Effect of Long-term Nitrogen, Phosphorus and Potassium Fertilization of Irrigated Corn and Grain Sorghum

This western Kansas study was initiated in 1961 to evaluate responses of irrigated continuous corn and grain sorghum to nitrogen (N), phosphorus (P), and potassium (K) fertilization. No yield benefit to corn from K fertilization was observed in the first 30 years and soil K levels remained high, so the K treatment in the corn study was discontinued in 1992 and replaced with a higher P rate. Nitrogen treatments for corn and grain sorghum were 0, 40, 80, 120, 160, and 200 lb/A. Phosphorus treatments for corn and grain sorghum were 0, 40, and 80 lb P₂O₅/A, and 0 and 40 lb P₂O₅/A, respectively. The K treatments for grain sorghum were 0 and 40 lb K₂O/A.

This research continues to show that P and N fertilizer must be applied to optimize production of irrigated corn and grain sorghum in western Kansas. Although adverse weather conditions in 2002 (hail, high temperatures, and low rainfall) reduced average grain yield of corn, N and P fertilization increased corn yields about 40 to 50 bu/A. The long-term average increase due to N and P is more than 100 bu/A. For the third consecutive year, corn yields tended to be greater with 80 than with 40 lb P₂O₅/A. Phosphorus improved apparent corn N use efficiency by about 38% at the optimal N rate (160 lb N/A). Both N and P fertilizers increased kernel size and number of kernels/ear, which increased ear weight. Grain sorghum yields in 2002 were greater than the long-term average, with near maximum yields obtained with 80 lb N/A when applied with P. Phosphorus increased sorghum yields by at least 20 bu/A, while K fertilization had no effect on yield. This project continues to generate excellent long-term yield response and environmental data, thus support will continue in 2003. KS-23F