Supplemental N during seed filling phase is essential to avoid the N limitations and consequently improve soybean seed protein concentration. Studies have suggested that late season N fertilization (N application after the beginning of seed filling stage) is more effective approach than early season or N application throughout the crop growing season to improve soybean seed protein concentration. Hence, late season N fertilization has emerged as a potential solution to address the declining soybean seed protein concentration trend. However, N fertilization could suppress biological N fixation, which is the most efficient source of N in soybean. Majority of nitrogen needs in soybeans come from biological N fixation (Salvagiotti et al., 2008), hence it is crucial to provide supplemental N fertilizations without negatively affecting its biological N fixation. Studies are undergoing to quantify the effect of late season N fertilization on soybean's biological N fixation and to optimize the late season N fertilizations dose and timing. Hence, late season N fertilizations should be used cautiously as a short-term alternative for now.
Cultural practices such as planting dates, plant density, row types, crop rotation and irrigation determine environmental conditions during crop growth and consequently influence soybean seed composition. Higher temperatures generally decrease soybean seed protein concentration while irrigation and crop rotation improve it. Supplemental bacterial inoculant (N fixing bacteria) application in the seeds or during in-season crop growth has also been tested to improve biological N fixation in soybean. Although it was generally found not effective under normal soybean growing conditions, studies have documented improved N availability to the plants under stressful environments and in the soils with no previous history of soybean cultivation. Soil amendments such as biochar increases root growth, root nodulation and biological N fixation in soybean indicating it could be useful to improve N availability to the plants. Research is undergoing testing the effect of soil amendments on soybean seed composition. Previous studies have made it clear that cultural practices can affect the protein concentration in soybean seeds. Hence, adopting cultural practices which can increase N availability to the plants is another strategy to improve soybean seed protein concentration.
Nitrogen limitations in high yielding modern U.S. soybean cultivars have resulted in decreased seed protein concentration. As a result, currently produced soybean meal in some U.S. locations is struggling to meet high quality animal feed standard. Supplemental N during seed filing stage is essential to increase soybean seed protein concentration and consequently produce high quality soybean meal for animal feed. Late season N application has emerged as a solution to avoid nitrogen limitation and improve seed protein concentration but its effect on soybean's biological N fixation is still not fully understood. Hence, late season N fertilization should be used cautiously as a short-term alternative. Adopting cultural practices which can increase N availability to soybean is another approach to improve its seed protein concentration.
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