Applicability of selective dissolution of manganese oxide by acidified 0.1 M NH2OH–HCl in Japanese soils

We investigated the extraction of manganese (Mn) oxides and iron (Fe) oxides from samples of three Andisols and seven other major Japanese soils with acidified 0.1 M NH2OH–HCl in order to evaluate this methodʹs applicability to selective dissolution of Mn oxide. The temporal changes in the amounts of Mn and Fe extracted revealed the following: 1. Mn oxides in non-volcanic-ash soil (low Alo + 1/2Feo) could be dissolved semi-completely upon extraction for 30 min, which is the customary extraction time. 2. Mn oxides in volcanic ash-containing soils (moderate or high Alo + 1/2Feo) continued to dissolve substantially after 30 min of extraction; 3. Fe oxides in all soils gradually dissolved throughout 4 h of extraction, although dissolution rates in volcanic ash-containing soils were much lower than in other soils. These results indicated that the solubility of Mn and Fe oxides in volcanic ash-containing soils were low, suggesting characteristic state of the oxides in these soils, and that the method is therefore unsuitable.