Soil pH Buffering Revisited

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Abstract
The buffering relationships of a wide variety of Vermont soils were investigated by adding either acid (H2SO4) or base (CaCO3). Soils were found to be very well buffered above pH 7 and below pH 4. When amendment added is expressed on an organic matter (OM) basis [mmol (H+ or 1/2 CaCO3) g−1 OM] and the check treatment is transposed to a common curve, all soils appear to follow a similar relationship (a unified buffer curve). Thus, OM appears to strongly influence the degree of pH buffering of Vermont soils. For practical purposes, pH buffering of soils in the pH 4.5 to 6.5 range is linear with added amendment. However, the change in slope of the unified buffer curve indicates that of the entire pH range soils may be least well buffered between pH 5 and 5.6. The low degree of pH buffering in this range may be due to a high degree of cation exchange capacity (CEC) buffering. The pH vs. percent base saturation (%BS) relationship has been misinterpreted as being the same as a pH vs. lime addition titration curve. In a soil without appreciable exchangeable Al, the %BS is essentially the soil's CEC at its current pH expressed as a percent of the CEC at pH 8.2. Ranges in pH where the %BS increases only slightly may be zones of low pH buffering.
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