Five active elements constituting the model are defined. They are amorphous sesquioxides (R), allophane (A), humus (H), crystalline sesquioxides (G), and layer silicate clays (C). Part of positive charge of R combines with negative charges of A, H, and broken bonds on lattice surface of C. Positive charge of A combines partially with H. Free positive charges of A and R absorb phosphate anion. Free negative charges are classified into two charges, i-charge (C) and o-charge (H, A, and C) (YOSHIDA, 1957) nearly equal to permanent and pH-dependent charges of Schofield, respectively. The model for amorphous matters is devided into type-I and type-II. Type-I is composed of R, H, C, and G, while type-II is composed of A, R, and H. The model is devised to explain changes in coefficient of phosphorous absorption (CPA) and o-charge resulted by successive treatment (H_2O_2-deferration-Tamm's reagent treatments) of humic soils. For instance, H_2O_2 treatment of soils with amorphous matters of type-I decreases remarkably both their CPA and o-charge. This is explained by that H with o-charge is decomposed and liberated R (positive charge) having combined with H is extracted. After H_2O_2 treatment, soils with amorphous matters of type-II, however, slightly increase their CPA because decomposition of H activates sufficient positive charge of A masked by H, to compensate contemporaneous loss of R, and moderately decrease their o-charge because of presence of A which has o-charge. Humic volcanic ash soils with amorphous matters of type-II are likely to be limited within vigorous ash fall area. Problematical humic soils, called by the author "Kuroboku" soils, have amorphous matters of type-I. The model suggests that R plays an essencial role in accumulation of humus exclusively for soils with amorphous matters of type-I and partially for soils with amorphous matters of type-II.
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