THE RELEASE OF SILICON, ALUMINUM, AND POTASSIUM DURING DECOMPOSITION OF SOIL MINERALS BY HUMIC ACID1

Publisher Website
Google Scholar
Abstract
Decomposition of primary minerals by humic and fulvic acids was investigated by shaking microcline, biotite, or muscovite with soil humic solutions at pH 7.0 and 2.5 for 0 to 1000 hours. Silica, aluminum, and potassium released by the primary minerals were determined in the extracts by atomic absorption spectrophotometry, and changes in mineral structure supporting possible decomposition were studied by X-ray diffraction analysis. At pH 7.0, humic and fulvic acids were capable of dissolving very small amounts of silica, aluminum, and potassium from the minerals by chelation, complex reactions, or both. The amounts released increased with time and reached a maximum at 800 to 1000 hours. At the termination of the experiment, the cumulative recovery of silica, aluminum, and potassium did not exceed 5 milligrams (or 0.2 millimoles) per gram of mineral, which was far less than that reported by other workers. The present findings conform better with natural conditions. The ease of mineral decomposition occurred in the following order: biotite > microcline > muscovite. The dissolution of the elements was enhanced considerably at pH 2.5. Fulvic acid extracted 10 times more silica and 6 times more aluminum at pH 2.5 than at pH 7.0. Statistical analysis indicated that this large increase in dissolution was the effect of an interaction between low pH and type of organic acid. X-ray diffraction analysis revealed the microcline to be a mixture of orthoclase and microcline. The X-ray curves yielded evidence that humic and fulvic acids had dissolved more readily the orthoclase component, the less resistant form of potash feldspar, leaving the stable form, or microcline, to accumulate in the residue.