Charge fingerprints of forest organic horizons from north-eastern USA

Abstract

The charge fingerprint procedure was used to characterise the exchange properties of high- organic forest soils from the north-eastern USA. Twenty-six Oa or A horizon samples were obtained from active research sites to provide samples with a range in carbon from 15·1 to 49·4% and pH (in 1 mM CaCl₂) from 2·8 to 4·2. Fingerprints were similar to those developed for other variably charged soils except that significant quantities of exchangeable H⁺ were found and no appreciable anion exchange capacity was measured. The magnitude of the ‘base’ cation capacity (CEC_B) varied linearly with pH, averaging >50% change per unit pH change. A good estimate of the CEC_B was obtained from the sum of cations removed by a single extraction with 1·0 М choline chloride (slope 1·03, r² = 0·91). Variations in exchangeable Al³⁺ or H⁺ with pH were related to the quantity found in untreated samples, probably because of the close association between exchangeable and organically complexed forms. The total CEC was closely related to the sum of cations removed by 0·1 M BaCl₂ (slope 1·05, r² = 0·96). Although low ionic strength may be difficult to maintain in low pH samples, the fingerprint method appears to allow precise interpolation of the organic soils’ exchange capacity at field pH.