“Moisture has the biggest impact on nutrient use,” explains Fabian Fernandez, University of Illinois soil fertility specialist. “Plants take nutrients from the soil solution. If it is dry, the plants are not growing and nutrients can’t be used.”

In Illinois, the Council for Best Management Practices funded a fall soil-sampling project for residual N lead by Emerson Nafziger.“There wasn’t a lot of information out there,” says Fernandez, one of the team members who analyzed the results. “We wanted to know where we are since there was the potential for a lot of N left in fields.”

Nearly all of the samples were taken following corn in 2012 and many were from fields where yields were low from dry weather. Survey results indicate high levels of residual N are common in many fields.

Since moisture was the limiting factor for yields in 2012, the amount of N used in a field and corn yield in that field didn’t match very well, according to the report by the Illinois team. The relationship between the amount of N applied and the amount found in the fall was quite poor. Plus, low yields were not associated with high amounts of leftover N among the fields sampled.

The report concludes, “We think this simply shows how complex the N interactions are in the soil. In a year like 2012, there is little N loss, N uptake ends early as the crop stops taking up water…Late summer and fall rainfall can produce new flushes of mineralized N long after crop uptake stops. It’s possible that soil moisture drives both yield and the amount of N in the soil and that these two things were more or less independent of each other.” This report confirms the need for an early-spring soil-test before applying N, Fernandez points out. Low yields don’t necessarily mean high residual N.

While there appears to be residual N in Illinois, there is wide variability in Minnesota, according to Gyles Randall, a recently retired University of Minnesota soil scientist. “Some Minnesota soil tests from last fall show no residual N.”