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

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 (Z_HSM, Z_HST) to be calculated; these are determined mainly by the binding of H+ and A13+. Concentrations of HS in solution, HS_aq], are given by the equation: where |Z_HSM| is the modulus of Z_HSM, n_HSM is the carboxyl group content of HSM, c_HSM 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 HS_aq], indicating a major influence of charge on release. The optimized value of c_HSM differs considerably among soils, whereas c_HIS varies by only a factor of about two. Total humic contents (c_HSM+ c_HSI estimated by model optimization are in approximate agreement with values estimated by extraction of the soils with NaOH.