| Research Abstract |
In order to examine characteristics of aluminum species toxic to plants and their concentrations in acid soils, soil solution extracted from soils of crop fields and forests was examined. Al species and their concentrations were calculated by applying the method of successive approximation reported by Adams (1971). In the present paper, we calculated Al species through the following mass balance equation, assuming that four kinds of polynuclear Al species which have been strongly suggested in several studies using aluminum salt solution can coexist.
Alt (total Al) =[Al^<3+>]+[AlOH^<2+>]+[Al(OH)_2^+]+[Al(OH)_3^o]+[AlSO_4^+]+[AlHPO_4^+]+[AlH_2PO_4^<2+>]+2[Al_2(OH)_2^<4+>]+3[Al_3(OH)_4^<5+>]+13[Al_<13>(OH)_<24>^<7+>]+6[Al_6(OH)_<15>^<3+>]
The following results were obtained.
1) If the pH value of soil solution pH is in the most acid range of 4.7 through 3.0 and the pAlt value ranges from 4.5 through 2.3, Al^<3+> became dominant Al species in Alt.
2) If the pH value of soil solution ranges fro
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m 7.3 to 5.3 and the pAlt value ranges from 5 to 2.3, then polynuclear species, Al_6(OH)_<15>^<3+>, becomes dominant Al species in Alt.
3) If the pH value of soil solution ranges from 7.3 to 5.3 and the pAlt value ranges from 7.5 to 4.5, then Al(OH)_3^o becomes dominant Al species in Alt.
4) In the pH value of range from 7.31 through 3.0 and in the pAlt value range from 7.31 through 2.3, neither Al_2(OH)_2^<4+> nor Al_3(OH)_4^<5+> becomes dominant Al species and the concentration of these species was one hundredth less than the Alt concentration. It is inferred that these polynuclear species can become dominant Al species in alminum salt solution where Alt concentration is larger than 10^<-2> molL^<-1> and the pH value ranges from 5 to 6.
5) The ratio of Al_<13>O_4(OH)_<24>^<7+> concentration to Alt concentration is about one to hundred million, and thus the existence can be ignored.
6) If the pH value of soil solution ranges from 6.5 through 5 and the pAlt value ranges from 7 through 4 and the pAlt value is almost equal to the pH value, then Al(OH)_2^+ species amounts to dozens of percentages in Alt.
7) In the case of cultivated field soils amended by high amounts of phosphate fertilizers, AlHPO_4^+ is recognized as dominant Al species.
Based on these results, it is assumed that one of Al^<3+>, Al(OH)_2^+, Al(OH)_3^o, Al_6(OH)_<15>^<3+>, and AlHPO_4^+ becomes dominant Al species in Alt in the pH and pAlt ranges examined. Less
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