RESIDUAL SOIL-NITROGEN IN HUMID REGION WHEAT PRODUCTION

The amount of soil mineral N (nitrate + ammonium) present when winter wheat (Triticum aestivum L.) is spring-fertilized is generally not taken into account when making N fertilizer rate recommendations in humid regions. This is partly because overwinter losses of soil N are thought to be extensive, resulting in little field-to-field variability in the amount of N fertilizer needed. Our objectives were to determine: whether significant levels of mineral N exist in the fall in fields planted to winter wheat in the Coastal Plain of Virginia; if so, rhe extent to which this mineral N remains within the rooting zone through the winter; and whether this residual N affects crop response to spring N fertilization. We measured soil mineral N and conducted N-response experiments in nine farmer fields cropped to winter wheat-five in 1989-1990 and four in 1990-1991. Total mineral N was measured to 4 ft shortly after planting (November) and approximately monthly thereafter until mid-March, at which time rapid crop N uptake typically begins in our climate. Soil mineral N in mid-November ranged from 62 to 212 lb N/acre, and in mid-March ranged from 17 to 149 lb N/acre. Seven of the site-years were responsive to spring N fertilizer applications; yield gains from N application ranged from 25 to 56 bu/acre. At the two sites with the highest soil mineral N levels, check yields were 70 and 75 bu/acre, and no yield response to N fertilizer was observed; in fact, yields were depressed considerably by the application of N fertilizer. We conclude that high levels of soil mineral N can occur in the fall, that they can persist until spring, and that they can have considerable influence on optimum spring fertilization strategy. The wide range in residual mineral N that we observed and its strong correlation with yield response to fertilizer N demonstrates the need for field-specific N recommendations for winter wheat in humid regions.