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

Abstract

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 (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 approximately one-half in the topsoil. Gypsum appeared to have minimal effect on pH or total Al (Al-T), although the amount of Al present as toxic monomeric Al decreased and the amount present as non-toxic AlSO+4 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 Al 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 Al-T/EC (where EC is electrical conductivity of a 1:5 soil:water extract), although the depth at which the relationship was strongest in the subsoil varied between sites. The ratio Al-T/EC was strongly correlated with the 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 the concentration of sulfate in the subsoil solution (which increased the ionic strength, thereby decreasing the activity of Al3+, and also increased the amount of Al present as the AlSO+4 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 aluminium toxic subsoils in sandy soils of low rainfall regions and that a simple soil test could be used to predict responses.

Lime and gypsum application on the wheat crop

“…In soils with Al toxicity in the subsurface layers, the improvement in growth and the increase in wheat yield with the use of gypsum have been related to the increase in the concentration of sulfate in the subsoil solution, which causes an increase in ionic force and a decrease in Al 3+ activity through the establishment of the ionic pair AlSO 4 + (Mclay et al, 1994). Since in the present work Al levels were not very high, this effect must not have been the most relevant.…”

Application of nitrogen in NO 3 − form increases rhizosphere alkalisation in the subsurface soil layers in an acid soil

“…In recent years, subsurface acidity has been given much attention due to the difficulties of amelioration (Hue and Licudine 1999;Farina et al 2000;Tang et al 2002. Conventional surface application of lime is known to be ineffective in combating subsurface acidity largely due to slow downward movements of lime (Conyers and Scott 1989;McLay et al 1994;Conyers et al 2003). New strategies are being tested to combat subsurface acidity (Liu and Hue 1996;Hue and Licudine 1999;Weligama et al 2008Weligama et al , 2010.…”

TaALMT1 promoter sequence compositions, acid tolerance, and Al tolerance in wheat cultivars and landraces from Sichuan in China

“…Below pH 5.0, the Al compound often solubilizes to form toxic trivalent cations (Al 3+ ) that limit root growth, water uptake, and absorption of nutrient elements (Kochian, 1995). Farmers are recommended to use alkaline materials such as lime to mitigate Al toxicity in acidic soils, which is not feasible (Mclay et al, 1994). The breeding and cultivation of Altolerant crops is an alternative way to address this issue.…”