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 N, P and K fertilization. Furrow irrigation was used through 2000, and sprinkler irrigation since that time. 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. Annual P treatments for corn were 0, 40 and 80 lb P₂O₅/A, and grain sorghum were 0 and 40 lb P₂O₅/A. The K treatments for grain sorghum were 0 and 40 lb K₂O/A.

Nitrogen applied alone in 2013 increased corn yield by up to 69 bu/A, while N and P applied together increased yield up to 150 bu/A. This is similar to the 10-year average, where N and P fertilizer together increased yields up to 150 bu/A. Application of 120 lb N/A (with P) was sufficient to produce 92% of maximum yield in 2013, which is similar to the 10-year average. The highest yield (220 bu/A) was produced with the highest N and P rates, 200 lb N and 80 lb P₂O₅/A. At this N rate, 40 lb P₂O₅ increased yield by 59 bu/A and 80 lb P₂O₅ increased yield by 81 bu/A. Nitrogen fertilizer alone increased sorghum yield by 57 bu/A, while N plus P increased yield by up to 84 bu/A. Application of 40 lb N/A (with P) was sufficient to produce about 80% of maximum yield in 2013. Potassium fertilization had no effect on sorghum yield, as has been the case throughout the study. This is one of the few continuous, long-term crop nutrition studies in the U.S., and support is planned to continue in 2014.