Perhaps more important than the composition of soil is the structure of the soil. The soil composition will determine the amount of micropores that are present in the soil. Soil absorbs water into its mircropores and holds it. Because of the small size of the micropores, water does not drain from them. Drainage occurs through larger spaces in soil known as macropores. It is the drainage of water through these larger spaces that allows air to be drawn into the soil. Without air (Oxygen) roots cannot live.

So soil structure essentially refers to the amount of macropores in the soil.

Soil structure can be compromised by breaking it down during construction, over tilling, compacting it, or killing off the organisms that provide the structure. Organisms that create structure can be complex organisms such as earthworms that create rather large spaces in soil by tunneling through it. There are also microorganisms that contribute to soil structure by creating a glue known as Glomalin that holds pieces of soil together and preserves the macropores in soil.

All of the organisms that contribute to soil structure require oxygen to live. So if oxygen is not constantly available in the soil there will be no organisms in the soil food web to preserve the structure of the soil.