Fjölrit RALA - 15.06.2004, Page 137

Aluminum solubility in nonallophanic Andosols from northeastern Japan
T. Takahashi and M. Nanzyo
Graduate School of Agricultural Science, Tohoku University, Japan
Introduction
Andosols are divided into two major groups on the basis of their colloidal compositions:
“allophanic” Andosols dominated by allophanic clay materials and “nonallophanic” Andosols
dominated by aluminum (Al) - humus complexes and 2:1 type aluminosilicates. Both groups
of Andosols show unique properties characteristic of volcanic ash derived soils, such as high
reactivity with phosphate and fluoride ions and a low bulk density. However, there are large
differences in soil acidity and A1 toxicity between the two groups of Andosols. Allophanic
Andosols are moderate to slightly acid even when the base saturation is very low and rarely
contain toxic levels of KCl-extractable Al. In contrast, nonallophanic Andosols are strongly
acid when the base saturation is low and possess a high KCl-extractable A1 that shows
toxicity to plant roots. The origin and status of the toxic A1 are not yet clear. In this study, we
analyzed A1 solubility of A horizons of nonallophanic Andosols from northeastem Japan.
Then, we investigated the relationship between 1 M KCl-extractable A1 and organically
complexed A1 that is a major A1 pool in nonallophanic Andosols. Finally, we examined the
effects of liming (CaC03 treatment) on Al-humus complexes.
Materials and methods
A horizon samples of nonallophanic Andosols were collected from northeastem Japan.
Sampling points were distributed in Aomori, Akita, Iwate and Miyagi Prefectures. As
comparisons, we used some allophanic Andosols and a Bhs horizon of a Spodosol.
An equilibrium study was conducted to determine the solubility of A1 as a function of
pH. A 0.01 M CaCl2 solution was added to soil samples and HCl or NaOH was added to
provide a pH range from 3 to 5. After 30 d incubation at 25°C, monomeric A1 concentrations
were determined and Al3+ activity was estimated.
Extractable A1 (1 M KCl and sodium pyrophosphate (Alp)) was determined to
characterize soil aluminum pools. The relationship between A1 saturation (KCl-extractable A1
/ effective CEC) and Alp was examined.
Based on the lime requirement with respect to a pH of 6.5, the mixture of soil samples and
CaC03 was incubated at field water capacity for 30 d. After air drying, the limed and unlimed
samples were used for determination of 1 M KCl-extractable Al, Alp, and so on.
Results and discussion
It is generally assumed that A1 solubility of mineral soils is regulated by a solid Al(OH)3
mineral phase (e.g. gibbsite). However, aqueous A1 concentrations in humus-rich soil
horizons are considered to be regulated also by humic substances. Figure 1 shows pH - pAl
relations obtained by the equilibrium study. A1 solubility of the allophanic Andosol was
nearly identical with that of synthetic gibbsite. The saturation index (SI) of imogolite for the
soil calculated from H+, Al3+ and H4SÍO4 activities showed +0.4 to +1.0, indicating slight
oversaturation with respect to imogolite. Thus, the allophanic Andosol horizon appears to be
in near equilibrium with both Al(OH)3 and imogolite. On the other hand, A1 solubility of the
nonallophanic Andosol A horizon and the Spodosol Bhs horizon was lower than gibbsite in
the lower pH range and show oversaturation in the higher pH range. These results strongly
suggest that the A1 concentration is controlled by ion exchange reaction of H+ and Al3+ ions
on negative charges of humus and that A1 solubility is regulated by Al-humus complexes.
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