Reaction Equilibration From Single-Addition Base Titration for Lime Requirement.

The University of Georgia Soil, Plant, and Water Laboratory (SPW) uses a single-addition titration for estimating soil lime requirement (LR).  The LR depends in part on a soil’s pH buffering capacity (described as lime buffer capacity (LBC) with units of mg CaCO₃ kg^-1 pH^-1) , which is estimated from the rise in soil pH after Ca(OH)₂ is added and allowed to react for 30 min. The 30-min reaction time has not been sufficient to reach equilibrium between the soil and Ca(OH)₂ resulting in an underestimation of LBC. As a corrective measure, SPW adjusts the ag lime recommendations to account for lack of equilibrium. This study investigates the timing of equilibration between soils and Ca(OH)₂. Specifically, the objectives of this study were to i) determine the time in which the Ca(OH)₂ treated samples reached pH equilibrium and ii) determine if a relationship exists between the LBC estimated from a 30 min equilibration and the LBC estimated from the time in which the sample has reached an equilibrium pH.  The 25 soils used in this study were equilibrated for up to 96 hours.  Results indicated that the soils had reached an equilibrium pH by 84 hours after addition of Ca(OH)₂. The ratio of the mean of 84 and 96 hour LBC and 30 minute LBC could be predicted with the equation 1.4745 + 0.0023 x 30 min LBC (r² = 0.83).  This relationship allows the current titration procedure with 30 minute equilibration time to be used to predict the equilibrium LBC, which in turn is used to calculate more accurate lime recommendations.