Soybeans require a lot of nitrogen (N), in fact, most legumes require more N than do other grain crops because of higher seed protein content, writes Mike Stewart, International Plant Nutrition Institute. For example, the N requirement per unit yield is about four times higher for soybean than for corn.
In a classic paper by Salvagiotti et al. (2008), an analysis of 637 datasets across 17 countries indicated that a soybean crop will take up an average of 4.7 lb N/bu in the aboveground biomass, and that soybean seed contains about 6.3% N. Using these figures, a 70 bu crop will take up about 330 lb N, and will remove about 265 lb N/A from the field at harvest. The same work determined that 50 to 60% of soybean N demand was met by biological fixation, with remainder of the crop’s N need supplied by soil reserves. Soybean may generally be considered neutral in relation to its contribution of N to the soil, but at high yields negative N balances can be expected.