ABSTRACT
In countries with capacity for excess food production maintenance of soil fertility is a requirement for both economic and environmental viability of the farming system, with production matched to national needs and export demands. The potential yield of food crops depends first of all on the genetic capacity of the plant. To produce these yields optimum climatic and soil conditions are required and the crop must be free from pests and diseases. Vertical bars have been marked on the right-hand side to indicate links between various factors. In soil Rhizobium bacteria exist in a free-living state and use a wide range of C compounds as food. A simple example would be differences in growth between a cold and a warm climatic region. Most agricultural soils are well supplied with P and K+. Responses in pot experiments are unlikely. Woodland soils may be suitable, but an availability index should be measured before time is spent setting up an experiment.
