Abstract
Soil acidity and aluminum (Al) toxicity are significant soil constraints for agricultural production worldwide. Soil acidification, or a decrease in soil pH, is a natural process increased by agriculture due to product removal and nitrate leaching. Its effect on plant production is measured by dividing the soil profile into layers based on nutrient availability. In most regions of Australia, a 0-10cm soil layer is used to define the topsoil, while the subsoil is defined as soil layers below 10cm. The soil acidification process results in an acidic layer typically forming in the 10-30cm soil layer. Hence, subsoil acidity or the occurrence of soil Al is a greater issue than topsoil acidity. Acidity is used to describe the overall problem, but soil Al is the soil factor resulting in reduced root growth and agricultural production. The main toxic species in soils is @equ_0001.eps@, which is abbreviated to Al3+. A soil pH measurement defines the degree of soil acidity, while soil Al3+ content measures the degree of Al toxicity. Both pH and Al3+ are commonly measured using 0.01M CaCl2 at a soil:solution ratio of 1:5 (pHCaCl2 and AlCaCl2). Topsoil with pHCaCl2 less than 5.0 and subsoil with pHCaCl2 less than 4.5 are classified as acidic. At the same time, soils with AlCaCl2 greater than 2.5-4.5mg Al/kg in 0-30cm soil layers restrict wheat (T. aestivum) production.
Lime application to the soil surface effectively treats topsoil acidity/Al3+ toxicity but is ineffective in treating subsoil Al3+ toxicity. The low effectiveness is due to lime's low solubility combined with soil, environmental, and lime quality restrictions which limit lime dissolution and alkalinity movement from the topsoil to the subsoil. Hence, subsoil Al3+ toxicity management requires combining lime with gypsum applications, using strategic tillage to increase lime dissolution in the topsoil and redistributing lime into the subsoil, and growing Al3+ tolerant crop species. Topsoil alkalinity movement into the subsoil occurs when sufficient lime is applied to maintain the topsoil pHCaCl2 between 5.5 and 7.5. Maintaining pHCaCl2 within this range creates a pool of excess lime alkalinity which is leached into the subsoil to mitigate subsoil Al3+ toxicity. The conservation farming system or the no-tillage seeding system results in the stratification of surface-applied lime in soil layers near the surface. Strategic tillage (surface and deep) is used to increase the effectiveness of surface-applied lime in ameliorating subsoil Al3+ toxicity. Strategic surface tillage increases lime dissolution in the topsoil to achieve the target pHCaCl2 range faster, resulting in greater rates of alkalinity movement. Strategic deep tillage practices redistribute a lime-rich topsoil layer into the subsoil, typically to a depth of 40cm. Gypsum is more soluble than lime, and the applied calcium and sulfate are leached rapidly to treat subsoil Al3+ toxicity. Its application with lime can increase alkalinity movement into the subsoil when measured using a change in AlCaCl2 at pHCaCl2 less than 4.5. Further research is required to understand the soil chemistry process involved and to identify soil types or properties where gypsum and other organic amendments applied with lime can improve alkalinity movement.