Evaluation of the Illinois soil nitrogen test in the North Central Region of the United States

Recently the Illinois soil nitrogen test (ISNT) was proposed as a means to identify fields where corn (Zea mays L.) will not respond to additional N fertilizer and which may also be used to predict the economic optimum N rate (EONR). Data from 96 corn N rate response trials across Iowa, Illinois, Michigan, Minnesota, Nebraska, and Wisconsin were compiled to evaluate the usefulness of the ISNT in identifying nonresponsive fields, predictingEONR, and estimating mineralizable N. At each trial site, multiple rates of fertilizer N were applied, including zero N and nonyield limiting rates. Corn was grown following several crops. 'Ihe ISNT could not accurately predict nonresponsive sites, nor could it reliably estimate EONR. Subsetting the data based on soil drainage class and previous crop did not improve the predictive capability of the ISNT even though ISNT values were significantly different among previous crops and soil drainage classes. The ISNT was strongly correlated to soil organic matter (OM) and was apparently measuring a constant fraction of total soil N JN). The lack of correlation between the ISNT and relative N uptake (check plot N uptake/N uptake at the maximum N rate) suggests that the ISNT is not measuring the readily mineralizable fraction of soil N. Based on results of this project, the ISNT is not suggested for use in adjusting N rate recommendations for corn in the North Central Region (Corn Belt) of the United States.