Variable rate fertilizer application for corn and soybean

Some have argued that variable rate application (VRT) of fertilizer is appropriate and profitable for the highly productive and fertile fields of the central cornbelt, but the proposition has not been tested. In this field experiment treatments of variably and uniformly applied phosphorus (P) and potassium (K) fertilizer and a control treatment with no P or K fertilizer were applied in a randomized complete block design at six locations in central Illinois in 1994. Individual experimental units were from 1,000 to 1,500 m in length and about 20 m in width. Fertilizer application rate was based upon a 100 m grid soil sampling. The variable rate treatment rate for each plot was based upon interpolated values of the soil fertility of each portion of the experimental unit. Uniform rate treatments were based upon the median soil fertility value for the entire field. Yield estimates were obtained with grain yield monitors. Neither corn nor soybean yield was significantly affected by fertilizer application treatment. We propose this lack of an effect is due to insurance built into the current fertility recommendations. A response to variable treatments will not be seen if there is no response to fertilization. Variable rate application will probably be most useful in fields with areas of very low soil tests.