1. It quickly attacks and splits up the organic matters and accelerates afterwards their decomposition and their utilisation by plants; and
2. It neutralises the excess of acid in the soil, and causes the latter to part with its carbonic acid.
[Sidenote: Decomposition proceeds quickest at or near the surface.]
The process of decomposition proceeds as a rule at a much quicker rate on the surface and in the upper layers of the soil, where, as already mentioned, the number of micro-organisms is greatest.
[Sidenote: When carefully worked there is no time limit to the purifying powers of the soil.]
It has been maintained that the soil of sewage farms will after a while silt up and cease to purify sewage, but the results obtained with carefully managed farms clearly disprove this, and under these conditions there appears to be no limit as to time to the purifying power of soil.
[Sidenote: Depth of soil necessary for purification.]
Concerning the depth of soil—evaporation zone—that is necessary for the successful retention, absorption and decomposition of sewage, no generally applicable rule can be laid down, as this will depend on a variety of factors, amongst which may be mentioned: the character and thickness of the top soil (humus), the nature and cultivation of the top soil; the character of the subsoil—its permeability for air and its retaining and absorbing powers; the surface slopes of the land and the level of the subsoil water.
[Sidenote: Greater depths than 4 ft. will be rarely necessary.]
On some farms a depth of 3 feet on an average has proved sufficient, and on others the drains have been laid at depths ranging between 3 and 6 feet, but very special reasons ought to be shown for all depths over 4 feet.
[Sidenote: Soil best suited for sewage farms.]
Whilst practically no soil is entirely useless for sewage farming, with the exception perhaps of peat, owing to the quantity of moisture it contains, a soil that combines great permeability for air with high retaining and absorbing powers—such as a loamy sand with fairly large grains—is probably the best.
[Sidenote: Clay soil not unsuitable for sewage farms, but it necessitates a greater area of land.]
It has been maintained that clay, owing to its impervious character, is totally unsuitable for sewage farming, but the experience of such farms as South Norwood, Wimbledon, Warwick and Leicester disproves this. It is true, however, that as the purifying powers of the soil are restricted in a vertical sense to the upper layers, it may become necessary in places to extend the area of the farm beyond what would be necessary with a more pervious soil.