Soil properties and weather conditions are known to affect soil N availability and plant N uptake; however, studies examining N response as affected by soil and weather sometimes give conflicting results. Meta‐analysis is a statistical method for estimating treatment effects in a series of experiments to explain the sources of heterogeneity. In this study, the technique was used to examine the influence of soil and weather parameters on N response of corn (Zea mays L.) across 51 studies involving the same N rate treatments that were performed in a diversity of North American locations between 2006 and 2009. Results showed that corn response to added N was significantly greater in fine‐textured soils than in medium‐textured soils. Abundant and well‐distributed rainfall and, to a lesser extent, accumulated corn heat units enhanced N response. Corn yields increased by a factor of 1.6 (over the unfertilized control) in medium‐textured soils and 2.7 in fine‐textured soils at high N rates. Subgroup analyses were performed on the fine‐textured soil class based on weather parameters. Rainfall patterns had an important effect on N response in this soil texture class, with yields being increased 4.5‐fold by in‐season N fertilization under conditions of “abundant and well‐distributed rainfall.” These findings could be useful for developing N fertilization algorithms that would prescribe N application at optimal rates taking into account rainfall pattern and soil texture, which would lead to improved crop profitability and reduced environmental impacts.

Nitrogen (N) fertilizer rates applied spatially according to crop requirements can improve the efficiency of N use. The study compares the performance of two commercial sensors, the Yara N-Sensor/FieldScan (Yara International ASA, Germany) and the GreenSeeker (NTech Industries Inc., Ukiah, California, USA), for assessing the status of N in spring wheat (Triticum aestivum L.) and corn (Zea mays L.). Four experiments were conducted at different locations in Quebec and Ontario, Canada. The normalized difference vegetation index (NDVI) was determined with the two sensors at specific growth stages. The NDVI values derived from Yara N-Sensor/FieldScan correlated with those from GreenSeeker, but only at the early growth stages, where the NDVI values varied from 0.2 to 0.6. Both sensors were capable of describing the N condition of the crop or variation in the stand, but each sensor had its own sensitivity characteristics. It follows that the algorithms developed with one sensor for variable-rate N application cannot be transferred directly to another sensor. The Yara N-Sensor/FieldScan views the crop at an oblique angle over the rows and detects more biomass per unit of soil surface compared to the GreenSeeker with its nadir (top-down) view of the crop. The Yara N-Sensor/FieldScan should be used before growth stage V5 for corn during the season if NDVI is used to derive crop N requirements. GreenSeeker performed well where NDVI values were [0.5. However, unlike GreenSeeker, the Yara N-Sensor/FieldScan can also record spectral information from wavebands other than red and near infrared, and more vegetation indices can be derived that might relate better to N status than NDVI.

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Impact of nitrogen fertilization on yield, hollow stem incidence and sap nitrate concentration in broccoli. Can. J. Plant Sci. 81: 765-772. This study, spanning 2 yr at two different sites, compared the effects of nitrogen fertilization on broccoli yield and quality in relation to the influence of site and production season. This research also evaluated the potential use of measuring nitrate in petiole extracts to determine broccoli nitrogen requirements when making a second fertilizer application 5 wk after transplant. Treatments consisted of factorial combinations of three nitrogen fertilization rates at transplant (N min , 50-N min and 100-N min kg N ha -1 , where N min was the soil nitrate concentration) and four nitrogen fertilization rates 5 wk after transplant (0, 50, 100 and 150 kg N ha -1 ). The marketable yield of broccoli increased with the rate of nitrogen application. Hollow stem incidence showed the same trend as yield, and was greatest when high levels of nitrogen were applied 5 wk after transplant. Interactions between the 5-wk nitrogen application and production year, as well as growing site were significant. The nitrate content of the sap was measured immediately before the 5-wk nitrogen application and 2 wk later. The nitrate concentration measured at 5 wk increased with transplant nitrogen, as did the concentration measured at 7 wk. Nitrate concentration at 7 wk increased curvilinearly with the 5-wk fertilizer application rates. Sap analysis provided clear evidence of the effect of nitrogen application rates, but Environment precluded determination of an absolute threshold of nitrate sufficiency in the sap.Key words: Brassica oleracea L. var. Italica, split application, quality, environment Bélec, C., Villeneuve, S., Coulombe, J. et Tremblay, N. 2001. Impacts de la fertilisation azotée sur le rendement, la présence de tige creuse et la concentration en nitrate de la sève chez le brocoli. Can. J. Plant Sci. 81: 765-772. Deux années d'étude sur deux sites différents ont permis de comparer les effets de l'application de fertilisants azotés sur le rendement et la qualité du brocoli par rapport à l'influence du site et de l'année de production. Cette recherche visait également à vérifier les capacités et les limites de la mesure du nitrate des extraits de pétioles dans la détermination des besoins en azote du brocoli au moment de réaliser une application complémentaire de fertilisant, 5 semaines après la plantation. Trois doses d'azote ont été appliquées à la plantation (N min , 50-N min and 100-N min kg N ha -1 , N min étant la concentration de nitrate dans le sol) et quatre doses cinq semaines après la plantation (0, 50, 100 et 150 kg N ha -1 ). Le rendement vendable du brocoli a augmenté avec les doses d'azote. L'incidence de la tige creuse a évolué dans le même sens que le rendement avec une augmentation importante reliée à l'application d'azote cinq semaines après la plantation. Des interactions significatives ont été mesurées entre les traitements et l'année de production ou le site de cultu...