Sequential versus parallel density fractionation of silt-sized organomineral complexes of tropical soils using metatungstate

A comparison of two density fractionation methods used for studying soil organomineral complexes was carried out on the silt-size fractions of two highly weathered soils from NE Brazil. The difficulties commonly encountered in isolating organomineral complexes in highly weathered soils were overcome when sodium metatungstate solution in combination with centrifugation on an angular rotor was used. The sequential method caused a greater loss of C (8–10%) than the parallel method (3–5%); however, the sequential method permits direct measurements on C concentration, complexing cations (Fe and Al), and mineralogical composition.