Soil N mineral content is not considered when recommending N for grain crops in southern Brazil due to the high variation according to environmental conditions. In the methodology described here soil N mineral is considered in adjusted N recommendations to increase efficiency and reduce production costs. The soil levels of NH4+ and NO3- were analyzed to a depth of 15 cm, in the pre-sowing and grain yield stage of 61 barley trials distributed in 20 counties in three regions of Rio Grande do Sul State (Depressão Central, Planalto Médio and Alto Uruguai) for four years (2002-2005). The yield potential was classified (very low, low, medium, high and very high) and related to the N mineral contents using the Gompertz regression model. The adjusted model indicated two N mineral values delimiting relative yields: 8 and 18 mg kg-1 soil. Low yields predominate below 8 mg kg-1 soil, indicating a direct effect of low N availability on yield potential very early in the season. At soil NH4+ and NO3- values over 18 mg kg-1 soil N was no longer decisive in the formation of yield, indicating a very high yield potential. These values may be useful in models of N recommendation.
nitrogen; spatio-temporal analysis; winter grains
