This thesis investigates the competitive relationships between two elemental pairs in a plant - soil system: thallium (Tl) and potassium (K), and selenium (Se) and sulphur (S). It considers the influence of these competing ions and, in the case of Se the exposure to selenate or selenite, on the accumulation and distribution of Tl or Se in spring canola and durum wheat grown in a sandy-clay soil. In general, it was observed that accumulation of Tl or Se was greatest in canola. Thallium (I) sorption to soil constituents and its accumulation and distribution in both plant species was influenced by K. Selenium accumulation and distribution in both plant species was influenced by the concentration and speciation of Se. Selenium sorption to soil and accumulation and distribution in wheat plants was not influenced by S while Se accumulation and distribution in canola was influenced by S. Applications to environmental risk assessment were considered.