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A soil test for estimating mineralizable N in soil is needed to improve fertilizer use efficiency. Our objectives were to: (i) test the effectiveness of two chemical indices in estimating potentially mineralizable N (N0) in soils, and (ii) determine if these indices could be used to differentiate the impact of cultural practices on the N‐supplying capacity of soils. We collected samples from the 0‐ to 0.15‐m depth of 42 soils representing all agroecological regions in Saskatchewan, Canada. We determined (i) N mineralized (Nmin), N0, and the rate constant (k) by aerobic incubation at 35°C for 24 wk, and (ii) the NH4‐N extracted by (a) 2 M KCl heated at 100°C for 4 h, and (b) a steam‐distilled phosphate‐borate solution buffered at pH 11.2. The association between N0 and the hot 2 M KCl extracted NH4‐N was close (r2 = 0.78, significant at P < 0.001); the relationship between N0 and the phosphate‐borate NH4‐N was slightly less precise (r2 = 0.73, P < 0.001). When NH4‐N extracted with cold 2 M KCl was subtracted from the NH4‐N extracted with hot extractants, the association with Nmin was weaker (r2 ≤ 0.50). The value of k for the 42 soils was generally constant (avg. 0.067 ± Sxt 0.05 = 0.040 wk−1). In a second experiment, we compared the two chemical extractants vs. the N mineralized in a 24‐wk incubation regarding their effectiveness in differentiating the effects of eight 37‐yr crop rotation treatments on N‐supplying capacity. These three indices performed effectively in this regard. For example, the 2 M KCl NH4‐N was closely associated with N mineralized in 24 wk (r2 = 0.92), as was phosphate‐borate NH4‐N (r2 = 0.88; both significant at P < 0.001). We concluded that the chemical extractants may provide a quick test for assessing N‐supplying capacity of the soil.

The absorption of K' by excised roots of barley ( (12,13). In particular, these improved techniques of experimentation with excised roots made possible a much more precise delineation of ion transport kinetics than was possible earlier (8).

, J. 1997. Use of hot KCl-NH 4 -N to estimate fertilizer N requirements. Can. J. Soil Sci. 77: 161-166. We need an easy-to-use chemical index for estimating the amount of N that becomes available during the growing season, to improve N use efficiency. This paper discusses how producers may, in future, use crop growth models that incorporate indices of soil N availability, to make more accurate, risk-sensitive estimates of fertilizer N requirements. In a previous study, we developed an equation, using 42 diverse Saskatchewan soils, that related potentially mineralizable N (N o ) to NH 4 -N extracted with hot 2 M KCl (X), (i.e., N o = 37.7 + 7.7X, r 2 = 0.78). We also established that the first order rate constant (k) for N mineralization at 35°C is indeed a constant for arable prairie soils (k = 0.067 wk -1 ). We modified the N submodel of CERESwheat to include k and N o (values of N o were derived from the hot KCl test). With long-term weather data (precipitation and temperature) as input, this model was used to estimate probable N mineralization during a growing season and yield of wheat (grown on fallow or stubble), in response to fertilizer N rates at Swift Current. The model output indicated that the amount of N mineralized in a growing season for wheat on fallow was similar to that for wheat on stubble, as we hypothesized. Further the model indicated that rate of fertilizer N had only minimal effect on N mineralized. We concluded that, despite the importance of knowing the N min capability of a soil, it is available water, initial levels of available N and rate of fertilizer N that are the main determinants of yield in this semiarid environment. The theoretical approach we have proposed must be validated under field conditions before it can be adopted for use. + 7,7 X, R 2 = 0,78. Nous avions également établi que la constante de taux du premier ordre (k) pour la minéralisation de N à 35°C est effectivement celle qui s'applique aux sols arables des Prairies (k = 0,67 sem -1 ). Nous avons modifié le sous-modèle N du modèle CERES-blé, de façon à y inclure les valeurs k et N o (ces dernières étant tirées du test au KCl bouillant). Complété par l'inté-gration des données météorologiques de longue durée : précipitations et température, le modèle est utilisé à Swift Current pour estimer l'importance probable de la minéralisation de l'azote durant une saison de végétation, ainsi que le rendement du blé cultivé sur jachère ou sur chaume, selon sur divers taux de fumure N. On a ainsi constaté, comme on l'avait supposé, qu'autant de N était minéralisé dans la saison pour le blé sur jachère que pour le blé cultivé sur chaume. De plus, le modèle a permis de voir que le taux de fumure N n'avait qu'un effet minime sur la quantité de N minéralisé. La conclusion qui s'impose c'est que même s'il est important de connaître les quantités de N potentiellement minéralisables d'un sol, les principaux déterminants du rendement du blé dans ces zones semi-arides demeurent les disponbilités en eau, les niveaux initiaux de N disponibl...