Evaluation of industrial by-products as soil acidity amendments:: chemical and mineralogical implications

Lime, mined gypsum and some gypsum-like by-products have been frequently applied to soil to counter acidity. We have examined the changes in the chemical and mineralogical properties of three acid soils resulting from the application of three industrial by-products (phosphogypsum, red gypsum and sugar foam). We did so in laboratory experiments on suspensions of soil in saturated solutions of the by-products. A sizeable fraction of the Al released by exchange with Ca of the amendments remained on the mineral surfaces as Al polymers, as suggested by changes in the CuCl2-, oxalic-oxalate- and DTPA-extractable Al contents. Polymerization of Al was promoted by corresponding neutral and basic pH of red gypsum and sugar foam saturated solutions, respectively. Sorption of these Al polymers was particularly favourable in those horizons with more clay fraction and variable-charge clay minerals. On the other hand, in most cases there was little or no detectable sorption of sulphate, thus excluding precipitation of crystalline or amorphous basic Al sulphates to alleviate Al toxicity. Based on the reduction of the Al saturation of the exchange complex in the soils, as well as on the small contents of heavy metals and natural radionuclides of the three by-products, these can be considered effective alternatives to mined gypsum and lime for alleviating soil acidity and reducing toxic concentrations of Al in agricultural, acid soils.