[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.
[Sidenote: 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.
[Sidenote: When, after the limit of the retentive power has been
reached, of water are poured upon the soil, a portion of the
previously stored water is driven out, and its place in the pores
taken up by the fresh supply.]
It is further of interest to observe here, that if after the limit of
the retentive power has been reached further quantities of water are
poured upon the soil, the water retained in the lower layers will
commence to drain away. This means that the water freshly poured upon
the soil will drive out a portion of the water previously stored in
the pores. It is important to bear this in mind when dealing with
polluted water, as owing to this action the water penetrating into
deeper layers will to some extent at least have become purified in the
upper layers.