Evaluation of Soil Test Methods for Available Nitrogen, Phosphorus and Potassium in Direct-Seeded Rice–Wheat Cropping Sequence

A study was carried out in an attempt to obtain simple and efficient soil test method(s) for determination of available nitrogen (N), phosphorus (P) and potassium (K) in Mollisols. Soil samples were collected before sowing of direct-seeded rice (DSR) and wheat in sequence to evaluate the soil test methods for available N, P and K. Methods used were wet-oxidation for organic carbon (OC); alkaline KMnO4 for available N; Olsen’s-P, AB-DTPA and Mehlich-I for available P; and NH4OAc-K, AB-DTPA and Mehlich-I for available K. Suitability of these methods for given soil nutrients was evaluated by comparing the R2 values (coefficient of determination) obtained from regression analysis. Results showed that the R2 values of obtained equations by using different combinations of soil test methods for the determination of available N, P and K in soil were highly significant in both the crops. Highest R2 value for DSR (0.442**) and wheat (0.898**) were observed with the combination of OC, Olsen’s-P, ABDTPA-K, and OC, Olsen’s-P, Mehlich-K, respectively. It showed that these combinations are more promising and superior over other methods.