SUMMARY Wheat and bailey plants were grown for 3 weeks in a constant environment chamber containing approximately atmospheric concentrations of carbon dioxide (0.03%) labelled with 14C. The roots of the plants were maintained under sterile or non‐sterile conditions in soil contained in sealed pots which were regularly flushed with air. This enabled the quantities of 14C‐labelled carbon dioxide produced in the soil by plant and microbial activity to be separately assessed. At harvest, the 14C and total carbon contents of the roots and shoots and of the water‐soluble and insoluble material present in the soil were measured. These procedures enabled both the amounts of organic materials released into soil by the roots of growing plants and the effects of micro‐organisms on the process to be determined. Under sterile conditions between 5 and 10% of the photosynthetically fixed carbon may be released by roots compared with 12–18% by the roots of plants growing in unsterilized soil. These latter values are equivalent to 18–25% of the dry matter increments of the plants. The results indicate also that the increased evolution of carbon dioxide by cropped as compared to fallow soils can largely be ascribed to the immediate utilization by micro‐organisms of substances released by roots.

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