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Fluid Journal : Fluid Journal 1999-2001
Spring 1999 Summary: Results from a two-year study at four irrigated sites in Kansas show that late-season application of N to soybeans at the R3 growth stage will increase soybean yields. These results, coupled with similar results in other states (the work of Flannery and Gascho), suggest that public and private labs and consultants who make fertilizer recommendations should consider N applications on irrigated soybeans with high-yield potential. Combined analysis of all sites revealed that soybean grain protein and oil concentrations were not consistently affected by late-season N fertilization. Likewise, leaf N concentration in samples taken two to three weeks after N application were not affected by late-season N fertilization. ________________________________ Previous research on N fertilization of soybeans has produced inconsistent results, showing both increased and decreased soybean yields. While irrigated soybean producers have routinely achieved yields of 60 bu/A, few studies have evaluated the impact of late- season N application on irrigated soybeans with high yield potential. In much of the past work evaluated, N was applied either preplant, at planting time, or early in the growing season. We know that a soybean crop that yields 70 bu/A must translocate 242 lbs/A of N into the seeds during pod fill. This period of high N requirement for soybeans is from the R3 to R6 growth stages. With the high-yield potential of irrigated soybeans, the question is can N fixation supply enough N to meet demand? Or can a late-season supplemental N fertilization increase yields? Little of this work has been conducted in the Midwest. Changes in soybean marketing strategies have also renewed interest in late-season N fertilization to increase grain protein and/or oil concentrations. The USDA Federal Grain Inspection Service has discussed implementation of oil and protein testing as official soybean marketing criteria. Soybean pricing structures in the future may well be influenced by these parameters. The question then naturally arises as to what effects late-season N fertilization might have on protein and oil contents of irrigated soybeans. The objective of the research in this discussion, therefore, was to evaluate the effects of late-season supplemental N on soybean grain yield, protein, and oil content of irrigated soybeans with high yield potential. Yield up During the two years of this research, late-season N fertilization significantly increased soybean grain yield at six of eight sites (Figure 1). Visual inspection of soybean root systems at all locations revealed excellent nodulation at the time of N fertilization. Leaf N concentrations of the check treatments, ranging from 4.96 to 5.70 percent, verify that soybean plants were not severely N stressed and suggest that nodules were effectively fixing N for the plant. The two non-responsive sites were at the Stafford County location, which was also the lowest yielding location. The soybeans there would have had a lower T.L. Wesley, Drs. R.E. Lamond, V.L. Martin, S.R. Duncan Applied N At R3 Stage Bumps Soybean Yields Nitrogen applications at R3 growth stage produce 11.8 percent average yield increase in two-year Kansas study. Table 1. Location and cultural practices for research sites, Wesley, et al., Kansas State University, 1994-95. Location Site Row spacing Cultivar Seeding rate inch Seeds/A Johnson County JO94 30 Asgrow A4138 160,000 Brucker Farm JO95 30 Asgrow A4138 160,000 Shawnnee County SN94 36 Asgrow A3935 180,000 Parr Farm SN95 36 Asgrow A3935 180,000 Reno County RN94 7.5 Asgrow A3935 200,000 Sock Farm RN95 7.5 Asgrow A3834 200,000 Stafford County SF94 30 Resnick 150,000 Sandyland field SF95 30 KS3494 125,000
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