Crop production is largely dependent upon the availability and use of adequate water and plant nutrients. Likewise, natural resource conservation is markedly affected by how water and nutrients are managed. Water runoff and soil erosion are major components of natural resources degradation, accounting for more than half the soil degradation worldwide (oldeman et al. 1991). Water deficiency (drought conditions) gives rise to wind erosion (28% of soil degradation) and to chemical degradation (12% soil degradation). The adequacy of vegetative cover to protect soils from erosion is determined to a considerable extent by the water and plant nutrient supply for plants.
Plant genetic research plays an important role in the management of water and plant nutrients. Enhancing the efficiency of water and plant nutrient utilization will be a challenge for genetic researchers. Tolerance to drought and toxic soil conditions is essential for both the production of crops and the residues that will be needed to protect soils. If plants do not grow on acid soils, for example, little food will be produced and there will be sparse vegetative cover to prevent soil erosion. If crop plant varieties could be developed that are tolerant to alumunium and other toxic elements in the 170 milion hectares of acid tropic soils (grimshaw et al., in press), great opportunities for increased food production would be opened up. Plants that grow well on acid alumunium-toxic tropical soils have deeper root systems (arija et al. 1992) and can utilize water from lower soil horizons.
Research to develop soil and water management systems that assure adequate vegetative cover of soils is imperative. Such cover will not only enhance the infiltration of water into the soil but also simultaneously decrease water runoff and soil erosion. This research must cover not only physical and biological issues but any socioeconomic constrains that limit the use of crop residues as soil cover.
The effect of soil erosion on crop productivity is generally greater in the tropics than in temperate zones. This is thought to be due to the greater concentration of soil organic matter in the surface horizons of tropical soils (Lal 1984). When erosion removes these upper layers, where much of the soil nutrients and water-holding capacities are found, soil productivity is negatively affected.
Research is needed to develop soil and crop management systems that maximize water use efficiency and minimize soil erosion. ICRISAT scientists have had some success in developing soil management systems for Vertisols that achieve these objectives. Recently in Mali, studies have demonstrated increased water use efficiency from ridge tillage and fertilizer application (kouyate and wendt 1992). The degree to which such results can be transferred to farmers’ fields need to be assessed.
One research area that should be more thoroughly explored is the possibility of using living grass barriers planted on the contour to diffuse the concentrated flow of water runoff from soils. Vetiver grass (vetiveria zizanioides (L) Nash) barriers have been heralded for their potential to trap sediment and thereby help create de facto bench terraces, without use of bulldozers or plows. A recent publication of the US National Academic sciences (NRC 1993) reviews the worldwide experiences in evaluating and using this hedging plant. The world bank has aggressively encouraged its use where it can have a comparative advantage. Research is needed to access not only this barrier plant, but also others that might conserve both soil and water.
Research also must be carried out to enhance the cycling of plant nutrients from the soil to plants and back to the soil. Such studies should increase the efficiency of use of native soil nutrients as well as those supplied by chemical fertilizers. The effectiveness of mychorrizal associations in enhancing nutrient availability should be further evaluated.
A major challenge, especially in Africa, is to increase the supply of plant nutrients to crop plants. Assessments must be made of systems such as alley cropping to enhance nutrient cycling and residue coverage of soils. Likewise, assessments should be made of the complementarity between organic and inorganic sources to maximize nutrient utilization efficiency. This will call for greater use of nitrogen-fixing crops.
The modeling of soil, crop, and atmospheric conditions should be further evaluated as means of better assessing options for management changes to increase productivity. Interdisciplinary approaches will be needed to carry out studies that can greatly increase the efficiency of technology dissemination from one ecoregion to another.