Properties of biologically formed manganese oxide in relation to soil manganese

Abstract

Manganese oxide, produced from the oxidation of manganous ions by bacteria at pH 6.5, was extracted with solutions commonly used to estimate the availability of soil manganese to plants. Reducing agents at pH 7.0 completely dissolved the manganese oxide as did sodium ethylene diaminetetra-acetate (EDTA) solutions more acid than pH 6.0. Other acidic solutions extracted part of the manganese in amounts in proportion to the amount of oxide present. Neutral salts extracted only traces of manganese; however, the addition of copper or zinc salts to neutral M ammonium acetate resulted in the release of appreciable amounts of manganese from the oxide. The fact that sodium EDTA solutions and solutions containing copper and zinc released manganese from the bacterial manganese oxide indicates that the practice of extracting soils with these solutions to estimate of organically bound manganese is unsound. The oxide, when pre-heated from 50 to 108°C, became progressively more soluble in neutral M ammonium acetate. Manganese was also rapidly released when the pH of the oxide in suspension was lowered from pH 7 to pH 4. Conversely the oxide rapidly adsorbed manganous ions as the pH was raised from pH 4 to 7. These changes in manganese solubility parallel those reported for soils receiving similar treatments, and it is postulated that the active manganese oxides in soil are similar to bacterial manganese oxide. It is also suggested that the acid-replaceable manganese associated with the oxide plays an important part in the availability of manganese in soils.