The influence of surface incorporated lime and gypsiferous by-products on surface and subsurface soil acidity. I. Soil solution chemistry

Lime, fluidised bed boiler ash (FBA), and flue gas desulfurisation gypsum (FGDG) were incorporated in the top 50 mm of repacked columns of either an Allophanic (the Patua sand loam) or an Ultic (the Kaawa clay loam) soil, at rates containing calcium equivalent to 5000 kg/ha of CaCO₃. Each column was leached with 400 mm of water. After leaching, the columns were sliced into sections for chemical analysis. In the columns of the variable-charged, allophanic Patua soil, topsoil-incorporated FBA ameliorated top and subsurface soil acidity through liming and the 'self-liming effect' induced by sulfate sorption, respectively. The soil solution pH of the top and subsurface layers of the Patua soil were raised to pH 6.40 and 5.35, respectively, by the FBA treatment, compared with pH 4.80 and 4.65, respectively, in the control treatment. Consequently, phytotoxic labile monomeric aluminium (Al) concentration in the soil solution of the FBA treatment was reduced to <0.1 μM Al, compared with 8-64 μM Al in the untreated control. FGDG had a similar 'self-liming effect' on subsurface of the Patua soil, but not the topsoil. Whereas FBA raised the pH of the Kaawa topsoil, no 'self-liming effect' of subsurface soil by sulfate sorption was observed on the Kaawa subsurface soil, which is dominated by permanently charged clay minerals. Application of FBA and FGDG to both soils, however, caused significant leaching of native soil Mg²⁺ and K⁺. These nutrients were displaced from the exchange sites by the relatively high concentration of Ca²⁺ released from dissolution of gypsum.