Technical Abstract: High concentrations of NO3 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 report describes how the late- spring test for soil NO3 and the end-of-season test for cornstalk NO3 can be used to measure N sufficiency levels attained (measures of N-management outcomes) across many fields and how the results can be used to evaluate management practices within a watershed. More than 3,200 samples of soil and cornstalks were collected over an 11-year period from fields that were planted to corn (Zea mays L.) and already fertilized by farmers using their normal practices. Analyses identified early season rainfall and associated losses of N as major factors affecting N sufficiency levels attained. Because these losses can be avoided by delaying applications of N, 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 individual farmers, the efficiency of N fertilization, and NO3 concentrations in rivers.