[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.