Nitrogen source and sulfur contribution effects on yield, protein, and oil content in soybean ( Glycine max )

This study investigates the effects of N topdressing treatments on soybean yield, seed protein, oil content, and test weight over three growing seasons on Prince Edward Island, Canada. The aim was to see whether the choice of N source, rate of application, and timing could help improve seed quality and yield in a short‐season, cool‐climate setting. A factorial field trial was used to test if various granular N sources (ammonium nitrate [AN], ammonium sulfate [AS], and urea), at rates of 0, 30, 50, and 70 kg ha −1 , applied at either the R1 (beginning bloom) or R3 (early pod) stages could impact seed yield or quality. The 30 kg N ha −1 of AS applied at R3 treatment, resulted in a 15.5% seed yield increase over the control. Seed protein increased by nearly 4%, with AS topdressing, particularly when applied at the R3 growth stage. In contrast, seed oil content decreased by 3% compared to the control when AS was applied at the R3 growth stage. Test weight showed no response from any treatments. The study suggests that responses to nitrogen topdressing were closely linked to sulfur availability, emphasizing the importance of managing both nutrients to optimize soybean yield and quality.