The reaction rate and residual value of particle size fractions of limestone in southern New South Wales

Grain yield is frequently constrained by soil acidity in southern Australia yet limestone crushing plants are few and distant, making the use of limestone costly. The efficient technology of agricultural liming is therefore critical to the continuation of the practice following its adoption during the 1980s. We hypothesise that finer particles are the most effective materials for ameliorating soil acidity even over the longer term, when the residual value of coarser particles might be expected to be greater. Finer particle sizes of limestone, particularly <0.075 mm, initially gave the largest increases in soil pH per tonne of limestone applied. Despite the rapid and large increase in soil pH with finer particles, there was no less residual value in surface soil pH after 7 years or in grain yield in the 7th and 8th growing seasons compared with coarser particles. Most particle size fractions of limestone converged to a similar soil pHca at 0–10 cm depth after about 6 years but the coarsest particle size fraction (2–5 mm) lagged the other five. Finer particles also resulted in better movement of alkali and Ca into the subsurface soil layers below the depth of incorporation (0–10 cm). The measurement of unreacted limestone in the soil showed that the dissolution of limestone took up to 3 years (1807 mm of rainfall) for the 2.5 t/ha rate and up to 6 years (3592 mm) for the 5 t/ha rate. The rapid increase in soil pH in Year 1, the slow ongoing reaction of limestone over 3–6 years as measured by unreacted limestone, the slow but measurable improvement in subsurface acidity, and the sustained residual value to grain yield over in excess of eight seasons, indicate that the use of finer liming materials should remain a viable practice for growers.