INTERACTIONS BETWEEN PLANT NUTRIENTS, WATER AND CARBON-DIOXIDE AS FACTORS LIMITING CROP YIELDS

Biomass production of annual crops is often directly proportional to t he amounts of radiation intercepted, water transpired and nutrients ta ken up. In many places the amount of rainfall during the period of rap id crop growth is less than the potential rate of evaporation, so that depletion of stored soil water is commonplace. The rate of mineraliza tion of nitrogen (N) from organic matter and the processes of nutrient loss are closely related to the availability of soil water. Results f rom Kenya indicate the rapid changes in nitrate availability following rain. Nutrient supply has a large effect on the quantity of radiation intercepted and hence, biomass production. There is considerable scop e for encouraging canopy expansion to conserve water by reducing evapo ration from the soil surface in environments where it is frequently re wetted, and where the unsaturated hydraulic conductivity of the soil i s sufficient to supply water at the energy limited rate (e.g. northern Syria). In regions with high evaporative demand and coarse-textured s oils (e.g. Niger), transpiration may be increased by management techni ques that reduce drainage. Increases in atmospheric [CO2] are likely t o have only a small impact on crop yields when allowance is made for t he interacting effects of temperature, and water and nutrient supply.