Soil Nutrient Management for Sustained Food Crop Production in Upland Farming Systems in the Tropics

Lloyd R. Hossner and Anthony S.R. Juo
Soil and Crop Sciences Department
College Station, Tennessee, 77843, USA, 1999-05-01

Abstracts in Other Languages: 中文, 日本語, 한국어

Introduction

The decline of food production in upland farming systems in the tropics has often been attributed to the lack of adoption of modern farming technologies. The limitation seems to be the farmer's inability to replenish nutrients lost in the continuous cultivation which has replaced the traditional bush fallow system.

"Slash and burn" agriculture remains the major food production system in the tropics. However, where fallow periods have been shortened below a critical level, the system can no longer sustain crop yield due to a decline in soil fertility. During the past few decades, efforts to improve food crop production in the tropics follow one of three models. These are:

• The increase in crop yield of irrigated rice and wheat on fertile alluvial soils through the use of high yielding varieties and chemical inputs - the Green Revolution of wheat and rice in Asia,
• The development of large-scale mechanized monoculture on well-structured upland soils _ maize production in the Tropical Highlands of East Africa and soybean production in the Cerrado Oxisols of Brazil, and
• Modification of traditional bush fallow on sandy and kaolinitic soils in the humid tropics, including agroforestry and judicious use of chemical fertilizers _ the low-input cropping systems of sub-Saharan Africa and the Amazon basin.

Much has been written about the agronomic and social impacts of the Green Revolution on irrigated wheat and rice production in tropical Asia. The successes and failures in developing large-scale monocropping in the tropics have also received much attention. In this Bulletin, we attempt to highlight recent developments in soil fertility management for sustained crop production on the widespread kaolinitic and sandy soils in the humid and subhumid tropics, with special reference to Tropical Africa.

Climate and Soils

Tropical agricultural regions comprise several ecological zones, which include wooded and grass savanna (or subhumid) zones and tropical forest (or humid) zones. Because of the cooler climate and the more fertile volcanic soils, tropical highlands are generally densely populated and intensively cultivated.

The growing season in rainfed farms in the semi-arid tropics is determined by the length of the rainy season. The semi-arid zone receives 200 to 700 mm of annual rainfall (Hance 1975, Grove 1978, Mughogho et al. 1986). Rainfall is characterized by high intensity, short duration, and large year-to-year variations in total rainfall (Sivakumar 1987). The subhumid or savanna zone receives approximately 700 to 1300 mm of rain during the rainy season, which spans five to eight months. The humid or forest zone has an annual rainfall that exceeds 1300 mm. The length of the rainy season in the humid zone can be eight months or more, allowing two successive crops per year.

Upland soil types in tropical regions are dominated by kaolinitic Alfisols, Ultisols and Oxisols with low effective cation exchange capacity, i.e. less than 12 meq/100g of clay (Sanchez 1976, Juo and Wilding 1996). With the exception of more productive volcanic soils in the highlands, prospects for increasing food production in sub-Saharan Africa is restricted by poor soil resources (Greenland 1981, Wilding and Hossner 1989, Juo and Wilding 1996). A unique feature of these soils is the small and easily consumed mineral nutrient pool associated with these soils under continuous cropping.

The semi-arid zone is characterized by sandy Entisols and Alfisols that are weakly structured. These soils have very low soil organic matter content (generally less than 0.3%), low water holding capacity, low nutrient content (CEC of surface soils rarely exceeds 3.0 meq/100g or cmol/kg), and are prone to water and wind erosion (Deckers 1993, Sivakumar et al. 1992). The subhumid savanna and the savanna-forest transition zones are dominated by the relatively more fertile kaolinitic Alfisols. Soil erosion and compaction are major management constraints (Lal 1993). In the humid forest zone, major upland soils are strongly weathered, kaolintic Ultisols and Oxisols (van Wambeke 1991, Juo and Wilding 1996). Both Ultisols and Oxisols are acidic, and thus contain very low levels of mineral nutrients (i.e. Ca, Mg, K, and P).

Cropping Systems

Major food crops in humid tropical regions include: plantain, banana, rice and root crops (such as cassava, yam, sweet potato and cocoyam) in the humid zone; sorghum, maize and cowpea in the subhumid zone; and millet and cowpea in the semi-arid zone (Okigbo 1980, Mudahar 1986).

Traditional cropping systems vary, since they have evolved in response to prevailing soil and climatic conditions and social and ethnological preferences (Ruthenberg 1976, Okigbo 1980, Kang 1986). Traditional farmers often plant more than one crop species in a small patch of cleared and burnt land after several years of bush fallow. Intercropping, the practice of growing two or more crops simultaneously in the same field, is common throughout the tropics. It is practiced in 80% of the cultivated areas in West Africa (Steiner 1984). The multi-story homestead gardens, where more than three annual crop and vegetable species are mixed with tree crops, are common in the humid forest regions (Juo 1989).

Rainfall distribution and solar radiation in the Savannah regions are better suited for a wider range of rainfed agriculture than the forest or semiarid zones. Most of the sorghum, millet, maize, cowpea, groundnuts and yams are produced on high base-status soils. In the humid region, which is dominated by low-base-status and acid Ultisols and Oxisols, systems based on trees, shrubs and root crops are more stable than cereal crop systems, as shown by the existence of highly successful tree crop plantations of rubber and oil palm (Sanchez 1976, Kang 1986, van Wambeke 1991). Systems based on cassava and plantain are prevalent in the humid region, which is dominated by acid and low-base-status soils. Systems based on maize and millet are more common in high-base-status soils in subhumid and savanna areas (Juo and Ezumah 1992). Generally, cropping systems in tropical Africa and Latin America may be grouped into five categories:

• Cassava-based cropping systems are mainly found on sandy soils of the coastal belt (mainly Ultisols) in the humid forest region, where other food crops perform less satisfactorily except for coconuts or oil palm. Cassava is mainly intercropped with maize or upland rice. These fast-growing cereals reduce nutrient loss through leaching, runoff and erosion by utilizing a substantial amount of N mineralized (100 to 300 kg N/ha) during the onset of the rainy season (Mueller-Harvey et al. 1985). These systems also recycle nutrients by returning residues to the soil.
• Cropping systems based on plantain or starchy banana are common in forested areas. Intercropped with plantain are cocoyam, maize and beans, planted so as to maximize light use efficiency.
• Yam-based systems are traditionally intercropped with a number of food crops, including cowpea, maize, cassava, vegetables, plantains, and groundnuts. Under upland conditions, cassava is intercropped with maize or upland rice during the second year as soil nutrient levels become inadequate to support a yam crop.
• Maize-based systems are widely practiced in the humid transitional zone as well as in the subhumid region and tropical highlands. In wetter areas, maize is usually intercropped with cassava, yam or sweet potato. In the subhumid regions, it can be intercropped with cowpea or beans. Commercial maize monoculture is found on volcanic soils and on the more fertile Alfisols in highland areas.
• Cropping systems based on sorghum and millet are typical of savanna zones and semiarid regions. These cereals are commonly intercropped with groundnut (peanut), cowpea or Bambara groundnut in Africa, and with beans in Latin America. Millet/cowpea intercropping is often found on sandy soils. Sorghum/legume intercropping is usually found on finer-textured soils. In areas where rainfall is less than 600 mm/year, millet monoculture and millet/cowpea intercropping are more common.

Soil Fertility Constraints to Agricultural Production

Hanson (1992) reported that of the three billion hectares of arable land in tropical Africa, only 14.7% is considered to be free of physical or chemical constraints. One third (32.2%) has physical constraints, 13.2% has limited nutrient retention capacity, 16.9% has high soil acidity, and 6.8% has high P fixation.

Nitrogen and phosphorus are the most serious limiting factors for cereals and food legumes, respectively (Jones and Wild 1975, Christianson and Vlek 1991, Manu et al. 1991, Takow et al. 1991). Deficiencies of potassium in root crops, sulfur and zinc in maize, and boron in cotton and groundnuts have been reported in continuously cultivated fields which have few or no inputs of crop residues or animal manure (Drosdoff 1972; Jones and Wild 1975; Friessen 1991; Hanson 1992). Furthermore, aluminum toxicity and related calcium, magnesium and phosphorus deficiency also limit the growth and yield of cereals and legumes in acid soils in both humid and semiarid regions (Friessen et al. 1982, Scott-Wendt et al. 1988, Pieri 1989, Wilding and Hossner 1989).

Throughout the tropical regions in the world, the slash and burn method has been widely used by small-scale farmers as a means of land preparation and soil fertility maintenance. Practiced in different forms in different regions, slash and burn agriculture involves manually clearing, burning and cropping a relatively small area of land (e.g. 0.5 - 1 ha.) for one or two years, followed by a long period of natural fallow (e.g. 15-30 years). The land is usually allowed to return to forest or savanna vegetation, in order to restore soil fertility (Nye and Greenland 1960, Allan 1965, Ruthenberg 1976, Sanchez 1976, Mokwunye and Hammond 1992).

Where the period of fallow has been shortened and cultivation has been extended for more than two years, crop yields generally decrease rapidly, creating a constant pressure to clear new land (Ayodele 1986, Sanchez et al. 1983). Burning means that most of the N, S, and C associated with organic matter is lost to the atmosphere. Large-scale clearing accelerates soil erosion, surface sealing and crusting (Lal et al. 1986, Kooistra et al. 1990, van de Watt and Valentin 1992). Subsequent cultivation may result in rapid deterioration of the biological, chemical and physical properties of the soil (Lavelle et al. 1992, Mokwunye and Hammond 1992, Allen 1985).

Continuous cropping of Alfisols, Ultisols and Oxisols in the tropics has resulted in a rapid decline in soil organic matter in the surface soil during the first few years following land clearing (Brams 1971, Juo et al. 1995b). Continuous cultivation also causes a significant decline in soil pH and exchangeable Ca and Mg levels. This is even more pronounced when acidifying fertilizers are used (Cunningham 1963, Adepetu et al. 1979, Juo and Kang 1989, Bache and Heathcote 1969, Kang and Balasubramanian 1990, Pichot et al. 1981, Juo et al. 1995a).

Cultivated highly-weathered soils commonly suffer from multiple nutrient deficiencies, and nutrient balances are generally negative (Tandon 1993, Mokwunye et al. 1996). Soil fertility management on small farms in the tropics has become a major issue, as a result of continued land degradation and rapid population growth (FAO 1981, Swaminathan 1983, United Nations 1989). Major arable soils are often poorly suited to high-input agriculture. Agricultural development efforts, therefore, must be directed towards the improvement of productivity and sustainability of smallholder production systems.

External nutrient inputs are essential to improve and sustain crop production on these soils. Nutrient inputs may either be from organic sources (i.e. crop residues, green manure, and animal manure) or from inorganic sources (i.e. chemical fertilizers and lime). Published results have shown that chemical fertilizers alone cannot sustain crop yields on poorly buffered kaolinitic soils.

The decline of crop yields under continuous cultivation has been attributed to factors such as acidification, soil compaction and loss of soil organic matter (Juo et al. 1995a). Thus, application of organic materials is needed, not only to replenish soil nutrients but also to improve the physical, chemical, and biological properties of soil. To a large extent, this may be achieved by managing the agroecosystem in such a way that nutrient sources are generated, recycled and maintained. Economically and ecologically viable alternatives in the humid regions of Latin America have been described by Nicholaides et al. (1985). Options for soil fertility improvement in the subhumid and semiarid regions of West Africa have been discussed in a recent publication edited by Renard et al. (1997).

Prospects for Fertilizer Use

Because of scarcity and high cost, most smallholders farmers in tropical Africa and Latin America rarely use inorganic fertilizers on food crops. Moreover, many low-yielding local cultivars are naturally developed to withstand low soil fertility and other environmental stresses, and are therefore less responsive to fertilizer use (McIntire 1986). Currently, an average of only 5 - 10 kg/ha of nutrients are applied as fertilizer on cropland in sub-Saharan Africa (Bumb and Baanate 1996, Larson and Frisvold 1996). Vlek (1993) estimates that at the current rate of fertilizer use (8.5 kg/ha), the soils of the African continent are effectively being mined for their nutrients, as they have been for decades. Nutrient inputs from chemical fertilizers are needed to replace nutrients which are exported and lost during cropping, to maintain a positive nutrient balance.

Moreover, continuous use of mineral fertilizer can have detrimental effects on soil properties. In temperate regions, continuous monocropping of cereals with optimum fertilizer use can sustain crop yields on fertile soils such as Mollisols and Alfisols with high activity clays (Jenkinson 1989; Oldman and Boone 1989; Unger 1982). But on the strongly weathered, poorly buffered soils of the tropics (e.g. kaolinitic Alfisols, Ultisols and Oxisols) continuous monoculture of cereals, using chemical fertilizers as the main source of nutrients, can lead to a significant decline in yields after only a few years of cropping because of soil acidification and compaction (Kang and Juo 1986).

Integrated Nutrient Management

Sustainable soil nutrient-enhancing strategies involve the wise use and management of inorganic and organic nutrient sources in ecologically sound production systems (Janssen 1993). The primary goal of integrated nutrient management (INM) is to combine old and new methods of nutrient management into ecologically sound and economically viable farming systems that utilize available organic and inorganic sources of nutrients in a judicious and efficient way. Integrated nutrient management optimizes all aspects of nutrient cycling. It attempts to achieve tight nutrient cycling with synchrony between nutrient demand by the crop and nutrient release in the soil, while minimizing losses through leaching, runoff, volatilization and immobilization.

Perhaps the central concept of integrated nutrient management in the tropics can be illustrated by the results of an earlier field experiment conducted by Fore and Okigbo (1974), who attempted to grow a high-yielding maize cultivar on a strongly acidic Ultisol (pH 4.6) in the subhumid region of eastern Nigeria (Table 1). Their results clearly demonstrated the need for balanced nutrient inputs from both organic and inorganic sources on this acidic and poorly buffered soil in a tropical environment.

Management of Crop Residues

Organic nutrient sources include plant residues, leguminous cover crops, mulches, green manure, animal manure, and household wastes. Under continuous cropping, recycling and reusing nutrients from organic sources may not be sufficient to sustain crop yields. Nutrients exported from the soil through harvested biomass or lost from soil by gaseous loss, leaching, or erosion must be replaced with nutrients from external sources. The judicious use of chemical fertilizer is essential to maintain soil fertility (Moorman and Greenland 1980, Tandon 1993, Ofori 1995, Hossner and Dibb 1995).

The beneficial effects of organic matter are well known. Physically, it improves soil structure and increases water holding capacity. Chemically, it increases the capacity of the soil to buffer changes in pH, increases the cation retention capacity (CEC), reduces phosphate fixation, and serves as a reservoir of secondary nutrients and micronutrients. Biologically, organic matter is the energy source for soil fauna and microorganisms, which are the primary agents that manipulate the decomposition and release of mineral nutrients in soil ecosystems.

Organic matter in soil exists as partially decomposed plant and animal residues, living and dead microorganisms, and humidified organic matter or humus. Stable humus constitutes 50 to 75% of the total soil carbon and is little affected by management. The labile soil organic matter pool, which is important for nutrient release during the growing season, can be manipulated through various soil management practices (van Faassen and Smilde 1985). In general, more than 95% of the total N and S and up to 75% of the P in surface soils are in organic forms (Fernandes and Sanchez 1990).

Rates of decomposition of both fresh plant residues and humidifed soil organic matter are three to five times greater in the humid tropical environment than under temperate conditions (Jenkinson and Ayanaba 1977; Mueller-Harvey et al. 1985). Therefore, in cultivated fields in the humid tropics, frequent application and larger quantities of organic materials are required to maintain adequate soil organic matter levels than in temperate regions (Mueller-Harvey et al. 1985, Juo and Kang 1989, Pichot et al. 1981, Pieri 1989, Bationo et al. 1993).

Strategies and practices for soil organic matter management include:

• Returning organic materials to the soil, to replenish soil organic carbon lost through decomposition (recycling of plant and animal residues, green manuring, cover crop rotation);
• Ensuring minimum disturbance of the soil surface (residue mulch, conservation tillage) to reduce the rate of decomposition;
• Reducing soil temperature and water evaporation by mulching the soil surface with plant residues; and
• Integration of multipurpose trees and perennials into cropping systems to increase the production of organic materials.

Application of millet residues as mulch at the beginning of the rainy season in the semi-arid zone increased millet yield from 100 to 650 kg/ha on Alfisols (psammentic Haplustalf) in Niger (Bationo et al. 1987, IFDC 1986). The combination of mineral fertilizer and residues has a significant additive effect on millet grain yield. When both residues and fertilizer were applied, the yield of millet grain was three times higher than when residue alone was used, and double that of fertilizer alone. Zaongo et al. (1997) found that mulching with sorghum residues (which were saved and stored from a previous crop) reduced evaporation at Maradi, Niger by 28%. Irrigation, mulch and nitrogen fertilization as sole amendments increased sorghum biomass yield by 55, 20, and 30% respectively, compared to the control where no amendment was used.

Crop residue management in semiarid regions is an effective practice for soil fertility maintenance only if crop residues are harvested and stored for use as mulch for the cropping season in the following year. In humid and subhumid regions, where cropping seasons are separated by a relatively short dry season, the usual practice is to leave crop residues in the field after each harvest. Yield increases from the use of organic mulches were reported by Jones et al. (1960) in Kenya, Matin (1935) in Uganda, and by Sanders (1953) and Bull (1963) in East Africa. Surface mulch has been observed to increase soil pH and exchangeable bases in acidic soil cropped with tea (Smith 1962a, 1962b).

Green Manure Crops and Intercropping

Timely applications of organic materials with a low C/N ratio, such as green manure and compost, could synchronize nutrient release with plant demand and minimize the amount of inorganic fertilizer needed to sustain high crop yields for short-cycle crops such as maize, rice, and soybean, all of which have a high nutrient demand (Sanchez et al. 1989, Lathwell 1990, Burle 1992). Fast-growing leguminous species such as mucuna (Mucuna utilis) and kudzu (Pueroria phseoloides) can be especially useful as cover crops for erosion control, weed suppression and for soil fertility restoration (Wilson et al. 1982).

Leguminous green manures and cover crops are able to:

• Enrich the soil with biologically fixed N;
• Conserve and recycle soil mineral nutrients;
• Provide ground cover to minimize soil erosion, and
• Require little or no cash input.

However, additional labor is required for timely establishment, maintenance and incorporation of the green manure crop. Most leguminous crops are better suited for high base status soils (e.g. Alfisols) containing adequate available phosphorus and calcium. In the humid lowland forest zones with bimodal rainfall distribution, it is possible to intercrop a slow-growing legume (e.g. Sesbania) with a food crop (e.g. maize) in the first season, and allow full growth of the legume in the second season to be incorporated as green manure in the first season of the following year (Balasubramanian and Blaise 1993).

Although some research workers have reported evidence of direct transfer of N in a maize/cowpea intercrop (Eaglesham et al. 1982), it is believed that the N benefits are mainly for subsequent crops after roots and nodules have rotted away and fallen leaves have decomposed (Agboola and Fayemi 1972, Ledgard and Giller 1995). It is generally known that soil conditions such as P, Ca and Mo deficiencies, A1 and Mn toxicities, and drough stress are limiting factors for N fixation.

Where certain legumes are not indigenous, compatible Rhizobium strains may not be present. Inoculation with an appropriate strain of Rhizobium may be required. In sub-Saharan Africa, legumes commonly used as intercrops with maize, sorghum or millet are cowpea, common beans and groundnut. African cowpea cultivars are mostly promiscuous, and nodulate with indigenous Rhozobium strains present in the soil. For common beans and groundnuts, Rhizobium inoculates are rarely used by farmers (Kang 1986, Ofori and Stern 1987).

Farmyard Manure and Household Waste

Farmyard manure and household waste are major sources of nutrients for food crops in many parts of the tropics. Cattle dung is also a potential source of plant nutrients, but only in areas where animals are tethered or penned, so that dung can be collected. Composting is a low-cost, efficient method of processing crop residues and household wastes through biological decomposition, although extra labor is required. The use of farmyard manure or compost as a nutrient source for food crop production depends largely on the prevailing farming system (Jones 1971, Pieri 1989). In some areas of East and Southern Africa, where crop and livestock production are somewhat integrated, farmyard manure could become a major nutrient source for food crops and reduce the need for fertilizer (Peat and Brown 1962a, b, Swift et al. 1994). In humid forest zones, where livestock may be limited to a few sheep or goats in each farm compound, the use of crop residues and green manure composts, though not common at present, could become a major source of nutrient input in the more permanent cropping systems which may replace slash and burn agriculture.

In the semiarid zone of West Africa, composting is not a common practice because of the lack of water and of crop residues (Poulain 1980). The use of animal manure and household wastes are limited to areas near the farm compound. Another system of manure utilization in West Africa is known as Kraaling. In this system, farmers invite nomadic Fulani herders to graze on their croplands during the dry season. Cattle are confined in a designated field during the night, to ensure a concentrated application of manure and urine. Generally, manure scattered when animals are grazing is insignificant for improving soil fertility (Taylor-Power 1991).

Chemical Fertilizers and Soil Amendments

Judicious use (i.e. lower rates, split application, banding) of inorganic fertilizers is needed on infertile kaolinitic and oxidic soils, to sustain high crop yields and maintain an optimum balance of nutrients. Continuous use of relatively high rates of nitrogen fertilizers on kaolinitic Alfisols, especially under cereal monoculture, can reduce soil pH (acidification) and seriously reduce soil fertility (Jones 1976, Nnadi and Arora 1985, Pieri 1989, Mokwunye and Hammond 1992, Juo et al. 1995a).

Acidification occurs mainly through the loss of exchangeable bases in leaching (Ca, Mg, K) and acid production during A1 hydrolysis and nitrification. For example, in a long-term experiment conducted on a Kaolinitic Alfisol in Nigeria, Juo et al. (1995b) reported that the rates of decline in soil pH and exchangeable Mg under three cropping systems with application of chemical fertilizers (NPK) were: continuous maize with NPK without residues > continuous maize with residue mulch > maize/cassava intercropping.

Without a residue mulch, soil pH (measured in water) dropped from 6.0 to about 4.5 after ten years (Fig. 1). Exchangeable Mg declined from 1.0 meq/100g to about 0.2 meq/100g after ten years of cropping (Fig. 2).

While fertilizer is needed to maintain soil productivity, it must always be used in conjunction with management practices that help maintain soil organic matter, such as return of residues or other organic materials to the soil, and minimum tillage. The use of N fertilizer was shown to increase water use efficiency of millet and sorghum in the semi-arid tropics (Sivakumar 1987, Payne et al. 1990, 1991, Zaongo et al. 1997). Fertilizer management, especially the type of nutrient and the application rate, is best based on site-specific experiments and farmers' experience.

Vast areas of arable land in the humid tropics are strongly acidic Ultisols and Oxisols (i.e. pH < 5.2 measured in water), with a high degree of exchangeable Al saturation (i.e. > 40% of effective CEC). Acid-tolerant root crops such as cassava and sweet potato are major food crops in these regions. For the cultivation of crops such as maize and beans, liming is needed not only to correct Al and/or Mn toxicity, but also to supply Ca and Mg as plant nutrients.

An important criterion for determining lime requirements is that acid tropical soils should reach a soil pH value of about 5.5, or to have the desired value of exchangeable Al saturation for a particular crop to be grown (Kamprath 1980).

The relationship between soil pH and the percentage of exchangeable Al saturation in five Puerto Rican Ultisols and Oxisols is shown in Fig. 3. This indicates that Al saturation is less than 10% at a soil pH value of about 5.0 or above.

Crop species generally show differential tolerance to Al toxicity. Liming is usually not required for food crops such as maize and beans in soils with an exchangeable Al saturation of 25% or less. Over-liming (e.g. liming soils to pH 7 or higher) can induce P and Zn deficiencies (Lathwell 1979). Thus, the annual application of relatively small doses of lime is usually recommended. For example, on a coarse-textured kaolinitic Ultisol (pH 4.3) in the high-rainfall area of southeastern Nigeria, lime requirements for maize and cowpea rotation can be as low as 0.5 mt/ha/yr (Friessen et al. 1982). On fine-textured acidic oxisols in Brazil and Rwanda, higher rates (1-2 mt/ha) are needed to give optimum maize and bean yields (Pearson 1975, Yamoah et al. 1992).

Phosphorus deficiency is widespread throughout the tropical regions. In kaolinitic and sandy soils (Alfisols, Ultisols, Inceptisols) in sub-Saharan Africa, the application of low to moderate doses (e.g. 20 to 60 kg P₂O₅/ha) is adequate to sustain yields of maize, sorghum and cowpea (Franzluebbers et al. 1998). However, in oxidic soils (Oxisols, Alfisols derived from ferromagnesian rocks and Andisols rich in allophanes), higher rates of P application (e.g. 100 to 600 kg P₂O₅/ha) are needed, because the Fe and Al oxides and allophanes in these soils have a high capacity for P immobilization or "fixation". A long-term P placement trial with maize in Brazil showed that banding application is more effective than broadcasting on these high P-fixing soils (Fig. 4) (Lathwell 1979).

Agroforestry

Agroforestry refers to all forms of land-use systems in which trees or woody perennials are deliberately planted on the same land management unit in association with livestock and/or annual crops, with significant ecological interactions between the woody and non-woody components (ICRAF 1983). Agroforestry systems in which tree legumes are interplanted with cereals can be effective in soil nutrient cycling and enhancement. Buresh and Tian (1997) pointed out the following benefits of tree-annual crop association:

• Retrieval of nutrients from below the rooting zone of annual crops.
• Reduction of nutrient losses from leaching, runoff and erosion.
• Legume trees increase the supply of nutrients within the rooting zone of annual crops through N input by biological N₂ fixation.

Alley Cropping

Alley cropping was developed by researchers at the International Institute of Tropical Agriculture (IITA) in Nigeria, as a alternative for slash and burn agriculture on the high base status Alfisols and Inceptisols of the subhumid tropics. In this system, food crops such as maize and cowpea are grown in alleys along the contours formed by hedgerows planted three to four meters apart, of fast-growing, leguminous shrubs and trees such as Leucaena (Leucaena leucocephala).

The hedgerows are periodically pruned during the cropping season to prevent shading. Prunings are used as mulch and green manure for the associated food crop (Kang et al. 1981). The woody portion of pruned branches can also be used as fuelwood or sticks for yams. Leaves may also be used as fodder during the dry season. Annual nutrient yields of two tree legume species grown in the alley system on Alfisols in Nigeria are shown in Table 2. Lucaena prunings gave a high annual N yield of 246 kg/ha.

When trees and shrubs, with their deep root systems, are planted along contours on sloping land, they are not only able to recycle soil nutrients, but also minimize water runoff and soil erosion (Lal 1989, Hauser and Kang 1993, Juo et al. 1994). A beneficial effect of alley cropping on acidic soils is that the use of prunings as leguminous green manure can reduce soluble and exchangeable Al in the soil by forming less soluble organo-Al complexes (Hue and Amien 1989, Wong et al. 1995).

Multi-Story Homestead Gardens

Multi-story homestead gardens may be the most ecologically viable farming system for indigenous people in the humid tropics (Marten 1986, Juo 1989). They are characterized by complete internal recycling of nutrients and organic matter. Plant species are maintained in ecological balance with livestock to meet human needs on a family farm. Homestead gardens comprise a diversity of crop, animal and off-farm enterprises, all of which contribute to the income of the farm family. The area around the house or farmyard is normally planted in a wide assortment of crops that require no purchased inputs and only a low level of management. In advanced farms, there may be as many as 50 to 60 economic plant species, including five or six tall tree species and the same number of medium height trees and bush or shrub species, together with four or five root crops and up to 30 shade-tolerant, short or vine-type annual crops (Okigbo and Greenland 1976, Okigbo 1980, Marten 1986).

Acacia Albida Parkland System

The tree legume Acacia albida (Faidherbia albida), otherwise known as "the fertilizer tree', is common in farmers' fields throughout the subhumid and semiarid zones of sub-Saharan Africa. Generally, tree stands are not planted but establish themselves naturally in farmers' fields in a random pattern. Most stands are found in fields located in river valleys or inland depressions, where groundwater is available during the dry season. Acacia albida has been protected from logging by traditional rules (e.g. the Emir of Zinder in Niger) as well as by the ordinances of national governments in Africa. The crop has been maintained in an agroforestry setting for centuries (NAS 1984).

Crops such as sorghum and millet grown in association with Acacia albida grow better underneath it than when they are outside its canopy. This tree legume has the unusual habit of growing new foliage during the dry season, and losing its leaves early in the rainy season. Nutrient amounts in the leaf litter of Acacia albida directly below the tree canopy were found to be equivalent to 110 - 185 kg N/ha, 4 - 40 kg P/ha, and 220 - 275 kg Ca/ha (Weil and Mughogho 1993). Other positive attributes of Acacia albida include recycling of nutrients from the subsoil, accumulation of windblown organic residues and mineral-rich soil particles near the tree trunk, and nutrient inputs when humans and livestock cluster under the tree during the dry season (Dommergues and Ganry 1986, Dancette and Poulain 1969).

Conclusion

The levels of nutrients in major soils of the tropics are inherently low. Plant production in natural ecosystems primarily depends on nutrient recycling and biological fixation of atmospheric nitrogen. Thus, sustainable crop production on these soils must strongly emphasize organic inputs and recycling. The choice of nutrient management strategies depends not only upon environmentally suitable crops and cropping systems, but also upon the available resources at the farm (Steiner 1984, Nicholaides et al. 1985, Juo 1989, Renard et al. 1997, Franzluebbers et al. 1998).

Three agroforestry systems appear to have good potential for maintaining soil organic matter at levels adequate for sustaining crop growth in the humid forest region, where root crops and tree crops are ecologically most suitable. These include multi-story homestead gardens, plantation/crop combinations, and alley cropping. All three systems produce large amounts of biomass, which provides an effective cover to the soil surface in litter and canopy as well as biological N fixation and recycling of mineral nutrients (Juo 1989, Young 1990).

Mulches of crop residues, minimum tillage and leguminous cover crops are promising technologies for improving nutrient and water use efficiency and sustaining high yields of maize, sorghum and cowpea in the subhumid and humid/subhumid transition zones (Lal et al. 1984, Juo and Kang 1989). In these zones, climate and soil constraints for crop production are less severe than in the humid and semiarid zones. Adoption of the principles and strategies of integrated nutrient management outlined above could produce a sizable surplus of food grains on small family farms in the African savanna. Sustainability of crop production under different soil management levels may be depicted by the four conceptual models shown in Fig. 5. Organic inputs and periodic fallow are the two most important management strategies for maintaining soil fertility and sustaining crop yield in the long term.

Improvement of millet and sorghum yields in the semiarid regions is severely limited by the lack of organic inputs. Crop residues are often harvested for other uses such as building materials, fuel, or fodder during the dry season. The use of crop residues as a mulch and nutrient source depends upon the availability of alternative sources of fuel and fodder in farming communities. Thus, the potential for increasing and sustaining food crop production in the semiarid zone is limited, and will depend upon successful integration of crop, livestock and fuelwood production on the same farm or in a watershed unit (Manu et al. 1994).

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Source: Juo et al. 1995

Source: Juo et al. 1995

Source: Pearson 1975

Source: Lathwell 1979

Source: Franzluebbers et al. 1998

Index of Images

• 

  Figure 1 Changes in Soil PH (0 - 15 CM) under Continuous Cropping and Fallow Over 13 Years in a Kaolinitic Alfisol in Nigeria. (<I>B</I> Is the Slope, and *, **, *** Denote Significance at P &LT; 0.001, 0.01, and 0.1 Level, Respectively NS = Not Significant

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  Figure 2 Changes in Levels of Exchangeable MG in the Surface Soil (0 - 15 CM) under Continuous Cropping and Fallow Over 13 Years in an Kaolinitic Alfisol in Nigeria.

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  Figure 3 Relationships between Degree of A1 Saturation with PH in Several Puerto Rican Ultisols and Oxisols.

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  Figure 4 Comparative Yields of Maize Growing on a Fine-Textured Oxisol in Brazil from Banding and Broadcasting Application of P Fertilizer at the Same Rates. Average Treatments Were 320,640 and 1280 KG P<Sub>2</Sub>O<Sub>5</Sub> Per Ha for Both Placement Methods.

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  Figure 5 Conceptual Models Showing Sustainability of Crop Yields under Five Different Management Systems

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  Table 1 Effects of Nutrient Inputs on Maize Grain Yield on an Acid Ultisol (PH 4.6) in Eastern Nigeria

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  Table 2 Annual Pruning and Nutrient Yields of Two Tree Legumes Commonly Used in Alley Cropping on High Base Status Soils in the Tropics

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