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

This long-term 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 N/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 inputs are critical to the optimization of irrigated corn and grain sorghum production in western Kansas. Corn yields in 2003 were higher than the 10-year average. Nitrogen alone increased corn yield in 2003 by as much as 62 bu/A, while N and P applied together increased yield by up to 120 bu/A. Historically, 160 lb N/A has been the economic optimum N rate for corn. However, only 80 lb N/A was required to obtain greater than 90% of maximum yield in 2003. Since the 2002 crop was damaged by hail, residual N likely contributed significantly to the higher yields at lower N rates in 2003. Yields overall tended to be only slightly higher with 80 compared to 40 lb P₂O₅/A. Grain sorghum yields were also higher in 2003 than the 10-year average. Nitrogen fertilizer alone increased yield by as much as 51 bu/A, while N plus P increased yield by as much as 66 bu/A. Potassium fertilization has had no effect on sorghum yield over the course of the study. This is one of the few crop nutrition studies in the U.S. that has spanned several decades. The results of long-term experiments that evaluate the effects of fertilization and nutrient interactions are becoming increasingly valuable as nutrient use comes under greater scrutiny. Since this project continues to generate excellent long-term yield response and environmental data, support will continue in 2004. KS-23F