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Fluid Journal : Fall 2016
12 The Fluid Journal Fall 2016 Dr. A. L. Shober and R. Taylor ▼ DOWNLOAD Figure 1. Conceptualized N budget for soybean is based on grain N uptake (grain removal + stover removal), maximum soil N mineralization of 100 lb/ac and maximum N fixation of 300 lb/ ac. Note that soil N + fixed N should meet crop N needs for grain yeld. Figure 2. Comparison of soybean nodule development on unfertilized plants (upper) and plants fertilized with urea at N rate of 100 lb/ac. Photo credit: Shawn Tingle, University of Delaware. application rate increases, such that application of 45 lbs/A N can lead to a 40% or greater reduction in N fixation over the maximum achievable N fixation when no supplemental N is applied. Applications of N at higher rates can further reduce N fixation. If nodules do develop in the presence of supplemental N, it is possible that those nodules will be inactive and will not fix N (Figure 3). If supplemental N and soil N pools are not sufficient to supply the entire amount of N needed for optimum yield, nodulation or reactivation of existing modules may be delayed and the plant will be unable to fix enough N to support maximum growth when the demand for N peaks during pod development. If plants are N deficient at the time of pod set/ seed fill, a significant loss of yield may occur. Research has indicated that supplemental N is most effective when applied near the R3 stage of plant development. Other risks While application of supplemental N might not reduce yield in all situations, it will often result in wasted money. In addition, some forms of N are easily lost to the environment in runoff of leachate. Application of supplemental N to soybeans may increase the rise for N losses, which may have negative impacts on water quality. Application of supplemental N should be avoided under the following situations because the economic and environmental risks are increased: • Non-irrigated soybeans: water will likely be more limiting than N (or any other nutrient) • Expected yield is <60-70 bu/A: there is probably enough N available in the soil and via fixation. • Soybeans have matured past R6 (full feed stage). The N requirement of soybeans is greatly reduced and supplemental N applications past this point are wasteful. • Fields with a history of soybean cyst nematode: yields will be more limited by the impact of nematode feeding than N. • Fields that have not produced soybeans for a long time (or ever). Skip the commercial or manure N and apply a good inoculant. Maximum benefit Application of supplemental N may provide a yield benefit for high-yield soybeans, but only in cases where expected yields are 60 to 70 bu/A or higher. If yields are consistently lower than 60 bu/A, skip the N and apply a good inoculant instead. Growers consistently exceeding 70 bu/A yield on irrigated soybeans should consider the following when considering applications of supplemental N: • Research suggests yield increases were greatest (when yield increase were noted) when N applications were <30 lbs/A. If applying N preplant or early season, use methods of application that won’t interfere with nodulation (e.g. deep placement of ammoniacal, slow- or controlled- release fertilizers). • Apply N in-season between growth stages R2 and R4 to provide N just before pod set, when N uptake is most rapid. However, applications of N at R2 to R4 must be done to minimize damage to the soybean plant, since