Proton interactions with soil organic matter: the importance of aggregation and the weak acids of humin

Samples of three organic-rich soils (ombrotrophic peat, podzol H-horizon, humic ranker) were extensively washed with dilute nitric acid, dialysed against deionised water, and then subjected to acid-base titrations over the pH range 3 – 10, in 0.3 – 300 mM NaNO3, and with soil concentrations in the range 2 to 150 g l-1. The results for the three soils were quantitatively similar. Comparison of the titration data with previously published results for humic acids isolated from the same soils showed the soil organic matter to have a greater ionic strength dependency of proton binding and to possess relatively greater buffering capacity at high pH, attributable to weak acid groups (c. 2 - 5 mmol g-1) in the humin fraction of the soils. To describe the soil titration data quantitatively, we modified Humic Ion-Binding Model VI-FD, which utilises a fixed Donnan volume to describe counterion accumulation, by increasing the content of weak acid groups. When artefacts in pH measurement due to the suspension effect were taken into account, the resulting Model VI-FD2 provided good or fair simulations of all the titration data. The results suggest that soil structure, specifically aggregation, plays a significant role in cation binding by organic soils in situ. The lack of dependence of the titration results on soil suspension concentration suggests that the findings can be applied to soils in situ.