Humic substances in acid organic soils: modelling their release to the soil solution in terms of humic charge

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Abstract
SUMMARY: Batch titration experiments were carried out with organic soil samples in order to investigate the release to the solution phase of humic substances (HS). Measurements were made of pH, dissolved organic carbon (DOC) concentration, and the concentration of mono‐meric (inorganic + organic) aluminium, as functions of added acid or base. DOC was taken to be entirely due to HS. The results can be interpreted in terms of a model in which the soil is considered to contain two types of HS–mobile or potentially mobile (HSM), and immobile (HSI). The binding of inorganic ions by the HS is calculated using humic ion‐binding model IV, previously developed in this laboratory. Model IV allows the charges on the HS (ZHSM, ZHST) to be calculated; these are determined mainly by the binding of H+ and A13+. Concentrations of HS in solution, [HSaq], are given by the equation: image where |ZHSM| is the modulus of ZHSM, nHSM is the carboxyl group content of HSM, cHSM is the soil content of HSM, β is a fitting parameter, and square brackets, [ ], indicate concentrations. For most of the soils a value for β of 3 gives acceptable agreements between measured and calculated values of [HSaq], indicating a major influence of charge on release. The optimized value of cHSM differs considerably among soils, whereas cHIS varies by only a factor of about two. Total humic contents (cHSM+ cHSI estimated by model optimization are in approximate agreement with values estimated by extraction of the soils with NaOH.

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