TIMING NITROGEN APPLICATIONS FOR CORN IN A WINTER LEGUME CONSERVATION-TILLAGE SYSTEM

Fertilizer N efficiency of corn (Zea mays L.) in conservation-tillage systems with winter legumes such as crimson clover (Trifolium incarnatum L.) can possibly be improved by better synchronization of legume-N release, fertilizer-N application time, and crop demand for N. The objective of this 3-yr (1986-1988) field experiment was to determine the effect of N application time on dry matter accumulation, N uptake, and grain yield of corn grown in a winter legume conservation-tillage system. Corn was planted with unit planters into crimson clover residue following in-row subsoiling. The clover was killed at midbloom every year. Treatments were a factorial arrangement of fertilizer N rates and application time. Nitrogen as NH4NO3 was broadcast at rates of 34, 67, and 134 kg ba-1. Zero-N checks were also included in both clover and rye (Secale cereal L.) plots. Application times were at planting, or 3, 6, or 9 wk later. In addition, split applications (113 at planting and the remainder 6 wk later) of the 67 and 134 kg N ha-1 rates were included. In 2 of 3 yr, dry matter accumulation was not affected by N application time. In 1987, however, dry matter production was greater when N was applied at planting compared to split applications or applications later than 3 wk after planting. Application time affected N uptake patterns during the growing season, but generally did not affect total N uptake at the end of the season. With the exception of the first year, split N applications resulted in equivalent or reduced N uptake compared to application of all N at planting. Based on linear regression models, maximum yield was obtained with 134, 116, and 93 kg N ha-1 in 1987, 1988, and 1989, respectively. After the first year, applying N later than 6 wk after planting reduced grain yield and split applications of N were not effective in increasing grain yield. These results suggest that the fertilizer N requirement of corn grown in winter legume conservation-tillage systems on Coastal Plain soils decreases with successive years in the system and that the optimum management practice for conservation of N, energy, time, and labor would be to apply all fertilizer N at planting.