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

This western Kansas study was initiated in 1961 to evaluate responses of continuous corn and grain sorghum grown under flood irrigation 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.
This research continues to show that P and N fertilizer must be applied to optimize production of irrigated corn and grain sorghum. Averaged across the past nine years, corn yields were increased more than 100 bu/A by N and P fertilization. In 2001, N and P increased yields up to 140 bu/A. Application of 160 lb N/A generally is sufficient to maximize corn yields. Phosphorus increased yields by 70 bu/A when applied with at least 120 lb N/A. Application of 40 lb P₂O₅/A has been adequate for corn until the past two years, when yields were increased by the higher P rate (80 lb P₂O₅/A).
Grain sorghum yields averaged across nine years were increased 45 bu/A by N and 20 bu/A by P fertilization. Application of 80 lb N/A was sufficient to maximize yields in most years. Potassium fertilization had no effect on yield. Application of 40 lb P₂O₅/A was not sufficient to maintain soil test P levels for corn but was for grain sorghum.
Phosphorus fertilization reduced accumulation of residual nitrate-N (NO₃^–-N) in sorghum by 50 percent, but had an inconsistent effect on corn. This project continues to generate excellent long-term yield response and environmental data. KS-23F