Abstract

Fifty commercial potato crops and associated soils were sampled. Soil solutions were extracted from rewetted soils by centrifugation, and solution composition was related to Cd concentrations in tubers. Soils were also extracted with 0·01 M Ca(NO₃)₂ and 0·01 M CaCl₂ solutions, and Cd²⁺ activities in the extracts were calculated by difference using the stability constants for formation of CdCl^2-n_n species.

The soils had saline solutions (>4 dS/m), and Cl^- and SO^2-₄ in solution markedly affected the speciation of Cd in soil solution, with chloro-complexes, in particular, dominating. While low soil pH was associated with high (>25 nM) concentrations of Cd in soil solution, chloro-complexation also led to high concentrations of Cd in solution, even at neutral to alkaline soil pH values.

Tuber Cd concentrations were not related to activities of Cd²⁺ in soil solution or to activities in dilute salt extracts of soil. Tuber Cd concentrations were related to the degree of chloro-complexation of Cd in solution. The relationship of tuber Cd concentrations to chloro-complexation in soil solution suggests that Cd species other than the free Cd²⁺ ion are involved in the transport through soil and uptake of Cd by plants.