A Buffer that Mimics the SMP Buffer for Determining Lime Requirement of Soil

The Shoemaker-McLean-Pratt (SMP) buffer used for determining lime requirement of soil contains chromium and p-nitrophenol, which classifies the solution as a hazardous waste. A buffer without hazardous chemicals producing the same pH as SMP buffer would eliminate hazardous waste and have no effect on agronomic interpretation. Chemicals chosen to replace chromium and p-nitrophenol were 2-(N-morpholino)ethanesulfonic acid monohydrate (MES) and imidazole. Acid titrations of SMP buffer and various mixtures of a new buffer were performed to duplicate the acid-base characteristics of the SMP buffer. The new buffer is adjusted to pH 7.70 and contains 69.6, 13.7, 31.4, 89.3, and 2000 mM of triethanolamine, imidazole, MES monohydrate, acetic acid, and KCl, respectively. Coefficients of determination (r ²) for linear regressions of soil-buffer pH with the new buffer versus SMP buffer on 255 Kentucky and 87 North American Proficiency Testing (NAPT) soils were 0.974 and 0.967, respectively. The linear regression equation for new buffer versus SMP buffer lime recommendations in Mg ha⁻¹ was y = 0.226 + 0.994x with an r ² of 0.935 for 158 Kentucky soils requiring lime and y = 1.42 + 0.876x with an r ² of 0.876 for 27 NAPT soils requiring lime. Repeated soil-buffer pH measurements were made using 10 Kentucky soils and 11 NAPT soils over 150 d. Only one of the soil samples showed a significant drift in soil-buffer pH with time using the new buffer, and the drift was also observed using SMP buffer. With 88 measurements of soil-buffer pH using the new buffer on 11 NAPT soils, 2 measurements fell outside NAPT warning limits for SMP soil-buffer pH. The new buffer without hazardous chemicals mimicked the soil-buffer pH obtained with SMP buffer, provided lime recommendations similar to that with SMP buffer, and had a laboratory shelf life of at least 150 d.