[Sidenote: Permeability depends first on the size of the pores, and secondly on the pore-volume.] The permeability of a soil for the passage of air and water depends, in the first instance, on the size of the pores, and is further to some extent influenced by the pore-volume. [Sidenote: Effect of large and small pores.] 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. [Sidenote: 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. [Sidenote: 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. 3. WATER CAPACITY OF SOIL. [Sidenote: Water capacity is equal to the pore-volume.] [Sidenote: 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. [Sidenote: 1 cubic yard of soil with particles of equal size will hold about 85 gallons of water.] As has already been stated, the pore-volume of a soil consisting of equal particles throughout, amounts to about 38 per cent. of the space occupied by it, and 1 cubic yard of such a soil—whether we have to deal with coarse gravel or fine sand—will hold about 85 gallons of water. 4. WATER-RETENTIVE POWER OF SOIL. [Sidenote: The water-retentive power of soil is a percentage of its water capacity.] The water-retentive power of a soil is expressed by that quantity of water which can be retained by it; it will always be a percentage or portion of the water capacity of this soil. [Sidenote: Soil with a large pore-volume and a large percentage of fine pores retains more water than soil with a small pore-volume and large pores.] [Sidenote: Clean gravel retains about 10 gal. and clean sand about 70 gal.] Soil with a large pore-volume and with a large percentage of fine pores will retain more water than soil with a small pore-volume and few fine pores. Clean gravel will retain about 12 per cent. of its water capacity, i.e. 10 gallons per cubic yard, whereas fine sand may retain as much as 84 per cent. of its water capacity, i.e. about 70 gallons per cubic yard. [Sidenote: Organically polluted soil retains more water than clean soil.] This will explain why a polluted subsoil containing a large amount of organic substances will retain more water than the same soil in a clean condition.