Abstract
The nitrogen cycle in soil is an integral part of the overall cycle of N in nature. The primary source of N is the atmosphere where the strongly bonded gas molecule N is predominately gas (78.08%). Total N content in soils ranges from 0.02% in the subsoil and more than 2.5% in peats; ploughed layers of most cultivated soils contain between 0.06 and 0.5%. The amount present in each case is, however determined by climate, type of vegetation, topography, parent material and activities of man. Over 95% of TN in surface soils is organically bound while the portion of non-exchangeable N is high in subsurface soil. Knowledge concerning the nature of organic N in soils is based on studies involving identification and estimation of N forms released by treatment with hot acids. Organic N forms were determined in three soils by acid hydrolysis. The total hydrolysable organic N for the 0-15 and 15-30 cm layers were 57.2 and 59.3% for Gituamba andosols; 56.9 and 61.9 for Kitale ferralsols; 39.0 and 42.1% for Katumani luvisols, respectively. Amide N ranged from 11.6 to 21.4% of total N; Hexosamine from 5.2 to 10.1% and Amino acid N from 26.2 to 37.1 %. Amino acid N therefore formed the highest portion followed by Amide N of the hydrolysable organic N.
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