Field calibration of lime requirement soil tests

Abstract

The incorporation of various rates of lime (0, 0.5, 1, 2, 4, 6 and 8 t ha-1) into the 0–10 cm soil depth at each of 27 field trial sites in southeastern Queensland, Australia enabled the field calibration of laboratory tests for lime requirement. Extractable Al (1M KCl) was measured in soil samples from each lime treatment, and the actual amount of lime required to reduce extractable Al to negligible levels was determined for each site. In addition to laboratory titration with Ca(OH)2, a range of buffer methods (SMP single and double-buffer methods, Yuan double-buffer method and the Mehlich single buffer method) were assessed for predicting field lime requirement to selected target pH values. To attain a target pH value of 5.5, the rates of lime varied from less than 0.5 t ha-1 to 7 t ha-1 highlighting the need for methods capable of predicting lime requirement for individual soils. The actual amounts of lime required to reduce extractable Al to negligible levels were far in excess of, and poorly correlated with, the amounts calculated from extractable Al in the unlimed plots. Laboratory titration with Ca(OH)2 and buffer methods were reasonable predictors of field lime requirement (r2 values ranged from 0.64 to 0.87), but there was no marked advantage in using double-buffer methods compared with single-buffer methods. Inclusion of soil pH measurement with the soil-buffer pH measurement in a multiple regression function improved the prediction of field lime requirement.