LIME REQUIREMENT OF ACIDIC QUEENSLAND SOILS .1. RELATIONSHIPS BETWEEN SOIL PROPERTIES AND PH BUFFER CAPACITY

The pH buffer capacity of 40 acidic surface soils (pHw<65) was determined from soil-CaC03-moist incubations. Buffer capacity values ranged from 02 to 54 g CaCC>3kg-1soil unit-1pH increase. Organic carbon, clay content, ECEC, 1mKC1 extractable acidity and Al, and the change in CEC with pH (ACEC) were measured and correlated with pH buffer capacity. Step-up multiplelinear regression indicated that the effect of ACEC on buffer capacity was highly significant(r2= 0-77, P<0-001), whereas that of exchangeable Al or exchange acidity was not. Thissuggests that deprotonation reactions, compared with exchangeable Al or exchange acidity, are considerably more important in determining buffer capacity. The major soil propertyaffecting ACEC in our soils was the organic carbon content and, when step-up multiple linearregression was used, ACEC could be best estimated by organic carbon plus clay content plusECEC (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 loworganic carbon (<2-5%) were considered. For the 33 soils with <2-5% organic carbon, ACECwas still the major determinant of buffer capacity(r2=0-76, P<0-001), although inclusionof exchange acidity in a multiple regression with ACEC significantly increased the varianceaccounted for (P2= 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 for85%of thevariance in buffer capacity, with organic carbon being the most important. © 1990 CSIRO. All rights reserved.