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Solution/Soil Ratio and Release of Cations and Acidity from Spodosol Horizons

Chemistry Dep., Royal Univ. of Veterinary and Agricultural Sciences, Thorvaldsensvej 40, 1871 Frederiksberg C, Denmark, and Dep. of Forest Ecology, Soil Science Section, Swedish Univ. of Agricultural Sciences, S-90183 Umea, Sweden

*Corresponding author.

ABSTRACT

The pH value, exchangeable acidity, and effective cation-exchange capacity (CEC_e) are often used to characterize soils. In addition to properties of the soil itself, these parameters are highly attributed to electrolytes and ionic strength in extraction solutions, as well as to the solution/soil ratio (s/s ratio, liters per kilogram). The influence of the s/s ratio on pH, exchangeable acidity, and cations were studied in soil suspensions of Spodosol O, E, and Bs horizon soil samples in distilled water, 0.01 M CaCl₂, 1.0 M KCl, and 0.5 M CuCl₂. An enhanced s/s ratio resulted in a release of H ions from O and E horizon soil samples in agreement with a weak acid equilibrium, whereas the Bs horizon samples showed a higher buffer intensity of pH. A simultaneous release of organic acids and Al and the fact that samples from the spodic horizons were close to the point of zero charge indicate a more complicated pH buffering mechanism in these horizons. The amount of Al ions released from O and E horizon samples in 1.0 M KCl (Al_e) were simply diluted when the s/s ratio was increased. Therefore, Al_e is suggested to be a quite finite subpool of totally bound Al (Al_t; extracted in 0.5 M CuCl₂). A solution of 0.5 M CuCl₂ extracted all exchangeable K⁺, Na⁺, Mg²⁺, Mn²⁺, Fe³⁺, and Al³⁺ in O horizon samples, independent of s/s ratio, whereas the release of Ca²⁺ was slightly enhanced with increased s/s ratio. Hydrogen ions exchangeable in 1.0 M KCl (H_e, centimoles of charge per kilogram) increased by 17 to 22% in O and E horizons and by 32 to 33% in Bs horizons when the s/s ratio increased from 100 to 150. Consequently, estimated H_e and CEC_e pertaining to different samples can only be accurately compared if they are obtained at the same s/s ratio or corrected for differences in the s/s ratio by specific functions for different soil horizons.

Received for publication August 3, 1993.