Nitrogen is one of six macro-elements that a grain crop takes up from the soil. As demand often exceeds supply, the crop usually needs nitrogen-containing fertilizer.
Leguminous crops such as soya bean are an exception; they supplement the soil nitrogen status through a symbiotic nitrogen-fixing process involving Rhizobium bacteria. These penetrate root hairs and live on the energy supplied by photosynthesis in the plant.
In turn, the bacteria fix nearly inert atmospheric nitrogen into compounds needed by the plant.
Because of this symbiosis, nitrogen fertilizer need not be added to soya bean, except on sandy soil where the residual nitrogen level is usually low. If you are growing soya bean on such soil, you can apply a small quantity of nitrogen to enhance the growth of young plants.
The Rhizobium species Bradyrhizobium japonicum is associated with soya bean. This does not occur naturally in South African soil and the soya bean seed (or the soil) must be inoculated with it.
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