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 the response of irrigated continuous corn and grain sorghum to nitrogen (N), phosphorus (P), and potassium (K) fertilization. It remains one of the few continuous, long-term crop nutrition studies in the U.S.A. For both crops, N treatments were 0, 40, 80, 120, 160, and 200 lb N/A and P treatments 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.

Grain yields were 25% and 8% lower for corn and sorghum, respectively, in 2017 compared to the 10-year average. Nitrogen applied alone increased corn yield by 70 bu/A, while N and P applied together increased yield up to 130 bu/A (compared with unfertilized control). Phosphorus alone increased yield by less than 10 bu/A. Maximum yield for corn (174 bu/A) was obtained with the addition of 200 lb N and 80 lb P₂O₅/A. At this same N rate, 40 lb P₂O₅ increased yield by 34 bu/A over no P application (P control), while 80 lb P₂O₅ increased yield by 60 bu/A. The addition of P (either 40 or 80 lb P₂O₅/A) almost doubled apparent fertilizer N recovery in grain (from 31 to 56%) over the zero-P control and averaged across all N rates. At the highest fertilizer rates, the apparent fertilizer N recovery was 42%, whereas the apparent fertilizer P recovery in the grain was 61%.

Nitrogen fertilizer alone increased sorghum yield by 53 bu/A, while N plus P increased yield by up to 67 bu/A. Phosphorus alone increased sorghum yield by 9 bu/Ac, and P plus K increased yields by 10 bu/ac. The application of K had no effect on sorghum yield in 2017 nor throughout the study period. Maximum sorghum yield for corn (137 bu/A) was obtained with the addition of 160 lb N, 40 lb P₂O₅/A, and no K. At this N rate, 40 lb P₂O₅/A without K increased yields by 17 bu/A. The addition of 40 lb P₂O₅/A fertilizer also increased the apparent fertilizer N recovery in grain from 36% (zero-P control) to 50% when averaged across all N rates. The addition of 160 lb N/A increased the apparent recovery of P fertilizer over the control from 18 to 70%. At the highest N, P, and K rate, apparent fertilizer recovery in the grain was 32% for N, 66% for P, and 39% for K.