Crops, plants breeding and seed production

The most important factor determining the type and abundance of vegetation in Africa is the amount and distribution of rainfall. The table shows the grain yield potentials of various rainfall zones in the tropics, assuming moderate phosphate and nitrogen fertilities, and no serious yield loss from pests and diseases. Moisture levels in the lowest rainfall zone shown in the first line of the table impose an upper limit on yields. Perhaps we my hope to make some progress in obtaining rather better yield stability by using drought enduring or drought avoiding crops: but even if a 20 percent mean yield increase ware to be obtained by this means, that would be worth only 260 kg. of grain per hectare. Further, such an improvement is likely to make little impact on the frequency of crop failures. Move to the next rainfall belt, and a 20 percent mean yield increase is worth 600 kg/ha of grain: and that might well lead to a slight reduction in the frequency of crop failure. Taking the whole range of the table, an average yield increase of 20 percent in the Higgins & Kassam^ have calculated the potential population that can be supported in Africa based on three levels of inputs - low, medium and high, taking soils and climate into account.