The air and other gases in spaces in the soil; specifically, that which is found within the zone of aeration. The aeration of soil by exchange with the free air above is a continuing process, but is accelerated by pressure and temperature changes (See normal aeration). The composition of soil air is modified by bacterial activity and by chemical processes due to the action of oxygen and carbon dioxide in the presence of soil moisture. Generally speaking, soil air contains more carbon dioxide than does free air. In addition to the gases originating in the soil, the following may be present in the soil atmosphere in measurable quantities: ammonia, hydrogen sulfide, methane, and other hydrocarbons resulting from decay of organic materials. Traces of helium, radon, thoron, and actinon from radioactive processes also diffuse through the soil layers into the atmosphere by the process of exhalation.