Early in spring after snowmelt or heavy rain, soil can reach its maximum water holdingt capacity, which means that all the pores are filled with water. As the soil dries out and water drains off, either naturally or with the help of subsurface drains, the soil reaches its field capacity.
Large pores empty
In this state of field capacity, the slightly larger pores are emptied of water and replaced with air, while the finer pores remain filled with water. The higher up in the soil profile the pores are, the more likely they are to be filled with air. In a soil with 50% solid material and 50% pores, at field capacity approx. 10-20% of the soil volume is filled with air and 30-40% with water.