LIME REQUIREMENT DETERMINATION OF ACID ORGANIC SOILS USING BUFFER-pH METHODS

Abstract

The aims of this study were to calibrate and assess the accuracy of seven buffer solutions for determining the lime requirement (LR) of organic soils using buffer-pH methodology. The various buffers were evaluated by comparing the precision of regression equations adjusted to the relationships between the LR rates required to achieve pH 5.0 (0.01 M CaCl 2 ; ≈ 5.4 (H 2 O)), as determined by incubation, and soil-buffer pH values. The buffers tested and their initial pH values were as follows: Shoemaker, McLean and Pratt (SMP), pH 7.5; Woodruff, pH 7.0; Yuan, pH 7.0 and 6.0; Mehlich, pH 6.6; 0.1 N Ca(OAc) 2 , pH 5.5; 0.1 N Ba(OAc) 2 , pH 5.5; and 0.1 N NH 4 OAc, pH 5.5.Results indicate that the liming material had reacted with the CaCO 3 neutralizable acidity of these soils within 2 mo as the pH values of the incubated samples did not change materially from the 2nd to the 8th month of incubation. Furthermore, the pH values of the soils utilized in this study increased linearly with increasing rates of applied CaCO 3 . Incubation lime requirements to achieve pH 5.0 (0.01 M CaCl 2 ) by the 34 unlimed and 12 partially limed organic soils utilized in this study to calibrate each buffer varied from about 1 to 28 meq CaCO 3 /100 mL soil (1–28 tonne/ha to a depth of 20 cm). Although, the SMP (r = −0.957**) and Woodruff (r = −0.943**) buffers were somewhat less precise than the Mehlich (r = −0.975), Ba-acetate (r = −0.966**), or ammonium acetate (r = −0.976**) buffers for determining the full range of LR rates encountered in this study, they were nonetheless as precise as any for determining the LR of organic soils needing less than about 20 meq CaCO 3 /100 mL soil to achieve pH 5.0 (0.01 M CaCl 2 ). Regression equations for calculating the LR of organic soils, adjusted for differences in their bulk densities, from soil-buffer pH values are provided in the text. Key words: SMP, Woodruff, Mehlich, Yuan, acetate buffers