THE INFLUENCE OF ORGANIC MATTER, CHALK, AND SESQUIOXIDES ON THE SOLUBILITY OF IODIDE, ELEMENTAL IODINE, AND IODATE INCUBATED WITH SOIL

Iodine in each of the forms iodide, elemental iodine, and iodate was added, at a rate of 5 mg/kg to a sandy loam and to mixtures of the soil with composted grass roots, chalk and sesquixoides, and its solubility determined after various periods of incubation. With iodide, solubility in both 0.01 M CaCl₂ and 1.0 M NK₄ acetate (pH 4.8) declined rapidly over the period o to 3 days and subsequently reached approximate equilibrium levels of 2.8 per cent solubility in CaCl₂ and 7.8 per cent in NH₄ acetate, these values being the means of samples incubated for 48, 103, and 160 days. The partial (5 per cent) replacement of the soil by composted grass roots had no appreciable effect on the solubility of added iodide, while chalk, incorporated at a rate of 5 per cent, depressed the solubility of iodide in CaCl₂ to 1.8 per cent but caused a slight increase in solubility in NH₄ acetate. The incorporation of 2 per cent hydrated ferric oxide or of 2 per cent hydrated aluminium oxide reduced the solubility of iodide in CaCl₂ to 0.1 and 0.3 per cent, and in NH₄, acetate to 3.8 and 5.7 per cent respectively. Elemental iodine was similar to iodide in its solubility in the two extractants and in its response to the various soil treatments. Iodate, however, differed considerably from the other two forms of iodine. With soil alone, and with the soil/chalk mixture, its decline in solubility with increasing incubation time was relatively slow, although after 160 days its solubility was similar to that of iodide and elemental iodine. The incorporation of composted grass roots caused a rapid reduction in iodate solubility, suggesting that the organic matter accelerated the reduction of iodate to elemental iodine or iodide. With the treatments involving the incorporation of ferric and aluminium oxides, there appeared to be considerable sorption of iodate during the 16 h extraction period and the effects of these materials on iodate solubility during incubation were therefore difficult to assess.