Non-symbiotic Nitrogen Fixation and Soil Fertility

Abstract

Soils in the semi-arid tropics (SAn are low in nitrogen and yet they continue to support most of the world's.milletsand sorghum production with virtually no addition df fertiliser. Legumes seem to play only a small role in the maintenance of soil fertility under the present management and cropping patterns. High levels of N can accumulate in soil under certain fallows planted to grasses such as Andropogon goyonus. Mineralisation mtes of 4 to 5 % per,annum provide up to 70 kg N I ha per annum as nitrate, some of which may leach to soil depths of I to 2 m. Nitrogen removal in pearl millet or sorghum crops in such soils can beas much as 150 kg N/ha, depending on the cultivar and the season. Since inputs from other sources such as rainfall are small, the maintenance of equilibrium levels of soil N depends on biological N2 fixation. Bacteria associated with the root systems of sorghum, millets and some tropical grasses are involved, as well as blue-green algal crusts. Long-term N balances for crop production, although difficult to measure,