In this experiment, we sought to verify the precision of the Shoemaker et al. (SMP), Yuan, Woodruff, and Mehlich lime requirement (LR) methods as proposed and modified for measuring the LR of 37 acid coarse-textured soils to attain pHs 5.5 and 6.0. The SMP buffer was tried at initial buffer pH levels of 7.5, 7.0, 6.5, 6.0, and 5.5 as a single-buffer (SB) method or double-buffer (DB; McLean et al.) modification. Results indicate that linearity of the relationship between incubation LR and soil-buffer pH increased as the initial buffer pH approached the desired soil pH. This comparison also disclosed that after recalibration the SMP-SB method was as accurate and precise as the more complex SMP-DB procedure for determining LR (5.5) and LR (6.0) rates. The most advantageous way of using the SMP-SB method for determining these LR rates was with the customary initial buffer pH of 7.5 but calculating the prescribed rates with a curvilinear instead of the more common linear buffer calibrations.
The Yuan buffer was evaluated at initial buffer pH levels of 7.0, 6.5, and 6.0 both with single- and double-buffer methodologies. This comparison revealed that the Yuan-DB procedure, though among the most precise tested for LR (6.0), was appreciably less precise for determining LR (5.5) rates. Nonetheless, the Yuan buffer was as precise for determining LR (6.0) rates and substantially more precise for prescribing LR (5.5) rates when used as a SB instead of its usually proposed more complex DB methodology.
The Woodruff buffer was as precise as any for determining LR (6.0) rates, but its precision was substantially lower for LR (5.5). As proposed, the Mehlich method was probably the most accurate method tested for determining LR (5.5) rates, because this method prescribed on the average 111% of the incubation values and the prescribed and incubation values were closely related (r = 0.938**). This study revealed that most buffers can be used for determining LR (5.5) or LR (6.0) rates after appropriate calibration. Regression equations are, accordingly, provided for calculating or adjusting LR rates to pH 5.5 or 6.0 for the methods and modifications tested. Results also indicate that buffer calibrations are valid for a wider range of mineral soils than used in this study.