Abstract
Rapid collection of soil-solution analytical data to activities of individual Al ionic species was made using a computer program. Soil-solution analytical data relating to the forest and upland soils were calculated. Al_6(OH)_<15>^<3+> as the polymeric hydrolytic product was added to the mass balance equation with respect to Al in addition to monomeric Al ionic species. The results obtained were as follows. 1) The maior ion-pair in soil solution was calcium nitrate in soils under the sugi stands, whereas aluminum nitrate and calcium nitrate were predominant in soils under the hinoki stands and around a sugi stem. 2) Al species in soil-solution was occupied by Al_6(OH)_<15>^<3+>. The percentage were more than 90% of the total Al in the pH range of 4 to 7 in the soils examined. 3) Al^<3+> activity in soil-solution could be described by the following equation : p(Al^<3+>)=2.58 pH-5.58 (γ= 0.994) where p(Al^<3+>) was highly correlated with pH in the range of 3.5 to 7. A plot of p(Al^<3+>) against the pH value showed that this solubility line was approximately intermediate between the solubility line of gibbsite and that of amorphous Al(OH)_3. At pH values below about 4,the main Al species was Al^<3+>. Furthermore, we inferred that Al^<3+> activity in soil-solution was in equilibrium with adsorption sites of solid phase containing soil humus. 4) The concentration of Al_6(OH)_<15>^<3+> could be calculated by the following equation : p(Al_6(OH)_<15>^<3+>)=0.48 pH+2.52 where the polymeric hydrolysis of Al^<3+> was assumed to be 6 Al^<3+>+15 H_2O⇄Al_6(OH)_<15>^<3+>+15 H^+ and the thermodynamic K for this equation was approximately 10^<-36>. Consequently, we proposed that the Al^<3+> and Al_6(OH)_<15>^<3+> activities were the main Al ionic species in the soil-solution and that they could be calculated from the observed pH values of the soil-solution.
