Cadmium (Cd) is a key environmental contaminant in New Zealand agricultural systems that is associated with the long-term application of high rates of superphosphate fertilizer. Although Cd is considered to be a non-essential element for plants, it is effectively absorbed by the root system of many plant species and can be subsequently transported throughout the plant. Recent studies have shown that the forage species chicory (Cichorium intybus L.) and plantain (Plantago lanceolate L.) accumulate a significantly higher shoot Cd concentration than the traditional pasture species ryegrass and clover. Forage crops are now common in New Zealand farm systems, and the potential implications of elevated Cd on food safety are actively being explored. The current study was designed to assess Cd uptake in forage species as a function of Cd soil chemistry in the rhizosphere. Cadmium complexed to organic compounds can be separated and quantified using gel exclusion and high performance liquid chromatography. However, the determination of free Cd²⁺ ions in rhizosphere soil and plant sap presents an analytical challenge. To achieve the aims of the current study, methodology needed to be developed to quantify different Cd species such as, free Cd²⁺ ion and Cd complexed with different organic compounds in both the rhizosphere soil and plant sap. Low-cost electrochemical methods such as stripping voltammetry (SV), which use working electrodes that have been selected for activity to target metals, show promise for monitoring low concentrations of free Cd²⁺ in environmental samples. In the current study a chemically modified carbon paste electrode to determine free Cd²⁺ ions was developed. The electrode was used with square wave anodic stripping voltammetry for analytical determinations. A carbon paste electrode was first prepared using graphite powder mixed with Bi₂O₃ as the electrode carbon source. Free Cd²⁺ was pre-concentrated on the surface of the modified electrode at −1.2 V vs. Ag/AgCl and then re-oxidized during a stripping step. The accuracy of measurements was validated against screen printed carbon paste electrodes. In initial work, the lower limit of detection of this electrode was 0.2 mg Cd²⁺/L solution. Further development of the electrode using sugar charcoal as the carbon source mixed with a novel Cd²⁺ ion binding ligand is currently in progress to reduce the detection limit to nanomolar levels. This paper explains the systematic development of a chemically modified carbon paste electrode that can be used for the determination of free Cd²⁺ ion concentration in rhizosphere soil solution and plant sap.