Fjölrit RALA - 15.06.2004, Qupperneq 137

Fjölrit RALA - 15.06.2004, Qupperneq 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. 113
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