Agronomic implications of the supply of lime and gypsum by-products to palexerults from western spain

A laboratory experiment with leaching columns reproducing the topmost portion (Ap and AB horizons) of a Palexerult from western Spain was conducted to compare the efficiency of limestone (L), gypsum (G), and various lime and gypsum by-products (sugar foam waste, phosphogypsum [PG], and red gypsum [RG]) to alleviate Al toxicity and improve soil productivity. The effect of the previous five amendments on composition changes in the exchange complex and the soil saturation extract was examined after one or two leaching cycles (equivalent to as many years of field precipitation). The productivity of the Ap horizon after the first leaching cycle was assessed by growing wheat (Triticum aestivum var. Jabato) in a greenhouse. The lime by-products increased the pH of the Ap horizon and the Ca content of the exchange complex to a greater extent than the gypsum amendments; in addition, the former minimized the Al content of the exchange complex and the saturation extract. Changes in the AB horizon were much less marked, and the sugar foam waste was a better liming material than limestone. The changes in the exchange complex and saturation extract from gypsum amendments followed a more complex pattern and affected both horizons (Ap and AB). The pH of the soil was initially decreased relative to the controls through a "salt effect" and then increased after the second leaching cycle, when most of the gypsum was leached from the soil. In both horizons, the Ca content was increased, and the Al content of the exchange complex decreased-albeit to a lesser extent than with the lime amendments in Ap; by contrast, exchangeable Mg and Na were lost in substantial amounts in the first leaching cycle; K losses were comparatively smaller. After the first leaching cycle, aluminum-fluoride ion pairs were the most active Al species in the extracts from Ap and AB in PG amended columns; on the other hand, the aluminum sulfate ion pairs were the most active in the extracts from the G- and RG-amended columns. After the second leaching cycle, the activity of Al-the most phytotoxic ionic species of Al-in the Ap extracts from all calcium carbonate-amended columns decreased, whereas that in the gypsum-amended soil only decreased in the PG-treated columns; in the AB horizon, however, the extracts from all gypsic amendments decreased the Al activity, with respect to control and lime amendments. The calcium aluminum balance () was increased by all treatments. All lime and gypsum amendments-particularly sugar foam waste-increased productivity relative to the controls. Of the three gypsum amendments, RG had the greatest impact on productivity; this can be ascribed to its Zn content, which was very low in the natural soil.