FURROW IRR IGATION AND N MANAGEMENT STRATEGIES TO PROTECT WATER QUALITY

Abstract

N management under furrow irrigation is difficult because nitratenitrogen (NO3-N) is frequently leached to groundwater. Banding and sidedressing N fertilizer on a non-irrigated side of a row of corn (Zea mays L.) might increase N uptake and minimize nitrate leaching potential by reducing the NO3-N in soil profiles at harvest, thereby protecting water quality. For two years in the field, we evaluated two N placements (broadcast vs. banded), two row spacings (0.76-m vs. a modified 0.56-m), and two ways of positioning irrigation water (applying water to the same side or alternating sides of the row with successive irrigations) for their effects on N uptake in corn silage and soil profile NO3-N (to the 0.9-m depth). In southern Idaho, we grew field corn in Portneuf silt loam (coarse silty, mixed superactive, mesic Durinodic Xeric Haplocalcid) by irrigating every second furrow nine times in 1988 and seven times in 1989. We measured N uptake by harvesting whole plants at physiological maturity and NO3-N in soil samples taken at two in-row locations in selected plots after each irrigation. Where irrigating alternating sides of the row, two-year average N uptake from 0.76-m rows was 131 kg ha 1, 15% greater (P < 0.001) than from 0.56-m rows. Where irrigating the same furrow all season, N uptake from banding equaled that from broadcasting the first year but was 21% greater (P < 0.001) the second. Applying water to the same furrow decreased profile N by about 170 kg ha 1 under 0.76-m rows by season's end in 1988. In 1989, irrigating the same furrow and banding N into an adjacent, never-irrigated furrow produced season-average profile N of a) 303 kg ha 1, the least under all fertilized 0.76-m rows, and b) 152 kg ha 1 under 0.56-m rows, half that under similarly treated 0.76-m rows. Our findings suggest that corn in 0.76-m rows should be fertilized by banding N into every second furrow and irrigated season-long using the remaining, non-fertilized furrows because those practices maintained or increased N uptake in silage and minimized residual NO3-N in 0.9-m soil profiles at season's end.