Soil pH buffering capacity: a descriptive function and its application to some acidic tropical soils

Abstract

Calculation of soil acidification rates requires knowledge of pH buffering capacity (pHBC), which is measured using titration methods. The pHBC is often quoted as a single value for a particular soil, implying a linear relationship between pH and the amount of acid or alkali added. However, over its whole range, the relationship is sigmoid rather than linear, and in many soils pH is low or high enough to be outside of the linear range. In this work we fitted a simple sigmoid function to pH buffer curves of 8 tropical Australian soils obtained using one titration method and 58 Papua New Guinean (PNG) soils obtained using another titration method. The function described the curves well for all soils (adjusted r2 > 0.93 for all samples and >0.99 for 90% of samples), irrespective of the titration method, allowing pHBC to be calculated as a function of pH across the range of pH values established. Using the function, the contribution of variable charge to pHBC was calculated for the PNG soils; on average it was 93% at the pH buffer curves’ inflection point, which corresponds with the soil’s minimum pHBC. Factors other than variable charge became important at pH (1 : 5, 0.002 m CaCl2) values <4.8 or >6.0. The relationship between pHBC and soil organic matter content was closest at pH 6.0–6.5. Application of the sigmoid function could facilitate more accurate assessments of acidification risks, acidification rates, and potential management interventions, particularly as soils become increasingly acidic.

Contrasting pH buffering patterns in neutral-alkaline soils along a 3600 km transect in northern China

“…2). These results were inconsistent with previous studies, which have shown that SOC content played an important role in soil pHBC (Aitken, 1992;Magdoff and Bartlett, 1985;Nelson and Su, 2010;Geissen et al, 2013). The discrepancy is probably caused by different types of organic matter involved (Nelson and Su, 2010).…”

“…Whereas acid inputs are fairly well documented, the patterns of soil pHBC across landscapes are not (Yang et al, 2012;Wong et al, 2013). Soil pHBC regulates the effect of acid deposition on terrestrial ecosystems by influencing the extent of soil pH change (Magdoff and Bartlett, 1985;Lu et al, 2015;Nelson and Su, 2010). Hence, measurement or estimation of soil pHBC is clearly beneficial for predicting the rate of soil acidification in response to predicted rates of acid deposition (Vet et al, 2014;Wong et al, 2013;Lu et al, 2015).…”

“…Previous studies have revealed that soil pHBC is governed mostly by acidic functional groups, dissolution/precipitation of carbonates and CEC, in soils with pH> 4.5 (Aitken, 1992;Bloom, 2000;Nelson and Su, 2010). Soil organic matter is a pH buffer, mostly due to weakly acidic functional groups such as carboxyl and phenol (Aitken, 1992;Bloom 2000).…”

“…Following Nelson and Su (2010), we used pHBC mid as the dependent parameter in the statistical analyses. The pHBC mid is the pHBC at the inflection point of the titration curve (i.e., A mid ).…”

“…For each titration, 2 g of air-dried soil (< 2.0 mm) was weighed into each of seven polyethylene tubes, and each tube received 20 mL of HNO 3 solution with a concentration of 0, 0.016, 0.032, 0.064, 0.080, 0.128 or 0.256 M. The suspensions were shaken at 25 • C for 24 h, left to settle at 25 • C for a further 6 days, re-suspended by shaking for 2 min, and then the pH was measured with a pH meter (S210 SevenCompact ™ , Mettler, Germany). Soil pHBC was calculated from the titration data following the method described by Nelson and Su (2010). The sigmoid function pH = pH min + a…”