A modified sequential chemical extraction procedure was developed for
partitioning particulate Cd into eight fractions: exchangeable, carbon
ate-bound, metal-organic complex-bound, easily reducible metal oxide-b
ound, organic-bound, amorphous mineral colloid-bound, crystalline Fe o
xide-bound, and residual. Results of experimental data on 16 surface s
oils of Saskatchewan, widely varying in physico-chemical properties, i
ndicate the presence of little exchangeable Cd. Cadmium in these soils
was predominantly in the form metal-organic complex-bound, accounting
for 31-55%, with an average of 40%, of the total Cd present in the so
ils, The average relative abundance of the different forms of Cd prese
nt in these soils is in the order: metal-organic complex-bound (0.107
mg kg(-1)) > carbonate-bound (0.052 mg kg(-1)) > residual (0.042 mg kg
(-1)) > organic-bound (0.035 mg kg(-1)) > crystalline Fe oxide-bound (
0.016 mg kg(-1)) > easily reducible metal oxide-bound (0.010 mg kg(-1)
) > amorphous mineral colloid-bound (0.002 mg kg(-1)), Statistical tre
atment of the Cd availability index, measured as ammonium hydrogencarb
onate-diethylenetriaminepentaacetoc acid (ABDTPA)-extractable Cd, with
different particulate-bound Cd species showed high correlation (r = 0
.916, p = 6 x 10(-7)) of the Cd availability index with the metal-orga
nic complex-bound Cd. The beta coefficients obtained from the multiple
regression analyses have given an insight into the importance of Al-o
rganic complex-bound Cd species in estimating the bioavailability of C
d in these soils, The relationship of the metal-organic complex-bound
Cd and the mobility acid bioavailability of soil Cd merits in-depth re
search in explaining the toxicity and food chain contamination of Cd i
n the environment.