Chemical attributes of some Queensland acid aoils. I. Solid and solution phase compositions

Abstract

Samples from surface and subsoil horizons of 91 acid soils in Queensland were analysed for pH, EC, Cl, exchangeable cations and organic C. Generally low values for EC, Cl, exchangeable Na and exchangeable K were found in surface soils and subsoils. Higher values of organic C, exchangeable Ca and pH occurred more frequently in surface soils, while exchangeable A1 and exchangeable Mg were generally higher in subsoils. A correlation matrix gave significant, but not strong, linear correlations between soil attributes associated with soil acidity (pH and Ca, Mg and Al saturations). Soil solutions were extracted from surface and subsoil horizons of 48 of these acid soils and analysed for pH, EC, Na, K, Ca, Mg, SO4 and monomeric Al. Ionic strengths and activities of monomeric Al species were calculated. Soil solution pH tended to be lower in subsoils than in surface soils, but very few values were <4.5. Ionic strengths were mostly <8 mM, with mean values of 5.3 mM for surface soils and 2.4 mM for subsoils. Subsoils had lower mean concentrations of Ca, Mg and K, and similar concentrations of Na, compared with surface soils. In both groups, molar concentrations followed the order Na > K ~ Mg > Ca. Mean values for activities of AlOH2+, Al(OH)2+, Al(OH)3 and AlSO4+ in surface soils exceeded those for subsoils, but activities of Al3+ were similar. Higher activities of Al species in surface soils resulted from higher monomeric Al concentrations, but it was concluded that the analytical method overestimated monomeric Al in surface soils, probably by including some of the soluble organic Al complexes present. The best correlation between pH and Al species was with Al3+ , particularly in subsoils (r2 = 0.913). Activity of Al3+ in subsoils was correlated with ionic strength (r2 = 0.666) when both were expressed on a logarithmic scale.