NITROGEN STATUS AND ITS SUPPLYING CAPACITY OF TROPICAL SOILS

Total Nitrogen content in tropical soils is generally low compared to the mosttemperate soils. About 98% total nitrogen is associated with soil organic matterwhich in turn subjects to mineralization and hence releases mineral nitrogen to thesoil. Thereby most tropical soils nitrogen supplying capacity or nitrogenmineralization potential is relatively low and declines rapidly during croppingseason unless nitrogen sources are incorporated frequently. This investigationconducted to find out the nitrogen status, distribution of different nitrogenfractions and the mineralization potential of some selected agriculturallyimportant soils of Sri Lanka.Sixteen soils collected from various locations representing different soil typeswere analysed for total nitrogen. Eight soils from sixteen soils were fractionatedand ana lysed for different nitrogen forms viz; available N, ammonia-N, aminosugar-N, amino acid-N, acid hydrolyzable unknown N and acid insoluble-N afteracid hydrolysis. In a laboratory incubation experiment, those eight soils weresubjected to follow the nitrogen mineralization potential and to determine themineralization rates according to the method described by Stanford and Smith(1972). The nitrogen mineralization data during 315 days of inclubation wereused in Stanford and Smith's first order decay model to calculate nitrogenmineralization rate(k) and nitrogen mineralization potential (No).Total nitrogen contents in the studied soils varied from 89.36 (Aralaganwila) to365 mg/l00 g soil (NuwaraEliya). Soil organic matter contents also gave similarvariation and showed close relationship with total soil nitrogen. The distributionof different forms of soil nitrogen also varied among the soils studied. A majorpart of organic nitrogen in Sri Lankan soils was found to be associated with aminoacid-N fraction followed by the ammonia-N fraction. This was consistent withthe other studies conducted in elsewhere (Stevenson, 1957; 1982)Nitrogen mineralization rate was ranged from 0.0069 to 0.026 N mg/lOO soil perday. Highest mineralization rate was given by two paddy soils (Kiribathkumburaand Pilimathalawa). Nitrogen mineralizations potential or nitrogen supplyingpower also varied from 5.33 to 38.52 N mg/lOO g soil. High mineralizationpotential was also recorded in paddy soils. This may be attributed to the highorganic matter contents in those soils. Further these different nitrogen supplyingpowers of soils reflect their inherit characteristics or potential in supplyingavailable nitrogen for plant usage. One should give serious consideration to thoseparameters of soil nitrogen when nitrogen fertilizer recommendation practices areconducted for crops to attain optimum yield.