Abstract Calcined attapulgite, a non-swelling clay mineral, has been used as a medium for plant growth when mixed with a nutrient solution in the proportion of 0.95 ml g −1. Attapulgite is an ideal model “soil” for ultrastructural studies, enabling large intact thin sections through root, rhizoplane and soil. Transmission electron micrographs are presented which illustrate the value of attapulgite for in situ studies of rhizosphere populations, for the demonstration of enzyme activities in individual bacteria and for specific staining of extracellular polysaccharide. A standard fumigation-respiration technique widely used for estimating soil microbial biomass is shown to give unreliable results for rhizosphere samples and should not be used to measure microbial biomass in close asssociation with living roots. The addition of a dilute soil suspension to the attapulgite medium caused a stimulation of root growth without any increase in shoot growth.