Air In Soil
About half of the total mass of soil is made of up vast, interconnecting cavities, or holes. These holes are filled with both air and water. The amount of air and the amount of water filling these spaces varies dramatically throughout the year and from location to location, but averages about 50% each.
The air found in soil is very different from the air found in the atmosphere. The air in soils is not exposed to moving air currents and is much more moist, or humid, than atmospheric air. It also tends to be very rich in carbon dioxide and poor in oxygen.
Oxygen is removed by living organisms, and carbon dioxide is left behind. This carbon dioxide leaks out of the soil, slowly replenishing the atmosphere's carbon dioxide supplies.