Surface and Subsurface Soil Acidity: Soybean Root Response to Sulfate-Bearing Spent Lime

Surface and subsurface acidity are of agronomic concern in the southeastern USA. The limits to crop production imposed by subsoil acidity need to be further defined for this region. Gypsum is being applied to ameliorate subsoil acidity and to overcome the problem of very slow lime movement from surface lime applications. Reduced crude conversion spent lime (RCCSL) containing anhydrite (CaSO₄) was evaluated for use as a liming material with specific consideration given to the movement of SO^2-₄ into the acid subsoil. Agricultural lime and RCCSL were applied at 0, 3.36, 6.72, and 10.08 Mg ha⁻¹ to an acid Trappist silt loam (clayey, mixed, mesic Typic Hapludult). Soybean [Glycine max (L.) Merr.] was grown in the second year following lime application. Soil cores to 0.90 m were taken at growth stage R5 for root length measurement and soil chemical analyses. Yields were increased by the soil amendments and were well correlated with total root length (R² = 0.84). Root growth below the surface limed layer was positively related to increased soil solution base cation (especially Ca) concentrations and SO^2-₄ movement into the subsoil. Previously described solution Ca plus Al species activity models were not well related to field rooting responses. There remains a need for soil test procedures that predict the need for, and the amount of, subsurface acidity amelioration.