To evaluate a sequential extraction procedure to separate chemical forms of soil microelements, I fractionated 16 soils, both surface and subsoils. The purpose of the sequential extractions was to solubilize the Mn, Cu, Fe, and Zn in the exchangeable, organic matter, Mn oxide, amorphous Fe oxide, and crystalline Fe oxide fractions. The solid portion remaining was separated into sand, silt, and clay sizes, and each was dissolved and analyzed for the above elements. Soil properties were determined, and whole soils were analyzed for total microelement content.
Manganese was primarily in the organic and Mn oxide fractions, whereas Cu and Zn were found mainly in the crystalline Fe oxide, silt, and clay fractions for the fine-textured soils. In sandy soils the Mn was predominantly in the sand, and Cu was in the Fe oxide fractions. Iron was relatively higher in the organic matter, and Zn was relatively higher in the exchangeable and organic fractions in the sandy soils, compared with the fine-textured soils. High correlation coefficients were found among such soil properties as CEC, organic matter, and clay versus metals removed from the various fractions. The sums of the fractions versus whole soil digestion values compared favorably. The fractionation scheme presented was found to be satisfactory for a wide range of soils.