CALCIUM CARBONATE AND MANGANESE DIOXIDE AS REGULATORS OF AVAILABLE MANGANESE AND IRON

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Abstract
Wheat was grown in sand culture to evaluate the effects and interactions among levels of .delta.-MnO2, CaCO3, and added Mn2+ and Fe2+ on yield and chemical composition of the crop. The experimental variables, at 5 levels each, were arranged in an incomplete factorial combination for response surface evaluation. Treatment effects on yield were not statistically significant, although the plants growing in unlimed pots were stunted and presented symptoms of Mn toxicity. The concentration of Mn in plant tops varied from 78 to 6140 ppm. Over 90% of the variation in Mn uptake was explained by the levels of Mn2+, CaCO3, and .delta.-MnO2, and by some of their quadratic and interaction terms. Calcium carbonate had a strong negative effect on the uptake of Mn, attributable to immobilization of the divalent cation through adsorption and precipitation, or formation of manganocalcite. Although .delta.-MnO2 was a source of Mn to plants at low levels of added Mn2+, it presented a negative influence at high Mn2+ level because of adsorption and oxidation of Mn2+ on its surface. Added Fe2+ did not significantly influence Mn uptake. The significant interactions, .delta.-MnO2 .times. Mn2+ and CaCO3 .times. Mn2+, were negative, indicating that the response to added Mn2+ was depressed by the presence of .delta.-MnO2 and CaCO3, both buffers of Mn2+ concentration. None of the factors studied influenced plant uptake of Fe. The concentration of Fe in plant tissues varied from 34 to 54 ppm, except for those of the no-line treatment, which contained 108 ppm. These results were attributed to sorption and oxidation of Fe2+ on the .delta.-MnO2 surface at all levels of added oxide, both reactions being favored by high pH.

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