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 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.

The 2010 results of this project were impacted by a hail event during the growing season, thus yields of both crops were reduced compared to other years and caution should be used in interpreting 2010 data. Nevertheless, N alone increased corn yield by a factor of 3.3, while N and P applied together increased yield by about five fold. Averaged across the past ten years, N and P applied together increased irrigated corn yield by 140 bu/A. Application of 120 lb N/A (with P) has on average generally been sufficient to produce greater than 90% of maximum yield. Phosphorus fertilizer at the lowest P rate more than tripled corn yield with 120 lb N/A, and application of the highest P rate increased yield by a factor of 3.8. The no fertilizer treatment in the sorghum study produced 51 bu/A. Nitrogen fertilizer alone increased sorghum yield by about 25 bu/A, while N plus P increased yield by 35 bu/A. Application of 40 lb N/A (with P) was sufficient to produce about 85% of maximum yield in 2010. Potassium fertilization had no effect on sorghum yield. This is one of the few continuous, long-term crop nutrition studies in the U.S., and support is planned to continue in 2011. KS-23F