Apparent pH independence of charge in forest organic surface soil horizons

Conventional wisdom states that the source of negative charge in organic soil horizons is pH dependent and, therefore, acidification will decrease charge and the ability to retain nutrient cations. Using a variety of methods, we found that the native cation exchange capacity (CEC) of northeastern US forest soils varied with the amount of soil carbon (about 0.5 cmol per %C), independent of field pH. However, individual soil samples exhibited dramatic charge variability if the pH was adjusted during CEC measurement, as much as 20 cmol_c kg^? per pH unit change. These last two statements appear to be mutually exclusive. Extrapolating from pH-adjusted samples, the point of zero “base” cation capacity was consistently about 1.5 pH units below the native pH. We hypothesize the amount of charge is at a steady state with humification and decomposition processes. Response of soils to long-term acidification may be much different than that of short-term laboratory adjustments.