COMPARATIVE STUDY OF THE FITNESS OF VARIOUS LIMESTONE AND GYPSUM BYPRODUCTS FOR AMENDING Al TOXICITY IN A PLINTHIC

The topmost part (Ap and AB horizons) of a Plinthic Palexerult from the Cañamero raña (western Spain) was reconstructed in percolation columns in order to compare the effects of the application of various byproducts including sugar foam waste (SF), phosphogypsum (PG) and red gypsum (RG) on the dynamics of Al3+ ion in the soil solution. Also, the action of these byproducts was compared with that of conventional amendments such as quarry limestone (QL), and gypsum (QG) Both QL, and SF were found to dramatically decrease the Al3+ content of the soil solution by raising the pH and precipitating the ion as insoluble hydroxides. Both (QG), and the gypsum-based byproducts increased the content in total Al upon application of the first few percolation rates; however, a sizeable fraction of Al was in the form of ion-pairs with sulphate and fluoride ions —the latter were displaced from the soil matrix by sulphate ions from the gypsum materials in the QG case—; this decreased the concentration and activity of Al3+ in the soil solution relative to the blank columns. With PG, F – displaced from the soil added to that contained in this byproduct, which increased the amount of Al–F ion-pairs formed relative to the other two gypsum-based byproducts. Unlike the calcareous amend-ments, the gypsum-based amendments also affected the AB horizon. After treatment with the amount of water equivalent to one year of precipitation in the field, soil from the Ap horizon in each column was subjected to a productivity test in pots that were sown with Triticum durum sp and received the same NPK rates used in the field tests but no external calcium or sulphate. The favourable effect of the calcareous and gypsiferous amendments resulted from the increased Ca and decreased Al content of the exchange complex and a increased Ca/Al ratio in the soil solution