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Relationships differ greatly between soil Mn solubility and pH, between plant uptake of Mn and soil pH, and between plant uptake of Mn and nutrient solution pH. The discrepancies suggest that soil Mn availability is controlled by neither soil nor plant characteristics per se but by the combined effects of soil properties, plant characteristics, and the interactions of plant roots and the surrounding soil.Experiments were done to evaluate the contributions of root/soil interactions, specifically those of root exudates, to Mn availability. The solubility of soil Mn and of MnO2 in root exudates and in rhizosphere and bulk soils was measured over the pH range of 4.5 to 6.5. The relationships between pH and MnO2 solubility in solutions of root exudates and selected root exudate components closely resembled those between plant uptake of Mn and soil pH. Similarly, the relationship between CaCl2‐soluble Mn in the rhizosphere soil and soil pH resembled that between plant uptake of Mn and soil pH. The pattern with pH of the CaCl2‐soluble Mn of the bulk soil was that of soil alone and fell below that of the rhizosphere soil. These relations establish that root exudates make an important contribution to plant uptake of soil Mn. Exudate compounds, such as hydroxy‐carboxylates, increase soil Mn solubility through reducing MnO2 and complexing the divalent Mn released. The effect is particularly marked in systems more acid than pH 5.5 and explains many of the apparent anomalies of soil Mn availability.

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The reactions of malic acid, an important constituent of root exudates, with the hydrous oxides of manganese (Mn) and iron (Fe) were studied to further our understanding of the processes controlling the solubility and plant intake of these elements. The reaction of malate with δ‐MnO2 followed either of two pathways depending on the pH‐controlled adsorption of carboxylates on the oxide surface. In acid systems, the first product of the oxidation of malate (oxaloacetate) is adsorbed on δ‐MnO2 surfaces and hydrolyzed to a molecule each of formate and malonate. These products are further oxidized to carbon dioxide (CO2) and water, and 6 mol of Mn2+ are released; the malonate is oxidized via tartronate, formate, and oxalate, and the formate is oxidized directly. Oxaloacetate not adsorbed by the δ‐MnO2 is rapidly decarboxylated to pyruvate, which is subsequently oxidized to acetate and CO2, and only 2 mol of Mn2+ are released. Although favored by acid conditions, adsorption of oxaloacetate by MnO2 is still significant at the pH of calcareous systems; thus, the reduction of MnO2 can proceed via both pathways in agricultural soils.The reduction of Fe(OH)3 by malate was less than that of δ‐MnO2 and was enhanced by acidity and addition to low levels of δ‐MnO2. Higher levels of added δ‐MnO2 diminished soluble Fe due to adsorption and reoxidation of Fe2+ by the δ‐MnO2.

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