In-season nitrogen fertilization of soybean

Nitrogen (N) fertilization is not a traditional nutrient management practice for soybean production in Iowa. Soybean is a legume plant and is assumed to adequately obtain needed N through symbiotic fixation. However, there has been interest over the years in using N fertilization to increase yield and grain protein due to the recognition of the large N requirement associated with high yields. Depending on the soil residual inorganic N level and soil N mineralization characteristics, approximately 40 to 75% of the N in a mature soybean plant is derived from the soil (Shibles, 1998). Also, soybean seems to require this soil derived N for high yield. Nitrogen fertilization research in the upper Midwestern U.S. (residual nitrate, preplant, inseason, or foliar) has shown that while soybean yield can be increased, responses are inconsistent, vary by variety, location, pest presence, or growing condition, and rarely are economical (Oplinger and Bundy, 1998; Randall and Schmitt, 1998; Lamb, et al., 2000)