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 f or soybean [Glycine max. (L.) Merrill]. The objective of our research was t o evaluate several in-season N fertilization strategies on soybean seed yie ld response as well as to measure the effect of fertilizer N additions on l ate-season plant N concentrations and accumulation, seed N removal, seed pr otein, and seed oil composition. The research was conducted at 12 sites in the southern soybean-growing region of Minnesota in 1998 and 1999. A combin ation of (i) application time (July vs. August), (li) placement method (bro adcast vs. knifed), and (iii) N source (urea vs. poly-coated urea) gave fiv e N treatments plus a control at all sites. Seed yield did not respond to t he fertilizer N treatments at an), of the 12 sites; however, a combined ana lysis indicated a significant increase (generally less than 0.06 Mg ha(-1)) from using polymer-coated urea or applying the area in August. Herbage dry matter (DM) and herbage N concentrations at the R6 stage is ere not affect ed 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 treat ments. In general, polymer-coated area, knifed applications, and August app lications increased soil NO3-N in the 0- to 30-cm layer at R6 relative to s tandard area, broadcast applications, and July applications. Even though in -season N fertilizer created greater levels of available soil N at all 12 s ites during soybean pod filling, seed yield was not improved compared with unfertilized control plots at any, site. As a result, the University of Min nesota does not recommended in-season N fertilizer applications for soybean production.