Short- and Long-Term Effects of Lime and Gypsum Applications on Acid Soils in a Water-Limited Environment: 3. Soil Solution Chemistry

Aluminum (Al) toxicity imposes a significant limitation to crop production in South Western Australia. This paper examines the impact of surface-applied lime and gypsum on soil solution chemistry in the short term (1 year) and the long-term (10 years) in water limited environments. In the experiments, we measured soil solution chemistry using a paste extract on soil profile samples collected to a depth of 50 cm. We then used the chemical equilibrium model MINTEQ to predict the presence and relative concentrations of Al species that are toxic to root growth (Al associated with Al3+ and AlOH2 or Toxic-Al) and less non-toxic forms of Al bound with sulfate, other hydroxide species and organic matter. A feature of the soils used in the experiment is that they have a low capacity to adsorb sulfate. In the short term, despite the low amount of rainfall (279 mm), sulfate derived from the surface gypsum application is rapidly leached into the soil profile. There was no self-liming effect, as evidenced by there being no change in soil solution pH. The application of gypsum, in the short term, increased soil solution ionic strength by 524-681% in the 0-10 cm soil layer declining to 75-109% in the 30-40 cm soil layer due to an increase in soil solution sulfate and calcium concentrations. Calcium from the gypsum application displaces Al from the exchange sites to increase soil solution Al activity in the gypsum treatments by 155-233% in the short term and by 70-196% in the long term to a depth of 40 cm. However, there was no effect on Toxic-Al due to Al sulfate precipitation. In the long term, sulfate leaching from the soil profile results in a decline in soil solution ionic strength. Application of lime results in leaching of alkalinity into the soil profile leading to a decreased Toxic-Al to a depth of 30 cm in the long term, but it did not affect Toxic-Al in the short term. Combining an application of lime with gypsum had the same impact on soil solution properties as gypsum alone in the short term and as lime alone in the long term.