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

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

Nitrogen and P fertilization increased corn yields more than 100 bu/A in 1998. Although there are some year to year variations, the optimum N rate (with P) for corn remained relatively constant at about 160 lb N/A. Fertilizer P at 40 lb P₂O₅/A increased corn yields by 75 bu/A when applied with at least 120 lb N/A. In 1998 grain sorghum yields were increased over 40 bu/A by N and about 20 bu/A by P fertilization. In sorghum production, the optimal N rate has varied considerably from year-to-year. However, application of 80 lb N/A is sufficient to maximize yields in most years. Potassium fertilization had no effect on grain sorghum yields. Phosphorous fertilization has reduced soil nitrate levels due to increased N use efficiency, thus, reducing the likelihood of nitrate leaching into groundwater. Long-term studies such as this are important in developing an understanding of trends in crop response and the economics of fertilization. KS-23F