Equations for calculating N-fertilizer rates for Khaw Dauk Mali-105 rice from soil analysis

Methods studied were: (1) measuring soil organic matter by Walkley and Black's method, (2) measuring total soil N by Kjeldahl's method, (3) extracting soil N with acidified K2Cr2O7 solution, (4) extracting soil N with basified KMnO4 solution, (5) extracting soil N with acidified KMnO4 solution, (6) extracting soil N with solution of CaCl2 and K2SO4, (7) extracting soil nitrate according to Keeney and Nelson (1982) and then measuring the extracted nitrate by Kjeldahl distillation, (8) extracting mineral N in soils with 2NKCl followed by distillation of NH3 with MgO and Devarda alloy, (9) measuring NH4-N production from incubation of soils under waterlogged conditions for 7 days, and (10) measuring NH4-N production from incubation of soils under waterlogged conditions for 14 days. Only the indices from the methods (9) and (10) gave significant relationships (at 95 percent confidence level) with the relative paddy yields, with Method (10) showing slight superiority over the method (9). None of the chemical methods gave significant relationships among the index and the relative dry matter and amount of N in plants. The equations for calculating rates of N fertilizer required for desired paddy yields were: (a) log (100-y)=2-0.0226b-0.0374x for method (9) and (b) log (100-y)=2-0.00533b-0.0584x for method (10); where is the desired grain yield (as percent of maximum yield), b availability index value for soil N (in ppm N), and x rate of fertilizer N required (as kg N/rai, 6.25 rais=1 ha). Both of the two equations gave highly significant correlation between the actual paddy yields and the predicted paddy yields. However, method (10) was more recommended than method (9) for it was more reliable than method (9) in prediction of the yield.