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Fractionation of Soil Manganese Forms Important to Plant Availability

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

*Corresponding author.

ABSTRACT

Prior to this study, no method had been developed specifically for characterizing soil Mn. The unique chemistry and reactivity of soil Mn forms were carefully considered in the development of this sequential chemical fractionation procedure, since incidental dissolution of Mn-oxides or carbonate Mn has been a pervasive problem in other fractionation methods, and can result in significant errors in estimating the Mn content of these soil fractions. Accurate determination of these fractions is particularly important for plant-soil studies, since these Mn forms interact with plant roots in the rhizosphere. A sequential-extraction procedure is described that quantitatively estimate those forms of Mn in soils of significance to plant Mn availability. The Mn fractions and their extractants are: (i) readily soluble Mn-0.05 M Ca(NO₃)₂; (ii) weakly adsorbed Mn-pH 8.5, CaDTPA-B₄O₇ (0.025 M calcium diethylenetriamine pentaacetate in 0.025 M Na₂B₄O₇); (iii) carbonate-bound Mn (calcareous soils)-rapid extraction with 1.6 M HNO₃; (iv) specifically adsorbed Mn (noncalcareous soils)—0.05 M Cu(NO₃)₂ in 0.05 M Ca(NO₃)₂, pH 4.4; and (v) oxide-Mn—pH 2.0, 0.1 M NH₂OH·HCl. The procedure differs from other methods in that evaluation of carbonate-bound Mn is included, use of extractants that reduce oxide-Mn is avoided, and the adsorbed forms are differentiated into weakly and specifically-adsorbed fractions.

Received for publication December 20, 1989.