Impact of Residual Mineral N in Soil on Grain Protein Yields of Winter Wheat and Corn1

Abstract

Fertilizer N has become a major cost input in the production of winter wheat (Triticum aestivum L.) and corn (Zea mays L.) with uncertainties remaining as to rates required for effecting optimum grain and protein yields without environmental pollution implications. For this reason investigations were conducted with the two crops to determine optimum rate of fertilizer N for grain and protein yields with varied levels of residual mineral N in the rooting profile of soil to 180 cm depth at planting. Fertilizer rates employed for wheat in 118 farmers' fields ranged from 0 to 67 kg N/ha by 22.5 kg increments broadcast in the spring and for corn in 17 farmers' fields from 0 to 225 kg/ha by 45 kg increments sidedressed at 30 to 45 cm growth stage.Residual mineral N was found to exert major influence on grain protein percentage and protein yield of both crops within nominal rates of N fertilizer application. The magnitude of residuals varies with the soil environment and amount of prior fertilizer use on the soil. The data reveal that grain yield response of wheat to applied N is unlikely when soil residual mineral N exceeds 120 kg/ha and for irrigated corn around 240 kg/ha at the yield levels of approximately 25 to 30 quintals/ha of wheat and 85 to 90 quintals of corn obtained in these experiments. Maximum protein yield for wheat required an additional 50 to 60 kg N/ha and for corn 60 kg, economics of which can be ascertained only with knowledge of the unit nutritional value of the produce in relation to the unit cost of N applied.