Soil spaces not occupied by soil water are occupied by soil air. Soil air is more humid than the air we breathe and has a higher carbon dioxide content. It carries oxygen from the surface to plant roots. Oxygen diffuses easily from the ground surface into the soil if the soil particles are large enough. The soil “breathes” oxygen in and carbon dioxide out through diffusion.
The amount of air in the soil depends on the size of the soil pores, which depends on the size of soil particles. Pores over a certain size do not have enough capillary strength to hold water against the pull of gravity. As the water drains from them, air is pulled in from the surface. The amount of air in the soil is not directly related to the amount of water. A coarse sand has excellent air circulation but holds very little water. A clay soil with poor structure can hold lots of water, but little air. However, a good potting mix holds lots of water and lots of air.
Anything that restricts the flow of air into the soil restricts the growth of plant roots. Because there is always more oxygen near the surface than deeper, plant roots grow more shallowly.
The most common causes of poor air circulation are compaction and poor drainage. Compaction in a garden is usually caused by foot traffic, which compacts the top couple of inches of soil. Cultivating relieves this compaction. Avoiding walking on the soil by using stepping stones or a system of permanent paths prevents compaction, removing the need to cultivate regularly.
Poor drainage can be caused by several factors, all of which keep water from leaving the soil. Because the soil is full of water, there is no room for air. Most plant roots die after a couple of days without oxygen.
If drainage is only partially restricted, some air will enter the surface of the soil. Plants adapted to using surface air can continue to grow in this situation, but deeper-rooted plants die. For example, annual bluegrass, a shallow-rooted weed in lawns, thrives in poorly-drained soil, but turfgrasses suffer.