After each sequential extraction step the suspension was centrifuged, the supernatant was separated and the solid phase was added
to the reagents for subsequent extraction. The residual fraction of soil
from RSES was determined by subtracting the sum of the first three
extractable fractions (i.e. exchangeable+reducible+oxidizable) from
the corresponding aqua regia extractable Cd. The residual fraction of
soil from BSES was digested with aqua regia according to McGrath and
Cunliffe (1985) and the summed concentrations of Cd sequentially
extracted by this procedure were from 96.2 to 104% of that determined
by the aqua regia digestion.