SOIL-PROFILE DISTRIBUTION OF PRIMARY AND SECONDARY PLANT-AVAILABLE NUTRIENTS UNDER CONVENTIONAL AND NO-TILLAGE

Nutrient distributions under no tillage (NT) compared with conventiona l disk-and-bed tillage (CT) management in the warm, humid region of th e southeastern USA need to be assessed so that future placement, quant ity, and type of fertilizers can be altered, if necessary, to efficien tly match crop demands. We determined soil-profile distributions of pH , N, P, S, K, Ca, Mg, Na, Zn, Fe, Mn, and Cu to a depth of 0.9 m at th e end of 8.5 years of continuous CT and NT management on a Weswood sil ty clay loam (fine, mixed, thermic Fluventic Ustochrept) in southcentr al Texas. Most dramatic changes occurred within the 0-0.05 m depth, wh ere soil under NT had lower pH, Fe, and Cu than under CT, but greater P, K, Zn, and Mn. Greater P and K under NT than under CT also occurred below the till-zone (0.15-0.3 m). At a depth of 0-0.3 m, soil under N T contained greater amounts of extractable P, K, Zn, Fe, Mn, and Cu th an under CT. Nitrogen fertilization had little effect on nutrient dist ributions, except resulting in greater extractable K at 0-0.05 m and g reater nitrate at 0-0.15 m. Few changes in soil-profile distributions were observed for extractable S, Ca, Mg, and Na. Long-term continuous use of NT on this fine-textured, high-fertility (except for N) soil ha d no apparent adverse effects on nutrient distributions relative to CT , but enhanced conservation and availability of P, K, Zn, Fe, Mn, and Cu near the soil surface where crop roots proliferate.