The production of ethylene and other hydrocarbon gases by soils under anaerobic conditions was measured by gas chromatography. Ethylene was the only hydrocarbon gas which occurred in physiologically significant concentrations; more than 20 ppm was found in several soils after 10 days at 20°C. These concentrations were considerably higher than those which were known to cause severe reductions in the extension of root axes of some plant species.

Experiments with sterilized and unsterilized soil indicated that ethylene was produced by enzyme activity and not by chemical action. The gas was found in soil when the oxygen concentration fell below 2 per cent; total evolution was correlated with organic matter content, and was affected by drying and rewetting and by the growth of plant roots. The rate of production was increased by raising the temperature and by addition of glucose or peptone; high concentrations of nitrate depressed the rate, but sulphate and phosphate had little effect.

It is concluded that ethylene may be a significant factor in causing injury to crop plants under waterlogged conditions and also in situations where anaerobic pockets occur within a mainly aerobic soil structure, provided that escape of the gas from the soil is impeded sufficiently to allow inhibitory concentrations to build up in the vicinity of plant roots.