Determination of the Charge Character of Selected Acid Soils

The components of the exchangeable cation suite and its degree of domination by aluminum (Al) are prerequisites for lime requirement determinations based on percent Al saturation. The usefulness of these data is enhanced if the determinations are made under conditions which approximate those of the “field” environment. Twenty acid soils were selected to evaluate several procedures using buffered and unbuffered electrolytes to determine soil charge characteristics. Soil pH values (1N KCl) and organic matter contents ranged from 3.4 to 5.2 and 0.4 to 6.6%, respectively. Exchangeable cations (Al, Ca, Mg, K, Na), cation exchange capacity (CEC), and anion exchange capacity (AEC) were determined by 0.2N NH₄Cl saturation followed by 0.2N KNO₃ saturation with correction for occluded salt. Extractable Al [1N KCl, 1N NH₄OAc (pH 4.8)], exchangeable bases [1N NH₄OAc (pH 7.0)], and buffered soil CEC [1N NH₄OAc (pH 7.0) saturation, ethanol wash, 0.2N KNO₃ saturation] were also measured. The effects of intervening ethanol and water washes on unbuffered electrolyte CEC values were determined. Positive charge was detected in 16 samples. Buffered acetate extractions for soil Al and CEC generally gave much higher values than did unbuffered electrolyte procedures. Exchangeable bases and Al were similar whether extracted with 1N NH₄OAc (pH 7.0) and 1N KCl or with 0.2N NH₄Cl (bulked saturation extracts). “Net CEC,” calculated as CEC-AEC using the unbuffered electrolyte method without an intervening wash, gave generally closer agreement to the total sum of exchangeable cations than a simple CEC value. A sum-of-cations calculation method utilizing an unbuffered electrolyte extraction for Al is recommended for soil testing facilities desiring the most unambiguous information regarding soil exchange character and the acidity associated with these exchange sites under field conditions.