SOIL PHOSPHORUS TEST METHODS CALIBRATION BY FIELD EXPERIMENTAL DATA FOR THE SOILS OF LOW COUNTRY INTERMEDIATE ZONE OF SRI LANKA

Green house studies showed that the 2.5 % Acetic acid (HAc), Olsen's bicarbonate (0.5 M NaHC03 ) , Anion Exchange Resin (AER) and Anion Exchange Resin + Cation Exchange Resin (AER + CER) methods were satisfactory for the estimation of plant available phosphorus (P) of soils of the Low Country Intermediate Zone of Sri Lanka. The objective of this study was to calibrate the four extraction methods by field experiments. Relative yield of Pueraria phaseoloides grown in field on five soil series treated with and without P fertilizer, but with basal treatment of nitrogen, potassium and magnesium fertilizer were employed as the plant index for soil P status. Soil P status of each soil prior to treatment was determined by the four extraction methods. The 2.5 % HAc - P values showed the best correlation with relative dry matter yield of Pueraria by fitting significantly to the Cate and Nelson model with R2 of 0.628 (P<0.01). Both AER - P and AER + CER- P values showed the poor correlation with R2 of 0.460 (P<0.05) compared to HAc - P. NaHC03 -P showed poorest correlation with R2 of 0.449 (P<0.05). The 2.5 % HAc - P was found to be superior to others as it accounted for 62.8 % of the variation of relative yield against 45 - 46 % by the other extractants. Threshold P levels as determined by the correlation data were 7.8, 3.84, 5.34 and 8.47 mg/kg for 2.5 % HAc - P, AER - P, (AER + CER) - P and NaHC03 - P respectively. The 2.5 % HAc method was found to be the most suitable P extraction method for Low Country Intermediate Zone of Sri Lanka