The atmosphere contains three main gases, namely oxygen, carbon dioxide (CO) and nitrogen. In the atmosphere, oxygen is 20%, nitrogen is 79% and CO is 0.15% to 0.65% by volume. CO increases with the increase in the depth of soil because of decomposition of accumulated organic matter and abundance of plant roots. The presence of oxygen in the soil is important because it helps in breaking down insoluble rocky mass into soluble minerals and organic humification.  Air in the soil is composed of gases that are present in the atmosphere, but not in the same proportions. These gases facilitate chemical reactions in microorganisms.  Accumulation of soluble nutrients in the soil makes it more productive. If the soil is deficient in oxygen, microbial activity is slowed down or eliminated.  Important factors controlling the soil atmosphere are temperature, atmospheric pressure, wind/aeration and rainfall.