AMELIORATION OF SUBSURFACE ACIDITY IN SANDY SOILS IN LOW RAINFALL REGIONS .2. CHANGES TO SOIL SOLUTION COMPOSITION FOLLOWING THE SURFACE APPLICATION OF GYPSUM AND LIME

Two field trials were sampled to investigate the changes to soil solut ion chemical properties of a yellow sandplain soil with an acidic subs oil following the application of gypsum and lime to the soil surface i n 1989. The soils were sandy textured and located in a region of low a nnual rainfall (300-350 mm). Soil was sampled annually to a depth of 1 m and changes in soil solution composition were estimated by extracti on of the soil with 0.005 M KCl. Gypsum leaching caused calcium (Ca), sulfate (SO4) 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 approximatel y one-half in the topsoil. Gypsum appeared to have minimal effect on p H or total Al (Al-T), although the amount of Al present as toxic monom eric Al decreased and the amount present as non-toxic AlSO4+ ion pairs increased. Magnesium (Mg) was displaced from the topsoil by gypsum an d leached to a lower depth in the subsoil. In contrast, lime caused pH to increase and Al to decrease substantially in the topsoil, but rela tively little change to any soil solution properties was observed in t he subsoil. There was an indication that more lime may have leached in the presence of gypsum in the first year after application at one sit e. Wheat yields were best related to the soil acidity index Al-T/EC (w here EC is electrical conductivity of a 1:5 soil:water extract), altho ugh the depth at which the relationship was strongest in the subsoil v aried between sites. The ratio Al-T/EC was strongly correlated with th e activity of monomeric Al species (i.e. the sum of the activities of Al3+, AlOH2+ and Al(OH)(2)(+)) in the soil solution. An increase in th e concentration of sulfate in the subsoil solution (which increased th e ionic strength, thereby decreasing the activity of Al3+, and also in creased the amount of Al present as the AlSO4+ ion pair) was probably the most important factor decreasing Al toxicity to wheat. The results indicated that gypsum could be used to increase wheat growth in alumi nium toxic subsoils in sandy soils of low rainfall regions and that a simple soil test could be used to predict responses.