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PRODUCTION PAPER

In-Season Fertilizer Nitrogen Applications for Soybean in Minnesota

^a Dep. of Soil, Water, and Climate, Univ. of Minnesota, St. Paul, MN 55108
^b Southern Research and Outreach Center, Univ. of Minnesota, Waseca, MN 56093
^c Dep. of Agronomy and Plant Genetics, Univ. of Minnesota, St. Paul, MN 55108

* Corresponding author (mschmitt{at}soils.umn.edu)

Received for publication July 27, 2000. 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.

Abbreviations: DM, dry matter

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