RELATIONSHIPS BETWEEN EXTRACTABLE A1, SELECTED SOIL PROPERTIES, PH BUFFER CAPACITY AND LIME REQUIREMENT IN SOME ACIDIC QUEENSLAND SOILS

The objectives of this study were to examine (1) interrelationships between various forms of extractable Al and selected soil properties, (2) the contribution of extractable Al to pH buffer capacity, and (3) investigate the use of extractable Al to predict lime requirement. Aluminium was extracted from each of 60 Queensland soils with a range of chloride salts: 1 M KCl (Al(K)), 0.5 M CuCl2 (Al(Cu)), 0.33 M LaCl3 (Al(La)) and 0.01 M CaCl2 (Al(Cu)). The amounts of Al extracted were in the order Al(Cu) > Al(La) > Al(K) > Al(Ca). Little or no Al was extracted by KCl or LaCl3 in soils with pH(W) values greater than 5.5, whereas CuCl2 extracted some Al irrespective of soil pH. The greater amounts of Al extracted by CuCl2 were attributed mainly to Al from organic matter, even though all of the soils were mineral soils (organic carbon less-than-or-equal-to 4.7%). Both Al(Cu) and Al(La) were significantly (P < 0.001) correlated with organic carbon, whereas none of the extractable Al measures was correlated with clay content. Al(K) and Al(La) were poorly correlated to pH buffer capacity. The linear relationship between Al(Cu) and pH buffer capacity (r2 = 0.49) obtained in this study supports the view of previous researchers that the hydrolysis of Al adsorbed by organic matter is a source of pH buffering in soils. However, the change in CEC with pH accounted for 76% of the variation in pH buffer capacity, indicating that other mechanisms such as deprotonation of organic groups and variable charge minerals are also involved in pH buffering. The ability of CuCl2 and LaCl3 extractable Al to estimate lime requirement depended on the target pH. The results suggest that lime requirements based on neutralization of Al(La) would be sufficient to raise pH(w) to around 5.5, whereas requirements based on neutralization of Al(Cu). substantially overestimated the actual lime requirement to pH(w) 5.5, but gave a reasonable estimation of the lime requirement to pH(W) 6.5.