots compared with 0.155% per year in ploughed treatments (Lal & Kang 1982).

The appropriate cultural operation and cropping systems for sustaining soil organic matter; and future research needs As a general rule, the most appropriate tillage operation and or cropping system for sustaining soil organic matter level under intensive land use in the low land humid tropics are those that ensure returns of adequate quantities of plant residues to the soil. Practices that can reduce the high rate of crop residue decomposition would also be appropriate

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a n d useful for soil o r g a n i c m a t t e r conservation. In effect, soil o r g a n i c m a t t e r c o n t e n t o f soils o f the tropics could be i m p r o v e d by practices such as residue m u l c h i n g , notillage farming, alley cropping, fallowing, a n d crop r o t a t i o n with a p p r o p r i a t e l y selected p l a n t species. L a n d clearing with heavy m a c h i n e r y and conventional double ploughing and h a r r o w i n g , leads to accelerated r e m o v a l o f p l a n t residues by o x i d a t i o n or physical t r a n s p o r t a t i o n . These practices s h o u l d be discouraged. F u t u r e research s h o u l d address m e a n s o f m i t i g a t i n g the intensity o f m a n u a l l a b o u r needs associated with m a n u a l l a n d clearing. Light weight items o f e q u i p m e n t , tools a n d m a c h i n e s should be d e v e l o p e d for l a n d clearing in the low land h u m i d tropics. T h e m a c h i n e s s h o u l d be such t h a t would n o t r e m o v e cut veget a t i o n out o f the field. T h e t h e r m o c h e m ical p a t h w a y s o f o r g a n i c m a t t e r decomposition and factors/enzymes/organisms associated with the rapid stepwise sequences, need be closely m o n i t o r e d . This might provide clues to possible interventions a i m e d at slowing d o w n d e c o m p o s i tion rates. M o r e work is needed on the develo p m e n t a n d refinement of alley cropping system to include the a p p r o p r i a t e a g r o f o r e s t r y species suitable a n d effective in accreting a n d stabilising soil o r g a n i c m a t t e r in the low l a n d h u m i d tropics.

Acknowledgement The a u t h o r s t h a n k f u l l y a c k n o w l e d g e the suggestion a n d c o n t r i b u t i o n o f Dr. B.T. K a n g (IITA I b a d a n ) in the p r e p a r a t i o n o f this paper.

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Address of authors: S.A. Ayanla|a J.O. Sanwo Ogun State University Ago-lwoye Nigeria

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