Amelioration of Subsurface Acidity in Sandy Soils in Low Rainfall Regions. II. Changes to Soil Solution Composition Following the Surface Application of Gypsum and Lime

Two field trials were sampled to investigate the changes to soil solution chemical properties of a yellow sandplain soil with an acidic subsoil following the application of gypsum and lime to the soil surface in 1989. The soils were sandy textured and located in a region of low annual rainfall (300-350 mm). Soil was sampled annually to a depth of 1 m and changes in soil solution composition were estimated by extraction of the soil with 0.005 M KCl. Gypsum leaching caused calcium (Ca), sulfate (S04) and the ionic strength to increase substantially in both topsoil and subsoil by the end of the first year. Continued leaching in the second year caused these properties to decrease by approximately one-half in the topsoil. Gypsum appeared to have minimal effect on pH or total A1 (AIT), although the amount of A1 present as toxic monomeric A1 decreased and the amount present as non-toxic Also; ion pairs increased. Magnesium (Mg) was displaced from the topsoil by gypsum and leached to a lower depth in the subsoil. In contrast, lime caused pH to increase and A1 to decrease substantially in the topsoil, but relatively little change to any soil solution properties was observed in the subsoil. There was an indication that more lime may have leached in the presence of gypsum in the first year after application at one site. Wheat yields were best related to the soil acidity index AIT/EC (where EC is electrical conductivity of a 1:5 soi1:water extract), although the depth at which the relationship was strongest in the subsoil varied between sites. The ratio AIT/EC was strongly correlated with the activity of monomeric A1 species (i.e. the sum of the activities of ~ l ~ + , A1OH2+ and AI(OH)$) in the soil solution. An increase in the concentration of sulfate in the subsoil solution (which increased the ionic strength, thereby decreasing the activity of ~ 1 ~ + , and also increased the amount of A1 present as the Also: ion pair) was probably the most important factor decreasing A1 toxicity to wheat. The results indicated that gypsum could be used to increase wheat growth in aluminium toxic subsoils in sandy soils of low rainfall regions and that a simple soil test could be used to predict responses.