Equations for Calculating N-Fertilizer Rates for Khaw Dauk Mali-105 Rice from Soil Analysis

Abstract

          Comparisons were made to assess the reliability of 10 chemical methods for evaluating the availability of N in soils to Khaw Dauk Mali-105 rice and for calculating rates of N-fertilizer for rice. The 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 2N KCl 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. The study was made with field-plot experiments at 18 sites inside and outside the Khaw Dauk Mali-105 producing areas.

             Only the indices from the methods (9) and (10) gave significant relationships (at 95% 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 y is the desired grain yield (as % 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.