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Amending subsoil acidity by surface applications of gypsum, lime, and composts

Crop growth on many highly weathered soils could be severely restricted by the subsoil acidity, which is characterized by low calcium (Ca) and high aluminum (Al) at depths below the plow layer. Since surface applied lime may not improve subsoil conditions, alternatives must be sought. Thus, effects of composts alone or in combination with lime on increasing Ca and decreasing Al in subsoil were evaluated in a simulated soil profile. An acid Ultisol was packed in a 50-cm long column with the top 15 cm being amended with either lime (CaCO3), gypsum (CaSO4·2H2O), a yard waste compost, a bio-solids-based compost (Nitrohumus), or a combination of compost and lime. The column was leached with 40 mL deionized water daily at a rate of 10 mL per 15 min for 27 days (40 cm water). Thereafter, the column was dismantled and cut into 15, 10, 10, 10 cm layers from the top for chemical analysis. Results showed that lime markedly increased pH and reduced exchangeable Al of the surface layer, but had little effect on subsoil pH. Only 7.6% of the applied Ca from lime moved from the applied layer to the next 10-cm layer, while more than 60% of the applied Ca from gypsum moved past the applied layer. Composts effectively reduced exchangeable Al of the top layer, and of the top two layers when applied together with lime. More Ca was found in deeper soil layers when lime and the Nitrohumus compost were applied together than when either material was applied alone. The downward Ca movement was mainly assisted by SO4 2− even in the compost treatments.