Applicability of selective dissolution of manganese oxide by acidified 0.1M 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 0.1M NH₂OH-HCl in order to evaluate this method's 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 30min, 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 30min of extraction; 3. Fe oxides in all soils gradually dissolved throughout 4h 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.