Predicting Calcite (CaCO3) Requirements of Sphagnum Peat Moss from pH Titration Curves

Liming materials are required to neutralize acidity in peat moss to make it a suitable substrate for growing container crops. A series of time-consuming incubations of peat:lime mixtures are typically used to determine the liming rate to achieve a desired pH. Our objective was to evaluate the utility of a rapid, acid-base titration method for predicting calcite (CaCO₃) requirement of sphagnum peat moss. In this method, peat moss with an initial pH of 3.7 was titrated in aqueous suspension with 0.1 M NaOH or 0.1 M HCl solution from pH 3 to 11 (forward titration), then from pH 11 to 3 (backward titration). Because of hysteresis, the forward and backward titration curves were averaged to predict calcite requirements. For comparison, peat moss samples were incubated in plastic bag for 13 days after applying calcite at rates between 0 and 1.5 mol CaCO₃·kg^-1 peat moss (dry weight basis), and pH was monitored until a steady-state was established. The pH achieved by incubation could be predicted up to pH 6.2 by the averaged acid-base titration curve. Above pH 6.2, calcite solubility limits caused a deviation between the two methods as the pH in the incubation experiment reached a plateau with increasing calcite additions. Geochemical speciation calculations showed that calcite solubility limits imposed by atmospheric CO₂ and dissolved Ca²⁺ activity restricts the maximum pH achievable. Nevertheless, the acid-base titration is a quick method for accurately predicting calcite requirements of acidic peat moss for target pH levels ≤ 6.2.