ABSTRACT
The movement of gases through the pore space of the soil is important in several respects. It plays a vital role in soil biological processes, in which the supply of oxygen, O2 , to respiring roots and microorganisms from the atmosphere above the soil is balanced by the outward flow of carbon dioxide, CO2. Impedance of this gas exchange is frequently damaging to plant growth, due to deficiency in O2 supply to the roots. Such conditions also give rise to emissions to the atmosphere of the microbially produced gases methane, CH4, and nitrous oxide, N2O, which, like CO2 , contribute to the greenhouse effect (Houghton et al., 1996); conversely, part of the methane in the atmosphere is removed by diffusion into well aerated soils, where it is oxidized by microorganisms. Soil fumigation to control diseases of horticultural crops depends on movement of the fumigant in the vapor phase; emissions of methyl bromide, the most widely used fumigant, contribute to stratospheric ozone depletion (as does N2O). In a very different context, emissions of the radioactive gas radon into buildings, following the decay of radium present in underlying soils, may be sufficient to constitute a health hazard in some localities.
