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Manganese Availability and Uptake by Rice in Acid Sulfate Soils

Soil Science Dep., Univ. of Florida, Gainesville, FL 32611

W. H. Patrick, Jr.

Lab. for Wetland Soils and Sediments, Louisiana State Univ., Baton Rouge, LA 70803

* Corresponding author.

ABSTRACT

Although Al and Fe toxicities have been reported to be major constraints to rice (Oryza sativa L.) growth on acid sulfate soils, it is unclear as to whether or not Mn reaches toxic proportions in these soils. The objectives of this study were: (i) to measure the solubility of Mn in flooded acid sulfate soils cropped to rice, (ii) to determine what controls Mn solubility in these soils, and (iii) to determine the relationships between soil parameters and Mn uptake by rice. Manganese availability and uptake by rice were evaluated in 134 flooded acid sulfate soils in the Central Plains region of Thailand and in a growth chamber experiment utilizing 50 of the same soils. Soil and plant metal analyses were conducted at the panicle differentiation stage of growth in both studies and in the soil prior to transplanting in the growth chamber experiment. Metal activities in soil solutions were determined using GEOCHEM. The results of this study showed that Mn solid phases such as oxides, hydroxides, carbonates, phosphates, or silicates were probably not controlling Mn solubility in these soils. Plots of E'-Mn (the ratio of Mn²⁺ to the sum of the divalent cations in soil solution) vs. E-Mn (the ratio of exchangeable Mn to the sum of the exchangeable divalent cations) indicated that the divalent charge fraction attributable to Mn²⁺ in soil solution was highly correlated to that on the cation exchange capacity. It was hypothesized that cation exchange was the dominant mechanism governing Mn²⁺ activities in these soils. Although Mn uptake was found to be correlated to Mn²⁺ activity and E'-Mn, it was more closely correlated to p(Mn⁺/Fe²⁺), indicating that an Fe/Mn interaction may be occurring. Leaf metal analyses also indicated that Mn toxicity probably did not occur.

Contribution from the Lab. for Wetland Soils and Sediments, Louisiana State Univ., Baton Rouge, LA 70803.

Received for publication May 23, 1988.