Soil with large pores will offer but little resistance to the passage of air and water, but when the pores are small these movements will be greatly impeded.
Permeability is proportional to the fourth power of the pore-diameter.
It has been ascertained that the permeability of soils is proportional to the fourth power of the diameter of the pores, so that it decreases very rapidly with the diminishing size of the pores.
In frozen soil permeability decreases rapidly.
In subsoil with small pores all movements of air practically cease when it is half full of water, and in frozen soil the decrease of the permeability is still more marked.
Water capacity is equal to the pore-volume.
Air can never be wholly driven out of the pores.
The water capacity of a soil is that quantity of water which can be stored in its pores; it is therefore equal to
the pore-volume. For very accurate measurements allowance must be made for a small amount of air, which even after filling remains in the pores and cannot be dislodged, but for practical purposes this can be overlooked.
1 cubic yard of soil with particles of equal size will hold about 85 gallons of water.