Cation exchange properties of acid forest soils of the northeastern USA

Negative correlations between soil pH and cation exchange capacity (CEC) or base saturation in soils of the northeastern USA and Scandinavia have raised questions regarding the nature of cation exchange in acid forest soils. Using data from three small‐catchment studies and an extensive regional survey of soils in the northeastern USA, I examined relationships among total carbon, effective CEC (CEC_e), soil pH_s (in 0.01 m CaCl₂) and base saturation. Organic matter is the predominant source of soil surface charge in these coarse‐grained, glacially derived soils. Correlation coefficients (r) between total carbon and CEC_e ranged from 0.43 to 0.74 in organic horizons and from 0.46 to 0.83 in mineral horizons. In all cases, the intercepts of functional relations between CEC_e and total C were near zero. In O horizons, the CEC_e per unit mass of organic carbon (CEC_e:C) was positively correlated with pH_s in three of the four data sets, consistent with the weak‐acid behaviour of the organic matter. However, CEC_e:C was negatively correlated with pH_s in mineral soils in two data sets, and uncorrelated in the other two. The CEC_e in mineral soils represents the portion of total CEC not occupied by organically bound Al. The negative correlations between CEC_e:C and pH_s can therefore be explained by increased Al binding at higher pH_s. Aluminium behaves like a base cation in these soils. When Al was considered a base cation, the relation between base saturation and pH_s could be effectively modelled by the extended Henderson–Hasselbalch equation. When modelled without Al as a base cation, however, there were no consistent relationships between pH_s and base saturation across sites or soil horizons. Because of the non‐acidic behaviour of Al, it is difficult to predict the effect of ongoing reductions in acid deposition on the base status of soils in the northeastern USA.