The Moore–Sikora Buffer for Lime Requirement Determinations

Abstract

Paranitrophenol in the Adams–Evans (AE) buffer used for determining lime requirement (LR) is considered a hazardous chemical due to its toxicity. The Moore–Sikora (MS) buffer, which mimics AE soil‐buffer pH, was developed without hazardous chemicals and is currently used for routine soil testing at Clemson University. The MS buffer was designed by considering probable chemicals with pKa values in the desired pH range, potential reaction of the components with soil, and longevity of the buffer during storage. Boric acid, 3‐(N‐morpholino)propanesulfonic acid (MOPS), and 2‐(N‐morpholino)ethanesulfonic acid hydrate (MES) were considered for the new buffer. Once chemicals were identified, a technique of finding appropriate concentrations was utilized by using an equation that could predict the acid titration of a buffer and refining pKa and concentration values until a match was found with measured pH titrations of the AE buffer. The MS buffer contains 212, 131, and 34.8 mmol L−1 of B(OH)3, MOPS, and MES, respectively. The buffer also contains 200 mmol L−1 KOH and 1 mol L−1 KCl. The MS buffer was compared with the AE buffer for determining soil‐buffer pH values on 222 South Carolina soils and 41 North American Proficiency Testing Program soils. The MS soil‐buffer pH was strongly related to AE soil‐buffer pH (r2 > 0.98). Soil‐buffer pH was slightly less with MS buffer than AE buffer, with a mean difference of 0.03 pH units. The lower soil‐buffer pH with MS buffer caused a slightly higher LR than the AE buffer, with a mean difference of 0.34 Mg ha−1 on South Carolina soils. Forty‐four percent of the samples indicating a need for lime did not have a difference in LR between the two buffers. The slightly higher LR with the MS buffer on some of the samples is not expected to raise soil pH too high above target pH values.