Reactions buffering pH in acid soils treated with lime

Abstract

Soil pH and exchangeable cation data from a glasshouse experiment with 18 acid soils treated with 6 rates of lime, was used to test a hypothesis that the pH buffering capacity (pHBC) of acid soils is dominated by reactions that cause changes in exchangeable Al (subsequently referred to as Al) and the effective cation exchange capacity (ECEC). The pH_Ca (pH in 0.01 M CaCl₂) varied from 4.0 to 7.5. The hypothesis was consistent with the data for weakly weathered soils such as black earths, red earths, red-brown earths and for strongly weathered soils that are low in solid phase SiO₂ such as krasnozems. For these soils the ratio (ΔAl + ΔECEC)/(Δlime) was not significantly different from 1 (all variables were expressed in cmol_c kg^-1 soil) where the mean pH for the lime increment (Alime) was less than 6.0. The ratio declined to zero near pH 7.4, within the pH range where CaCO₃ becomes saturated in 0.01 M CaCl₂. For strongly weathered soils that were also high in solid phase SiO₂ (<70%) the ratio was less than 1.0 from pH_Ca 4.0 to 6.0.

Among this class of soils the degree of decline in the ratio was weakly weathered podzolic and solodic soils > strongly weathered podzolic and solodic soils > acid earthy sands. Lower amounts of evapotranspiration from the SiO₂ dominated soils in the pH range 4 to 6 were consistent with the proposition that limestone dissolution was incomplete in these soils due to less water flow past the limestone particles. The possible involvement of the precipitation of alumino-silicates was therefore discounted.