PREDICTING THE FIELD PERFORMANCE OF 12 COMMERCIAL LIMING MATERIALS FROM SOUTHERN AUSTRALIA

The relative performance of 12 commercial liming materials from southe rn Australia was evaluated in a field trial. The materials included 6 calcitic limestones, 3 coralline limestones, 1 earthy limestone and 2 dolomites. All 12 materials were chemically and physically analysed an d assessed for their ability to change soil pH. Chemical efficiency% w as taken to be the traditional CaCO3 - equivalent mass. The particle s ize analysis of each material was used to predict its physical efficie ncy% on the basis of our particle size model for calcitic limestones: Delta pH = a - b log (particle diameter) where a = 2.875 and b = 0.697 for a 5 t ha(-1) application rate at this trial site. The relationshi p is mathematically valid for particle diameters between 1 and 13,000 mu m. All commercial materials were applied at 5 t ha(-1). The particl e size model was determined independently but concurrently at the same site. The model for physical efficiency % was expressed relative to a microfine grade of limestone (99% < 75 mu m) which makes it transfera ble to other sites. The product of relative physical efficiency % and chemical efficiency (CaCO3 equivalent)% was used to predict the total efficiency % of each liming material in increasing soil pH. A comparis on of the predicted total efficiency % was made with the actual total efficiency% of the 12 materials as measured in the field. During the f irst cropping season the model predicted the performance of the 6 calc itic limes: predicted = 0.94 actual + 3.95 r = 0.97 The slope was not significantly different to unity nor the intercept to zero. The dolomi tes performed, on average, 15% poorer than predicted by the calcite-ba sed model while the softer limes (coralline and earthy) performed 20% better than predicted. By 12 to 24 months these differences were atten uated and 10 of the 12 products were well predicted by the model. The performance of liming materials in increasing soil pH is considered to be the result of chemical composition (base content), particle size a nd the solubility of the dominant mineral. We propose the use of our p redictive model to evaluate the relative total efficiency % of liming materials.