The oxygen isotope signature of sulphate (delta O-18(sulphate)) is increasingly used to study nutritional fluxes and sulphur transformation processes in a variety of natural environments. However, mechanisms controlling the delta(18)O(sulphate)signature in soil-plant systems are largely unknown. The objective of this study was to determine key factors, which affect delta(18)O(sulphate)values in soil and plants. The impact of an(18)O-water isotopic gradient and different types of fertilizers was investigated in a soil incubation study and a radish (Raphanus sativusL.) greenhouse growth experiment. Water provided 31-64% of oxygen atoms in soil sulphate formed via mineralization of organic residues (green and chicken manures) while 49% of oxygen atoms were derived from water during oxidation of elemental sulphur. In contrast,delta(18)O(sulphate)values of synthetic fertilizer were not affected by soil water. Correlations between soil and plant delta(18)O(sulphate)values were controlled by water delta O-18 values and fertilizer treatments. Additionally, plant delta S-34 data showed that the sulphate isotopic composition of plants is a function of S assimilation. This study documents the potential of using compound-specific isotope ratio analysis for investigating and tracing fertilization strategies in agricultural and environmental studies.