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

Long term research conducted since 1962 has shown that P and N fertilizer must be applied for optimum grain yields and profitability of irrigated corn and grain sorghum in western Kansas. The objectives of this study were to determine the optimum rate of N, P, and K fertilizer for irrigated corn and grain sorghum.

Various levels of N fertilizer ranging from 0 to 200 lb/A in 40 lb increments were applied to corn and grain sorghum with 40 lb P₂O₅/A and without P fertilizer. A K treatment was included in the grain sorghum experiment. Both studies were damaged by hail in July. The corn plots did not recover from the hail damage; therefore, no results were reported from this part of the experiment in 1999. Long term analyses of corn yield data from 1962 to 1992 have shown that the optimum N rate is about 160 lb/A, and at this rate, 40 lb P₂O₅/A increased yield by an average of 55 bu/A and profit by well over $100/A. Grain sorghum yields were variable in 1999, possibly because of the hail damage in early July. Phosphorus increased yields by an average of 20 bu/A across all N rates. This is similar to yield responses in earlier years. Grain yields were not increased by N rates above 40 lb/A when applied with P, although typically 80 to 120 lb N/A are needed to maximize yields. There was no grain sorghum yield response to K applications. KS-23F