NITROGEN AND IRRIGATION MANAGEMENT-PRACTICES TO MINIMIZE NITRATE LEACHING FROM IRRIGATED CORN

Careful management of both N fertilizer and irrigation water is required to minimize NO3- leaching below the root zone in irrigated corn (Zea mays L.) production. Practices related to management of fertilizer N and irrigation water were evaluated in a series of studies conducted at 79 sites in Nebraska, from 1984 through 1988. Practices evaluated included N credit from NO3- in soil, N credit from NO3- in irrigation water, realistic yield goal selection, and irrigation scheduling according to crop water use. Nitrogen was applied in field length strips at the recommended rate, and at rates 50 lb N/acre above and below the recommended rate. Groundwater NO3--N concentrations at sites ranged from 0.5 to 46.1 ppm. The procedure for determining the recommended fertilizer N rate provided adequate N without reducing yields. Averaged over 79 sites, yield goal was 170 bu/acre; recommended fertilizer N rate was 130 lb/acre; yield was 173 bu/acre; and fertilizer N reduction due to accounting was 45 lb N/acre. Because of the often high NO3--N concentrations in irrigation water and substantial amounts of NO3- in soil (ranging from 15-265 lb/acre in 4 ft), grain yield was relatively insensitive to fertilizer N rate. With average values for soil and irrigation water N credits, increasing the fertilizer N rate by 100 lb/acre increased yield by only 1.3%. Decreasing the fertilizer N rate by 50 lb/acre decreased yield by only 2.6%. At the three primary N rates used in these studies (-50, 0, and 50 deviation from the recommended rate), irrigation water NO3--N concentration, irrigation water amount, and soil NO3- level all influenced yield more than fertilizer N rate.