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Plant Effects on Soil Manganese Availability¹

ABSTRACT

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.

¹ Contribution from the Dep. of Land, Air & Water Resources, Univ. of California, Davis, CA 95616.

² Former Graduate Student and Professor of Soil Science, respectively. The senior author is now Research Assistant, Office de la Recherche Scientifique et Technique Outre-Mer. BPV51 ABIDJAN Ivory Coast, W. Africa.

Received for publication December 17, 1979. Accepted for publication April 18, 1980.