Calibration of rapid lime requirement determination methods for acidic soils in South Australia

Context Liming is widely used to alleviate soil acidity but its effectiveness is often limited by the difficulty of accurately estimating the amount of lime required to neutralise the acidity. Aims This study aims to evaluate and calibrate several rapid chemical-based laboratory methods and predictive equations, including 30-min titration, 3–5-day equilibration, Mehlich single buffer, and predictive equations based on soil properties (e.g. clay and organic carbon content) against the moist-soil incubation method, which was used as the reference. Methods Lime requirements of seven acidic soils were determined using both rapid methods and the incubation method. The rapid methods were then calibrated against the incubation method using linear and polynomial regression models. Results The lime buffer capacity of the soils ranged within 500–1750 mg CaCO3 kg−1 pH−1 (3–5-day equilibration) and 603–2546 mg CaCO3 kg−1 pH−1 (incubation). The 30-min titration provided the lowest predicted lime rates, ranging from 0.34 t ha−1 for the Yumali (10–20 cm) soil to 1.40 t ha−1 for the Sandilands soil. The 3–5-day equilibration method resulted in higher lime estimates than the other rapid methods. The moist-soil incubation method produced the highest lime requirements (1.52–5.74 t ha−1), exceeding those estimated using the rapid methods by averages of 1.07–3.15 t ha−1. Polynomial lime–pH relationships provided better predictions of equilibrium pH for most soils (R2 ≥ 0.971). Calibration of rapid methods against the incubation results revealed that the 3–5-day equilibration (R2 = 0.947, RMSE = 0.32 t ha−1) and Mehlich single-buffer (R2 = 0.936, RMSE = 0.35 t ha−1) methods achieved the strongest polynomial model fits. Conclusions Overall, calibration improved the predictive accuracy of all rapid methods, and this approach offers a practical tool for large-scale and field-relevant lime requirement estimation for acidic soils. Implications By using calibrated laboratory methods and predictive equations, farmers in the region can reliably estimate the lime requirements of acidic soils.