WATER-QUALITY - NITRATE AND WATER PRESENT IN AND FLOWING FROM ROOT-ZONE SOIL

From 1990 through 1992, we studied water and nitrate-nitrogen (NO3-N) present in the soil and flowing into a subsurface drainage system. Til lages were chisel plow (CP), moldboard plow (MB), no-till (NT), and ri dge-till (RT). Crops were continuous corn (Zea mays L.) and a corn-soy bean [Glycine max (L.) Merr.] rotation. Tillage and crop only slightly influenced soil NO3-N and water. Drainage water flows were highest fo r treatments that minimized soil disturbance and maximized crop residu es produced during the previous year. The 3-yr total flows by crop wer e 38 cm water for rotation-corn plots, 56 for rotation soybean, and 59 for continuous corn, Flows by tillage were 41 cm water for MB plots, 52 for RT, and 55 for both CP and NT. In continuous corn, NT plots had more monthly water drainage than MB plots for most of 1990. Ridge-til l and CP plots had more drainage than MB plots for part of 1991. Crop rotation had the greatest effect on NO3-N drainage loss. The 3-yr tota l NO3-N losses were 77 kg ha(-1) for rotation corn plots, 84 for rotat ion soybean, and 164 for continuous corn, Tillage losses were 95 kg ha (-1) for RT, 102 for MB, 106 for NT, and 131 for CP. No-till and RT pl ots always had the lowest NO3-N concentrations in drainage water; thei r yearly NO3-N losses were usually smallest. Leaching can be best mini mized by applying fertilizer in amounts to just meet crop N demand and at times closest to peak uptake.