Effect of Planting Date and Starter Fertilizer on Soybean Grain Yield

In the Midwest, soybean [Glycine max (L.) Merrill] is planted over a large window of time. Starter fertilizer may be beneficial at earlier planting dates when cool soils limit nutrient availability or at later planting dates when the growing season is shortened. The objective was to evaluate the effect of planting date and starter fertilizer on soybean nodulation, biomass, canopy closure, grain yield, and grain quality in high-yielding environments. A trial was conducted at the Western Agricultural Research Station (WARS) in 2013 and 2014 and the Northwest Agricultural Research Station (NWARS) in 2014. Treatments included three to four planting dates ranging from May through early July and five starter fertilizers (none, urea, triple superphosphate, urea + triple superphosphate, and diammonium phosphate) applied 2 inches below and 2 inches beside the seed at planting. Starter fertilizer had a very small effect on soybean nodulation, biomass, canopy closure, and grain quality. In 2013 and 2014 at the WARS location, yield decreased an average of 0.58 bu/acre per day from the first to last planting date. Fertilizer did not increase soybean yield at any of the site-years. All sites had soil phosphorus (P) levels greater than the critical level of 15 ppm. At soil P levels >15 ppm, a yield response to fertilizer is unlikely. Nitrogen fixation and mineralization were adequate to meet the crop demand. Planting in early May without starter fertilizer provides the greatest chance to consistently maximize soybean yield and profitability when N and P are not limiting.