Alleviation of aluminum toxicity by phosphogypsum

Since the movement of lime down the soil profile is extremely limited and the incorporation of lime into subsoil requires large amounts of energy, the use of a soluble, surface applied ameliorant that would move into the deeper soil horizons resulting in amelioration of the subsoil is highly desirable. Phosphogypsum (PG), a by‐product of the phosphoric acid industry, has been evaluated for this purpose, and yield responses have been highly promising, although the mechanism of PG induced amelioration of subsoil acidity is not clear. Effects of PG on subsoil solution properties and aluminum (Al) speciation have been evaluated in this study. A subsoil sample from the Appling series’ (Typic Hapludults) was treated with either increasing levels of PG (2, 5, and 10 Mg ha‐1 PG), reagent‐grade CaSO4.2H2O (2 Mg ha‐1), or CaCl2.2H2O (2 Mg ha‐1) and incubated (22 ± 2°C) at ‐0.01 MPa moisture potential. Soil solution was extracted after 14 and 40 d by centrifugation at 2500 g. Soil solution pH was 5.67 in untreated soil, while increasing application of PG from 2 to 10 Mg ha‐1 decreased the soil solution pH from 5.08 to 4.47. The soil solution pH was higher in soils treated with similar rates of PG or CaSO4.2H2O than CaCl2.2H2O. Increasing levels of PG increased the concentrations of Ca, Mg, K, P, Na, Si, Mn, F and SO4 in the soil solution. The concentration of total Al in soil solution was 0.02, 1.95 and 5.25 ppm in soils treated with 2, 5 and 10 Mg ha‐1 PG, respectively. However, Al speciation predicted by the GEOCHEM computer program revealed that at the 5 Mg ha‐1 PG treatment, 99% and 0.6% of total Al was complexed with F and SO4, respectively, while only 0.3% was in Al3+ form. At the 10T ha‐1 PG treatment, although 10% of total Al was in Al3+ form, the activity of Al was only 0.11 ppm. Therefore, an increase in concentrations of F and SO4 in soil solution in PG treated soils may alleviate Al toxicity by formation of less phytotoxic Al‐F and A1‐SO4 complexes. The toxicity of Al may be further decreased by a reduction in activity of Al3+ due to an increase in soil solution ionic strength in PG treated soils.