Soil Lime Requirement by Direct Titration with a Single Addition of Calcium Hydroxide

Abstract

Direct titration of acid surface soils is often used to measure soil acidity. However, titration has not been used for routine determination of the lime requirement (LR) in soil testing laboratories, because it is too time‐consuming, requiring multiple additions of base and long equilibration times between additions. Since soil pH as a function of added base can be described well by a linear equation, titration may be adopted by soil testing laboratories using a minimum number of base additions. Our objective was to evaluate the accuracy of a simplified titration procedure, based on an initial pH reading and a second reading following the addition of one dose of Ca(OH) 2 , followed by extrapolation to the target pH. Seventeen soils were titrated with Ca(OH) 2 in both water and 0.01 M CaCl 2 with a 30‐min interval between additions. The slopes from the first two data points of the titration in water were frequently in error for estimation of the slopes regressed by all data points to pH 6.5. However, the first two data points in the 0.01 M CaCl 2 titration were found to be more reliable for estimating the slopes. Both the initial pH in water and in 0.01 M CaCl 2 were used to calculate the LR with the two‐point slope in 0.01 M CaCl 2. The LR predicted using the initial pH in 0.01 M CaCl 2 gave better estimates of the LR than the initial water pH when compared with the standard 3‐d Ca(OH) 2 incubation. A linear regression of LR by the single addition method in 0.01 M CaCl 2 ( Y ) on LR determined with the 3‐d Ca(OH) 2 incubation ( X ) gave the linear equation Y = 0.88 X , r 2 = 0.93. It is concluded that the LR predictions with the single addition method in 0.01 M CaCl 2 are sufficiently accurate to justify further evaluation under routine laboratory conditions.