We identified and quantified the immediately available (intensity factor) and the reserve forms (capacity factor) of Fe and Mn in 55 cultivated organic soils to assist selection of a reliable soil test for Fe and Mn in such soils. Soil Fe and Mn were sequentially partitioned into (1) H2O-soluble, (2) Ca-exchangeable, (3) free-oxide-associated, (4) weakly complexed, (5) strongly complexed, (6) carbonate-associated, and (7) sulfide-associated extractable forms by dissolution in (1) H2O, and aqueous solutions of (2) CaCl2, (3) CaCl2 + hydroquinone, (4) CH3COOH; (5) DTPATEA, (6) HCl, and (7) KCN, respectively.
On an average, 66% of the total Fe was unextractable, 26% strongly complexed, 4% sulfide-associated, 3% in carbonates, 0.5% weakly complexed, 0.4% in association with free oxides, 0.2% H2O-soluble, and 0.1% Ca-exchangeable. Of the average total Mn, 57% was unextractable, 18% associated with free oxides, 10% weakly complexed, 6.5% each strongly complexed and Ca-exchangeable, 0.9% in carbonates, 0.4% in sulfide, and 0.5% in H2O-soluble forms. Of the average total Zn, 8.7% was in free oxides, 4.5% Ca-exchangeable, and 1.9% in H2O-soluble forms.
The immediately available forms of Fe in H2O-soluble and Ca-exchangeable factions (intensity factor) were indicated to be replenishable mainly by the strongly complexed form (capacity factor). The capacity factor for Mn was suggested to be more than one form of soil Mn.
The data suggested that, although total Fe may be useful as a soil test value, the effect of pH on Mn oxides may render total Mn itself unreliable as a soil test value, even among the similar aerobic and acidic organic soils.
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