NPK fertilizer management for corn: Decision aids and test kits

Simple and rapid soil test for NPK is being developed for use in the field. Correlation studies were undertaken to select a single extraction solution for NPK determination in soils for routine analysis. A highly significant correlation between Mehlich 1 extractable ammonium+nitrate and corn dry wight (CDW) was obtained (r values of 0.74**). The highly correlation between ammonium+nitrate with uptake with r values of 0.77** was also observed. The highest, signifiant correlations were between extractable K and both CDW and K uptake using Mehlich 1 was selected for the extraction of the three nutrient elements (N, P and K). The colorimetric determination of ammonium, nitrate, and phosphorus by spectrophotometer were modified and developed for use with a standard color chart. Suitable correlations were obtained between spectrophotometer and color chart for the NPK determination. The process of soil analysis was simplified and developed to be soil test kit. In the field experments, the actual and predicted yield of corn (Zea mays, L.) in seven soil series across four provinces was compared. The N fertilizer quantity was generated by the CERES-Maize model while P and K were calculatd by a Mitscherlich-Bray equation. An agreement index of 0.86 between actual and predicted yield was obtained, indicating excellent correspondence. The PDSS (Phosphorus Decision Support System) was used to predict the P requirement and was tested with the 15 plots of the farmers field in the 5 provinces of corn production area and the four on-farm experiments in Nakorn Ratchasima province. The amount of fertilizer P predicted by PDSS was one-half of the amount obtained by calculation of Mitcherlich-Bray equation. The yield of the CERES-Maize-PDSS prediction was higher than with the farmer's practice. The economic analysis indicated that the CERES-MB and CERES-PDSS treatments gave the higher benefit compared to the farmer's practice.