Comprehensive Perspective On the ISNT Approach.

High fertilizer prices and awareness of the environmental implications associated with over-application of nutrients have increased the need for reliable predictions of nitrogen (N) applications for corn (Zea mays L.). Soil testing has long been the basis for P and K guidelines but for N, a yield-based N recommendation approach with adjustments for rotations and manure use has predominated for several decades. Such a yield-based approach is likely to result in under or over-fertilization since soil N supply is not taken into account. The Illinois Soil Nitrogen Test (ISNT) was developed as a quick and simple alternative for determining soil N supply potential. Since its release in 2001, several research groups have evaluated the ISNT as a tool for identifying the N responsiveness of corn. These studies have shown mixed results. Careful evaluation of the literature and an understanding of N cycling reveal why such contrasting results are not surprising: the ISNT approximates ammonium-N and soil N supply potential from labile organic N whereas crop responsiveness to N fertilizer and optimum N rates depend on timing and amount of soil nitrate-N available compared to crop N needs. As the ISNT does not measure nitrate, ISNT-N alone cannot be used to predict total N availability. Nor will it be successful in determining N needs of corn across diverse agricultural rotations and nutrient management systems. However, New York research showed that when ISNT-N results were interpreted as estimation of potential soil N supply, and when critical values were adjusted based on the soil’s overall organic matter content (LOI), the ISNTxLOI combination was 84% accurate in predicting if sidedress N was needed for 2nd or higher year corn. The project included nine responsive and ten non-responsive sites, indicating great potential for N fertilizer savings with ISNT-adjusted corn nitrogen management.