Corn and Soybean Response during 30 Years of N, P, and K Biennial Fertilization.

A balanced fertilization program is essential to optimize corn (Zea mays) and soybean [Glycine max (L.) Merr.] yields and maximize profits over time. The objective of this study was to evaluate corn and soybean yield response to biennial applications of combinations of N, P, and K fertilizer on high-testing soil under irrigation. A long-term study was established in 1972 and was continuously planted to soybean until 1982. During this period, P and K fertilizer rates were applied and soil test P and K increased to high levels. From 1983 through 2011 (28 years), a biennial fertilization treatment was established on a corn-soybean rotation with treatments applied before corn. Biennial fertilizer treatments included three N fertilizer rates, three P fertilizer rates, and two K rates in a complete factorial arrangement. The experimental design was a randomized complete block with four replications. Analysis across years showed significant yield response to N fertilizer for both corn and soybean, suggesting a possible soybean response to residual N. Corn and soybean response to P and K fertilization was inconsistent from year to year; however, analysis across years showed a significant soybean response to P fertilization, whereas corn showed no yield increase when analyzed across years. Response to K fertilization was also inconsistent and less frequent than response to P. Corn and soybean showed significant response only across years from 1983 through 1995. Despite increased yield levels in recent years, corn and soybean fertilization for P and K should be based on soil test levels. The high nutrient removal rates with high yields generate a significant decrease in soil test values over time. Biennial fertilization can be effective over time; however, application rates should be adequate for both crops to avoid decreases in soil test values.