Field application of industrial by-products as Al toxicity amendments:: chemical and mineralogical implications

Lime, gypsum and various gypsum-like by-products have long been applied to soil surfaces as ameliorants of soil acidity and aluminium and manganese toxicity. We examined changes in chemical and mineralogical properties at two different depths in two acid soils one year after the application of gypsum, phosphogypsum + dolomitic residue, red gypsum + dolomitic residue, sugar foam, and sugar foam + mined gypsum. All treatments were found to increase the proportion of Ca2+ and decrease those of Al3+ and Mn2+ in the exchange complex of the surface and subsurface horizons, thus reducing its effective Al and Mn saturation. However, the mined gypsum treatment resulted in losses of Mg2+ from the Ap horizon of the soils, and the sugar foam treatment was not so effective with the AB horizons as the other treatments. The combined application of both gypsum-like by-products and the dolomitic residue proved the most effective choice with a view to reducing the effective Al and Mn saturation of the exchange complex in the Ap and AB horizons. In addition, both treatments reduced Mg2+ losses at both depths. Finally, all treatments resulted in the formation and retention on mineral and organic surfaces of a large fraction of the Al3+ released by exchange with Ca2+ as Al polymers. This is quite consistent with the observed changes in the CuCl2-, oxalate- and DTPA-extractable Al contents as well as by SEM and EDS analyses. Based on these results, the use of the appropriate mixtures of these by-products is an effective alternative to that of mined gypsum and lime to alleviate soil acidity and reduce toxic concentrations of Al3+ and Mn2+ in agricultural acid soils.