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Fluid Journal : Fluid Journal 1999-2001
Spring 1999 N demand than at the other three locations because of lower yields (below 50 bu/A). The six responsive sites showed an average soybean yield increase of 6.9 bu/A or 11.8 percent with late-season N fertilization. Yields at these six sites ranged from a low of 56 bu/A to a high of 83 bu/A. Even though soybean grain yields were increased by late-season N application, differences between the 20 and 40-lb/A rates of N were minimal. When averaged across all site years, the 20-lb/A rate produced a yield of 62.4 bu/A compared to 61.9 bu/A for the 40-lb/A rate. Results also corroborate other work suggesting that soil N is crucial during periods of peak N demand such as pod fill. The study shows that producers of high yielding soybeans (greater than 55 bu/A) would benefit from a late-season application of N at the 20-lb/A rate of N. Assuming a $7/bu soybean price, the 6.9 bu/A yield increase would bring an added return of $48.30/A. Assuming an N cost of $0.30/lb, applying N at rate of 20 lbs/A would cost $6/A, leaving a net economic benefit of $42.30/A. Content unaffected Protein. Although soybean protein content was increased significantly by late-season N fertilization at four of eight sites, combined analysis indicated non- significant effects (Figure 2). At the responsive sites, protein increases were about 1 percent. Over the course of this two-year research project, N rate had no significant effect on soybean seed protein at any of the eight sites. Oil. Late-season N fertilization significantly increased soybean oil content at three sites, but combined analysis indicated overall effects were non-significant (Figure 3). At the responsive sites, oil content was increased by 0.3 to 0.5 percent. (This contrasts with other work that has found that N fertilization tends to lower oil levels.) Nitrogen rate effects on oil content at any of the other study sites were not significant. If soybean marketing eventually requires a minimum protein plus oil level, late-season N fertilization may have to be further evaluated. Currently, significant premiums are not offered for increased protein or oil in soybeans. Leaf N concentration. Soybean leaf N concentrations at two to three weeks after late-season N applications were not Table 2. Selected soil characteristics of research sites, Wesley, et al., Kansas State University, 1994-95. Organic Matter Profile N Site pH Bray--1P K 0-6 in. 0-6 in. 6-24 in. ------------ppm------------ % -- -- -- -- --ppm---------- JO94 6.9 41 125 0.7 4.1 - - JO95 6.8 44 165 0.8 3.0 5.5 SN94 7.3 65 305 2.8 6.7 - - SN95 7.7 67 240 3.1 7.9 6.3 RN94 6.8 50 210 1.2 2.7 - - RN95 6.8 48 190 1.7 3.0 2.2 SF94 6.9 31 140 0.9 3.1 - - SF95 6.7 52 130 1.3 7.8 4.5 Figure 1. Effect of late-season N application on irrigated soybean yield, Wesley, et al., Kansas State University, 1994-95. 67 65 63 61 59 57 55 53 Check UAN Am. Nit. Urea Urea + NBPT N Source Yield--bu/A 0 lbs/A 20 lbs/A 40 lbs/A
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