The last zone before the level of the subsoil water is reached is the capillary zone, in which the pores are partially or wholly filled by the upward movement—due to capillary attraction—from the subsoil water. The extent of this filling will depend on the size of the pores.
When the descending water has finally reached the subsoil water it either comes to a standstill altogether on the impervious layer or moves along the same, if the latter is not horizontal, until it may eventually leave the subsoil again by issuing therefrom in the form of visible or invisible springs.
[Sidenote: Rate of downward movement governed by pores.]
The rate of movement of any liquid—rain-water, sewage or other polluting liquid—is largely governed by the size of the pores. Where these are large, as for instance in coarse gravel, the descent of the water will be comparatively rapid, but when they are small it may take a very long time before the water reaches the level of the subsoil water, and in that case it will have undergone material changes as regards its chemical or bacterial composition.
[Sidenote: With a high level of subsoil water zones become indistinguishable.]
With a high level of subsoil water the zones may become indistinguishable, one zone reaching into the other, with the result that the whole of the soil becomes very wet.
When subsoil has been artificially drained the amount of water reaching the subsoil water below the general level of the drains will depend on the size of the latter and the distance between them. In such a case the downward movement of the water through undrained soil, previously described, may be further interfered with through the ventilation of the subsoil by drains, and the drying up action caused thereby.
[7] The term subsoil water is here used to denote that portion of the water in the pores of the soil, which is either at rest on or moves along the inclined plane of an impervious layer.
[Sidenote: Soil probably original home of micro-organisms.]
[Sidenote: Distribution of micro-organisms in soil.]
The soil is probably the original home of all micro-organisms, from which they have emigrated into other media. It contains vast numbers, and, according to some observers, 1 ccm. may hold 100,000 germs. By far the greater number is found on or near the surface, and in lower layers the numbers gradually diminish, until at last a depth is reached, which depends on local conditions, where the soil is perfectly sterile. The aerobes live near the surface and carry on their work in this region, whereas the anaerobes are at work lower down in the soil.