Lime requirement of acidic Queensland soils. I. Relationships between soil properties and pH buffer capacity

The pH buffer capacity of 40 acidic surface soils (pHw <6.5) was determined from soil-CaCO3- moist incubations. Buffer capacity values ranged from 02 to 5.4 g CaCO3 kg-1 soil unit-1 pH increase. Organic carbon, clay content, ECEC, 1M KCl extractable acidity and Al, and the change in CEC with pH (ÄCEC) were measured and correlated with pH buffer capacity. Step-up multiple linear regression indicated that the effect of ÄCEC on buffer capacity was highly significant (r2 = 0.77, P <0.001), whereas that of exchangeable Al or exchange acidity was not. This suggests that deprotonation reactions, compared with exchangeable Al or exchange acidity, are considerably more important in determining buffer capacity. The major soil property affecting ÄCEC in our soils was the organic carbon content and, when step-up multiple linear regression was used, ÄCEC could be best estimated by organic carbon plus clay content plus ECEC (R2 = 0.77, P < 0.001). To ascertain whether exchangeable Al (or exchange acidity) would contribute to buffer capacity in soils with less variable charge, soils of relatively low organic carbon (<2.5%) were considered. For the 33 soils with <2.5% organic carbon, ÄCEC was still the major determinant of buffer capacity (r2 = 0.76, P <0.001), although inclusion of exchange acidity in a multiple regression with ÄCEC significantly increased the variance accounted for (R2 = 0.80, P < 0.001). Of the soil properties that could be routinely measured, a multiple regression equation combining organic carbon, clay content and exchange acidity accounted for 85% of the variance in buffer capacity, with organic carbon being the most important.