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. Furrow irrigation was used through 2000, and sprinkler irrigation since 2001. No yield benefit to corn from K fertilization was observed in the first 30 years and soil K levels remained high, thus 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 project 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 2005 were slightly less than the 10-year average due to hail damage in August. Grain sorghum yields were also reduced by hail in 2005. Nitrogen alone increased corn yield by as much as 60 bu/A, while N and P applied together increased yield by up to 142 bu/A. Historically, 160 lb N/A has been the economic optimum N rate for corn. However, 120 lb N/A was required to obtain greater than 95% of maximum yield in 2005. Phosphorus fertilizer increased yield by an average of 96 bu/A at 120 lb N/A...from 66 bu/A with the zero P control to 162 bu/A with the application of 40 lb P₂O₅/A. Corn yield tended to be only slightly higher (5 bu/A) with 80 compared to 40 lb P₂O₅/A. Nitrogen fertilizer alone increased sorghum yield by as much as 28 bu/A, while N plus P increased yield by as much as 50 bu/A. Potassium fertilization has had no effect on sorghum yield over the course of the study.

This is one of the few long-term crop nutrition studies in the U.S. The results of such experiments are becoming increasingly valuable as nutrient use comes under greater scrutiny. Support will continue in 2006 since this project continues to generate excellent long-term yield response and environmental data. KS-23F