Biomass production of annual crops is often directly proportional to the amounts of radiation intercepted, water transpired and nutrients taken up. In many places the amount of rainfall during the period of rapid crop growth is less than the potential rate of evaporation, so that depletion of stored soil water is commonplace. The rate of mineralization of nitrogen (N) from organic matter and the processes of nutrient loss are closely related to the availability of soil water. Results from 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 scope for encouraging canopy expansion to conserve water by reducing evaporation from the soil surface in environments where it is frequently rewetted, and where the unsaturated hydraulic conductivity of the soil is sufficient to supply water at the energy limited rate (e.g. northern Syria). In regions with high evaporative demand and coarse–textured soils (e.g. Niger), transpiration may be increased by management techniques that reduce drainage.
Increases in atmospheric [CO<sub>2</sub>] are likely to have only a small impact on crop yields when allowance is made for the interacting effects of temperature, and water and nutrient supply.
Gregory, P.J.; Simmonds, L.P.; Warren, G.P. Interactions between plant nutrients, water and carbon dioxide as factors limiting crop yields. Philosophical Transactions B: Biological Sciences (1997) 352: 987-996.