The effect of topographical position on the yield response of canola (Brassica rapa var. Maverick) to N fertilizer was evaluated in a 3‐yr field study in Saskatchewan, Canada. The hummocky, glacial till research site was stratified into three topographically defined landform complexes (convex, linear, and concave). Five N treatments (0, 0.5, 1.0, 1.5, and 2 times the recommended soil test rates; treatments will be referred to as 0×N, 0.5×N, 1×N, 1.5×N, and 2×N, respectively) were randomized in replicated blocks, and each block spanned the three landform complexes. Substantial yield differences occurred among the study years and were associated with year‐to‐year differences in spring soil moisture. Canola seed yields (averaged across years and treatments) increased from 0.53 Mg ha−1 in convex complexes to 0.95 Mg ha−1 in linear, and 1.42 Mg ha−1 in concave landform complexes. The greatest yield responses to N occurred in the concave landform units in years where spring soil moisture was high. The N fertilizer rate required to achieve the economically optimum yield was significantly correlated to spring available moisture (required N rate [kilogram per hectare] = 40.9 + 14.87 [spring available water {in centimeters} water to 60‐cm depth], R2 = 0.73, sig. = 0.003) but was not significantly related to spring (i.e., preseeding) available soil N. The results confirm earlier studies indicating the importance of spring available water for crop production in the northern Great Plains, and suggest that intensive spring sampling for soil moisture conditions may be the most useful diagnostic tool for the implementation of a variable rate N program in this region.

Walley, F., Pennock, D., Solohub, M. and Hnatowich, G. 2001. Spring wheat (Triticum aestivum) yield and grain protein responses to N fertilizer in topographically defined landscape positions. Can. J. Soil Sci. 81: 505-514. A 3-yr field study was initiated in 1996 to examine the different grain yield and grain protein responses of wheat to varied N fertilizer rates in a typical glacial till landscape in Saskatchewan, Canada. Our objective was to assess the agronomic and economic feasibility of variable rate fertilizer (VRF) N application for wheat. Results suggest that spring soil water status largely determined the yield and the protein content of wheat both within different years of the study and between different landscape positions within a given year. Although grain yield was strongly related to spring soil water and was predictable on that basis, the grain yield response of wheat to fertilizer N additions was highly variable due, in part, to the dual role that N played in determining both grain yield and grain protein content. As a consequence of the unpredictable nature of the varied response of wheat to N fertilizer additions, there was little economic rationale for using VRF strategies in the 3 yr of this study. However, in the long-term, we believe that VRF N application strategies can be employed to manage N inputs from the perspective of managing and replacing harvested N. En 1996, les auteurs ont entrepris un projet de trois ans visant à évaluer la variation du rendement grainier et de la concentration de protéines dans le grain après application d'une quantité variable d'engrais azoté sur un till glaciaire caractéristique aux terrains de la Saskatchewan, au Canada. L'idée était de vérifier la faisabilité agronomique et économique d'un taux variable de fertilisation (TVF) azotée pour le blé. Dans le cas du blé, les résultats donnent à penser que le bilan hydrique du sol au printemps détermine largement le rendement et la concentration de protéines du grain, tant d'une année à l'autre que d'un endroit à l'autre, la même année. Bien qu'il présente une forte corrélation avec la teneur en eau du sol au printemps, et qu'on puisse se servir de ce paramètre pour le prévoir, le rendement grainier du blé varie considérablement avec l'addition d'un engrais azoté, en partie parce que le N intervient doublement, soit dans le rendement grainier et dans la concentration de protéines du grain. À cause de la nature imprévisible de cette réaction, un TVF s'est avéré relativement peu indiqué sur le plan économique au cours des trois années de l'étude. À plus longue échéance cependant, les auteurs croient qu'un régime de fertilisation à taux variable pour le N faciliterait la gestion des apports d'azote lorsqu'on souhaite réduire ou remplacer l'azote perdu à la récolte.

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