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.
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.
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.
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.
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.
The retentive power of a soil is due to its surface attractions.
The retentive power of a soil is due to the surface attraction of its particles, and when the space between them is small, or when, in other words, the pores are small, this attractive power will be all the greater.