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

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+/H₂O₂-extractable quantities between the two depths ranges from. 0.1 to 2.3 ppm Co, 0.1 to 2.2 ppm Cu, and 0.5 to 9.8 ppm Zn. H+/H₂O₂ is known to dissolve orgamc matter, any manganese oXides, and some Al and Fe in these soils and the above increments in extractable Co, Cu, and Zn are shown to be consistent with their accumulation from fertiliser additions and plant and faecal deposition. Fertiliser addition is the major contnbutor to changes in soil Co, and Cu in plant and faecal 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 fertiliser, 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 0.01M NaNO₃ are greater than 90% at pH 5 to 6, which 18 the normal field pH range. The similarity of the sorption reactions of hydrolysable divalent cations is examined for Co and Cd, where the amounts sorbed at pH 3.7 are highly correlated (r = 0.97*** 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.