The measurement of root exudation under natural conditions has numerous technical difficulties includ- ing binding of exudates to soil components, assimila- tion and mineralisation of exudates by microorgan- isms and an inability to separate root and microbial derived exudates. Hence, most exudate studies have been carried out in axenic sand or solution culture that excludes microorganisms. However, the pres- ence of both free-living (Barber and Martin, 1976; Biondini et al., 1988) and symbiotic organisms (In- gham and Molina, 1991) have been shown to affect root exudation both qualitatively and quantitatively (Section 2.4). Until the development of suitable methods for measuring exudation into soil, any model system of root exudation should consider the inclu-