. 2008. Relationship of the Illinois soil nitrogen test to spring wheat yield and response to fertilizer nitrogen. Can. J. Soil Sci. 88: 837Á848. 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 correlated with both unfertilized wheat yield (r 00.467, P 00.01) and fertilizer N response (r 0(0.671, P00.01) 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 (r00.721, P 00.01) and fertilizer N response (r 0(0.690, P00.01) 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. Le test du N du sol de l'Illinois (ISNT) sert a`diffe´rencier les sols de l'Illinois qui re´agissent bien ou mal aux engrais azote´s. Les auteurs voulaient e´tablir si ce test et les me´thodes plus classiques servant a`ve´rifier la fertilite´du sol, notamment la concentration de N-nitrate au printemps et la teneur en carbone organique du sol (COS), pouvaient servir a`pre´voir le rendement et la re´action aux engrais N du ble´(Triticum aestivum) cultive´sur les terrains mamelonne´s de la Saskatchewan. Pour cela, ils ont e´value´les liens entre le N-ISNT et le rendement du ble´ainsi que la re´action de cette culture aux engrais azote´s graˆce aux donne´es et aux e´chantillons de sol recueillis dans le cadre d'une e´tude sur la variation du taux d'application des engrais. Ils ont analyseć es sols pour e´tablir leur concentration de N-ISNT. L'objectif ultime e´tait de de´terminer si le N-ISNT pourrait conduire ad e meilleures pre´visions du rendement des cultures. Bien que le N-ISNT soit corre´le´au rendement des cultures de ble´non bonifie´es (r00,467, P00,01) et a`la re´action aux engrais N (r 0(0,671, P00,01) quand on combine les donne´es de tous les sites e´tudie´s, ces corre´lations varient avec l'emplacement dans le paysage et avec le site. La plus e´troite corre´lation existant entre le N-nitrate et le rendement du ble´non bonifie...