In-season fertilizer nitrogen applications for soybean in Minnesota

Predicted physiological factors and a limited number of field studies have resulted in debate regarding the recommendation of in-season fertilizer N for soybean [Glycine max. (L.) Merrill]. The objective of our research was to evaluate several in-season N fertilization strategies on soybean seed yield response as well as to measure the effect of fertilizer N additions on late-season plant N concentrations and accumulation, seed N removal, seed protein, and seed oil composition. The research was conducted at 12 sites in the southern soybean-growing region of Minnesota in 1998 and 1999. A combination of (i) application time (July vs. August), (ii) placement method (broadcast vs. knifed), and (iii) N source (urea vs. poly-coated urea) gave five N treatments plus a control at all sites. Seed yield did not respond to the fertilizer N treatments at any of the 12 sites; however, a combined analysis indicated a significant increase (generally less than 0.06 Mg ha(-1)) from using polymer-coated urea or applying the urea in August. Herbage dry matter (DM) and herbage N concentrations at the R6 stage were not affected by any of the N fertilizer strategies. Although soybean seed protein was statistically different among the treatments, protein was only increased 0.4 g kg(-1). Soybean oil concentration was not affected by fertilizer treatments. In general, polymer-coated urea, knifed applications, and August applications increased soil NO3-N in the 0- to 30-cm layer at R6 relative to standard urea, broadcast applications, and July applications. Even though in-season N fertilizer created greater levels of available soil N at all 12 sites during soybean pod filling, seed yield was not improved compared with unfertilized control plots at any site. As a result, the University of Minnesota does not recommended in-season N fertilizer applications for soybean production.