Soybean (Glycine max L.) seed is a major source of protein, oil, carbohydrates and other nutrients that are important for human and animal nutrition. Producers have considered applying nitrogen (N) fertilizer to soybean crop to maximize seed yield; however, its effect on seed composition is not well understood. The objective of this two-year (2015 and 2016) study was to evaluate the effects of N fertilizer sources and application rates (45, 90, 135 and 179 kg N ha −1 ) on soybean seed composition on two soil textures (clay and silt-loam) in Mississippi. The three fertilizer sources included in this study were urea with N-(n butyl) thiophosphoric triamide (Urea+NBPT), polymer-coated urea (PCU), and ammonium sulfate (AMS). Nitrogen application at 179 kg ha −1 on clay soil reduced seed protein by 1.05% compared to unfertilized soybeans in 2016. However, N application at 179 kg ha −1 increased oil content by 0.7% on clay soil compared to the unfertilized soybeans only in 2016. Nitrogen applications reduced stachyose content on both soil textures in 2015. The fatty acids showed variable response to N applications. Since, seed quality is not a trait from which growers receive an economic incentive, they are unlikely to adopt this practice for standard soybean production.
Core Ideas
Nitrogen applications increased soybean yields by 4% for sandy/silt loam soil.
Nitrogen applications increased soybean yields by 8% for clay soil.
Yield components response to N applications parallels the trends observed for yield.
Nitrogen application is not an economically feasible way to improve soybean yield.
Trials were conducted in 2014 and 2015 at Delta Research and Extension Center in Stoneville, MS, to evaluate soybean [Glycine max (L.) Merr.] response to N sources (ammonium sulfate [AMS], polymer‐coated urea [PCU], urea+urease inhibitor [Urea+NBPT, N‐(n‐butyl) thiophosphoric triamide]), N application rate (0, 45, 90, 135, 179 kg N ha−1) and timing (V4, R1), on two soil textures (sandy/silt loam and clay) that are commonly used for soybean production in Mississippi. Urea+NBPT produced 5 and 8% greater aboveground biomass than AMS and PCU on sandy/silt loam soil. Nitrogen applications at the V4 growth stage resulted in 6% greater biomass production than the R1 application timing on sandy/silt loam. Averaged over N rates where a seed yield increase was observed, N applications increased soybean yields by 4% for sandy/silt loam soil and by 8% for clay soil compared to unfertilized soybeans. No differences were observed for soybean yields between N rates of 45, 90, 135, and 179 kg N ha−1. Soybean yield from PCU was 2.5% greater than urea+NBPT on sandy/silt loam soil. Nitrogen rates influenced soybean yield components including pods and seeds plant−1 on sandy/silt loam soil as well as seeds and seed mass plant−1 on clay soil. Yield component response to N fertilization parallels the trends observed with overall soybean seed yield increases on both soil textures. Despite increases in seed yield observed with N application, analysis of partial returns suggests that N application is not an economically feasible way to improve soybean seed yield.
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