In-Season N Fertilizer on Soybean: Where's the Yield?.

The recent interest in using N fertilizer on soybean during the growing season is based on the logic that substituting exogenous N for that fixed in bacteria-containing nodules increases the amount of photosynthetically-fixed energy available for plant growth. By this reasoning, a high-yielding soybean crop should respond more to N fertilizer than would a lower-yielding crop. Across 22 recent trials in Illinois with yields ranging from 2,645 to 5,865 kg/ha and averaging 4,420 kg/ha, the response to N fertilizer (applied as urea, typically at about 50 kg N/ha) ranged from -267 to +409 kg/ha, with an average response of 20 kg/ha. The only significant response was 409 kg/ha, which occurred in a trial with a yield level in the middle of the range at 4,325 kg/ha. Higher yields did not mean more response to N; in fact, there was a modest trend (r=-0.39, p = 0.07) toward more positive responses to N at lower yield levels. These results call into question the environmental and economic wisdom of making in-season N applications to soybean, at least with our current state of understanding. Soils may provide more N than we realize, and the photosynthetic capacity of soybeans under field conditions may be underestimated.