Testing Soils and Cornstalks to Evaluate Nitrogen Management on the Watershed Scale

High nitrate (NO3–N) concentrations in Iowa rivers have been linked to areas of intensive row crop production, but they have not been experimentally linked to specific management practices used during row crop production. This study demonstrates how the late-spring test for soil NO3–N and the end-of-season test for cornstalk NO3–N can be used to measure N sufficiency levels across many fields and how the results can be used to evaluate management practices within a watershed. More than 3200 soil and cornstalk samples were collected over a 12-yr period from fields planted to corn (Zea mays L.) and already fertilized by farmers using their normal practices. Results showed that early-season rainfall and associated N losses were major factors affecting N concentrations in soils and cornstalks. Evidence for NO3–N movement from fields to rivers was provided by an inverse relationship between annual means for NO3–N concentrations in soils and rivers. Because these losses can be avoided by delaying N applications, the practice of applying N several weeks or months before plants grow was linked to inefficient use of fertilizer and manure N by crops. Results of the study demonstrate how aggregate analyses of soil and cornstalk samples collected across many farms and years make it possible to identify the major factors affecting N management outcomes and, therefore, N management practices that are likely to produce the best outcomes within a watershed or region. This approach seems to have unique potential to interrelate the management practices of farmers, the efficiency of N fertilization, and NO3–N concentrations in rivers.