Lime requirement of acidic Queensland soils.II. Comparison of laboratory methods for predicting lime requirement

A number of laboratory procedures for predicting lime requirement were evaluated by using 40 acidic surface soils from eastern Queensland. The methods were compared on the basis of their ability to predict the lime requirement to pHw values of 5.5 and 6.5 obtained from soil-CaCO3-moist incubations. The laboratory methods evaluated included 1M KC1 extractable Al, equilibration of soil : water suspensions with varying amounts of added Ca(OH)2, the Shoemaker, McLean and Pratt (SMP) single-buffer method, the SMP double-buffer method, the Yuan double-buffer method and the Mehlich single-buffer method. Aluminium extracted ,with 1M KCl was a poor predictor of lime requirement to pH, 5.5. In most of the soils tested, the actual amounts of lime required to reduce Al to a predetermined level far exceeded those calculated on the basis of 1 M KCl extractable Al values of untreated soils. Batch equilibration of soil : water suspensions containing Ca(OH)2 proved a reliable but relatively time-consuming method of determining lime requirement. All of the buffer methods were reasonably well correlated with lime requirement (0.61 < r2 < 0.82). Buffer methods which had a high initial buffer pH and a relatively high buffer strength were less well correlated with lime requirement than weaker buffers of lower initial pH. The Mehlich single-buffer method (initial pH 6.6) fitted both these latter criteria and gave good correlations with lime requirements to pHw 5.5 (r2 =0.78) and pHw 6.5 (r2 = 0.80). Compared with the single-buffer methods, neither of the double-buffer methods (which require two pH measurements) was better correlated with lime requirements.