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Optimizing Soil and Fertilizer Nitrogen Use by Intensively Managed Winter Wheat. II. Critical Levels and Optimum Rates of Nitrogen Fertilizer

Agro-Economic Div., Int. Fertilizer Development Ctr., Muscle Shoals, AL
Agronomy Dep., Virginia Polytechnic Inst. and State Univ., Blacksburg, VA 24061

^* *Corresponding author

The assessment of optimum N fertilizer rates for winter wheat (Triticum aestivum L.) production is a major unsolved problem, particularly in humid climates. Eight field experiments were conducted in the Coastal Plain and Ridge and Valley regions of Virginia during the 1981–1982 through 1985–1986 winter wheat growing seasons to determine critical N levels and optimum rates of N fertilization. The treatments in these experiments consisted of various N fertilizer rates applied at Zadoks growth stage (GS) 25 and GS 30. Plant N concentration (NCON30) and crop N uptake (NUP30) at GS 30 were measured immediately prior to the N fertilizer application. Nitrogen critical levels were defined as the NCON30 or NUP30 values that produced 90% of maximum grain yields in each experimental site with no further N fertilizer application. Two nonlinear models were developed to determine critical levels at GS 30 using NCON30 and NUP30 as the independent variable. The calculated critical levels for the models using NCON30 and NUP30 were 39.5 g N/kg and 95 kg N/ha, respectively, with R² values for the models being 0.87 (NCON30) and 0.79 (NUP30). A family of response curves of grain yield to N fertilizer applied at GS 30 was fitted at each ex-

Received for publication July 6, 1987.

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