Spring Nitrogen Fertilizer Recommendation Models for Dryland Hard Red Winter Wheat

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Abstract
Nitrogen fertilizer recommendations for dryland hard red winter wheat (Triticum aestivum L.) are more reliable when based on spring rather than fall soil and plant measurements. Objectives of this research were to develop spring N recommendations based on soil available N, plant tissue N, and/or climatic variables measured in the spring. Grain yield variation was described by simple and multiple regression analysis with various levels of complexity. Soil NH4‐N levels in the spring explained more grain yield variation than did NO3‐N. Spring soil sampling to depths >60 cm explained no additional grain yield variation. Percent sand in the soil surface (0‐30 cm) explained more grain yield variation between experimental sites than did other site variables. Simple Cate‐Nelson III models were more accurate than complex multiple regression equations. Cate‐Nelson III models, for available soil NH4‐N plus NO3‐N (0‐30 and 0‐60 cm) and Feekes 5 leaf N, predicted required N rates in the spring within ±22 kg N ha−1, 87 to 95% of the time. Spring N recommendations were improved when spring N rates were based on both available soil N and Feekes 5 leaf N. These results indicate (i) that analysis of available soil N in the spring must include soil NH4‐N to prevent over‐fertilization, and (ii) that simple Cate‐Nelson models are sufficient for calibration of spring N rates to available soil and plant N levels measured in the spring.