Soil chemical properties as influenced by tillage and nitrogen source, placement, and rates after 10 years of continuous sorghum

Tillage and nitrogen (N) management practices can have profound effects on the variations in plant nutrients and other soil chemical properties with d epth in the soil. Relatively few studies have reported on such effects with grain sorghum (Sorghum bicolor (L.) Moench) compared to crops such as corn (Zea mays L.). These changes can influence crop yields, nutrient utilizati on, and soil and water quality. The purpose of this study was to determine the effects of tillage and N management practices on selected soil chemical properties with depth after 10 years of grain sorghum production under no- till (NT) and chisel-disk (CD) systems. Ammonium nitrate and urea ammonium nitrate (UAN) solution were evaluated at 34, 67, and 135 kg Nka, The UAN so lution was applied either broadcast on the soil surface or knifed below the soil surface and ammonium nitrate was broadcast on the soil surface. Soil samples were collected from 0 to 15 cm by 2.5 cm increments, 15-30 cm by 5 cm increments, and from 30 to 40 cm. Soil pH decreased in the upper 5 cm of soil for NT with increasing N rates from surface broadcast applications of ammonium nitrate or UAN solution with no such effects for subsurface place ment of UAN. The largest decrease was from 6.7 to 5.2 at 0-2.5 cm for ammon ium nitrate. For CD treatments receiving 135 kg N/ha, soil pH was significa ntly lower in the upper 10 cm compared to treatments receiving less than or equal to 67 kg N/ha. The lowest pH found at the soil surface for NT was 5. 2 compared to 5.8 for CD. Extractable P was significantly higher for NT com pared to CD at 0-2.5 and 2.5-5.0 cm while similar results were obtained for extractable Ca down to 10 cm. Extractable K was not influenced by tillage, but extractable Mg was higher for CD compared to NT below 15 cm. For NT, o rganic carbon (C) concentrations were higher near the soil surface compared to CT and increased with increasing N rates. The largest increases were se en at 0-2.5 cm for broadcast ammonium nitrate where organic C increased fro m 14.5 to 19.2 g/kg. Organic C concentrations were higher for treatments re ceiving N than for the controls with CD. This study demonstrates that tilla ge and N source, placement and rate can influence the distribution of soil chemical properties with depth. (C) 1999 Elsevier Science B.V. All rights r eserved.