Cobalt, copper, and zinc in yellow-hrown pumice soils under grazed, permanent pastures

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Abstract
The 0-7.5 cm depth of soil from 29 pastures on yellow-brown pumice soils contains more H+/H2O2- and EDTA/NH4Cl-extractable Co, Cu and Zn than the corresponding 7.5-15 cm depth. The differences in H+/H202-extractable quantities between the 2 depths ranges 0.1-2.3 ppm Co, 0.1-2 ppm Cu and 0.5-9.8 ppm Zn. H+/H202 is known to dissolve organic matter, any manganese oxides and some Al and Fe in these soils, and the above increments in extractable Co, Cu and Zn are consistent with their accumulation from fertilizer additions and plant and fecal deposition. Fertilizer addition is the major contributor to changes in soil Co, and Cu in plant and fecal residues is the major contributor to changes in soil Cu. For Zn, both factors make significant contributions. Laboratory experiments support the conclusion that these elements, derived from fertilizer, accumulate in the surface 7.5 cm of soil. At a solution concentration of 10-5 M the percentages of divalent Co, Cu and Zn, sorbed from 0.01 M NaNO3 are greater than 90% at pH 5-6, which is the normal field pH range. The similarily of the sorption reactions of hydrolyzable divalent cations is examined for Co and Cd, where the amounts sorbed at pH 3.7 are highly correlated (r = 0.97 *** [P < .001] for 0-7.5 cm and 0.94 *** for 7.5-15 cm samples). The amounts sorbed appear to depend on the organic matter content of the soil as well as the pH, but the influence of manganese oxides is probably small.