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.
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.
Changes observed in the heavy clay land at Leicester since sewage treatment was commenced.
It may not be without interest to draw attention here to some of the changes that have taken place on the Leicester sewage farm since the land has received regular dressings of sewage. When I was engaged in laying it out in 1888 my powers of locomotion over the land were greatly impeded during wet seasons by the inordinate amount of clay that adhered to the boots; but when engaged again for some considerable time on the land during the winter 1900 to 1901 this unpleasant peculiarity had completely disappeared even on land that had recently been sewaged. Through the action of the sewage the very dense clay had been disintegrated and become so pliable that, when trod upon, it crumbled to pieces. The colour of the soil had been changed from a yellowish-brown to a greyish-black, and altogether the land had been greatly improved by the application of the sewage.
Movement of liquid through the passage and capillary zones to the impervious layer.
If more sewage is poured upon the land than the effluent drains can deal with—and here it may be well to bear in mind that on sewage farms in our climate on a broad average throughout the year about one-third of the total quantity is lost by evaporation—the excess will pass down between the drains from the evaporation to the passage zone, and if the flow of the sewage is not discontinued the downward movement in the passage zone may be continued until, after having traversed the capillary zone, the level of the subsoil water is reached.
Length of downward movement of water may be very great.
What length of time may elapse before this level is reached will entirely depend on local circumstances, but it will be clear from the preliminary remarks that the completion of this downward movement may in places and under certain conditions take a very long time.
Displacement of sewage held by the pores of the land by the fresh discharge of sewage upon the surface of the land.