ABSTRACT

Soil contains significant reserves of elements but in general they are not directly assimilated by plants. For example, one hectare of land contains 2 to 20 tons of nitrogen (Bockman et a\., 1990) but only a part of it is liberated through biological activity and chemical processes in the soil. Harvesting of crops reduces the nitrogen content in soil and to compensate for this loss, mineral, nitric, or ammoniacal nitrogenous fertilisers are added or, more commonly, a combination (100 to 250 kg nitrogen per hectare).