Relationship of the Illinois soil nitrogen test to spring wheat yield and response to fertilizer nitrogen

Steckler, S. J., Pennock, D. J. and Walley, F. L. 2008. Relationship of the Illinois soil nitrogen test to spring wheat yield and response to fertilizer nitrogen. Can. J. Soil Sci. 88: 837-848. The Illinois soil N test (ISNT) has been used to distinguish between soils that are responsive and non-responsive to fertilizer N in Illinois. We examined the suitability of this test, together with more traditional measures of soil fertility, including spring nitrate-N and soil organic carbon (SOC), for predicting yield and N fertilizer response of wheat (Triticum aestivum) on hummocky landscapes in Saskatchewan. The relationship between ISNT-N and wheat yield and fertilizer N response was assessed using data and soils previously collected for a variable-rate fertilizer Study. Soils were re-analyzed for ISNT-N. Our goal was to determine if ISNT-N could be used to improve the prediction of crop yields. Although ISNT-N was cot-related with both unfertilized wheat yield (r=0.467, P=0.01) and fertilizer N response (r=-0.671, P=0.01) when data from all study sites were combined, correlations varied according to landscape position and site. Stronger correlations between nitrate-N and both unfertilized wheat yield (r=0.721, P=0.01) and fertilizer N response (r=-0.690, P=0.01) indicated that ISNT-N offered no advantage over nitrate-N. Although both tests broadly discriminated between sites with high or low N fertility, few relationships were detected on a point-by-point basis within a field. Stepwise regression equations predicting yield and yield response did not include ISNT-N, due in part to the high degree of collinearity between ISNT-N and other variables such as SOC, suggesting that ISNT-N alone was not a key indicator of soil N supply.