This process of retention, absorption and decomposition of organic impurities is called “the self-purifying power of soil.”
After conversion substances are removed out of the soil by the plants, by the subsoil air and subsoil water.
The substances thus converted do not remain in the pores, but they are removed either by the plants, for which they act as food, or by the currents of subsoil air, or by the subsoil water, and as the removal of fertilising substances by the subsoil water indicates a waste it must be the aim of a careful management to utilise them as much as ever possible for the benefit of the plants.
Process of digestion. “Sewage sick.”
The whole of these intricate and very complicated changes may be likened to the process of digestion in animals, and when these digestive powers are overtaxed signs of sickness may be noticed as the inevitable result, which increase until, in sewage phraseology, the land becomes “sewage sick.” In this condition it remains until the flow of the polluting liquid is stopped, when after a period of rest—recreative period—the digestive powers gradually return and begin to do their work afresh.
Action of lime.
When the soil of a sewage farm has got into this
state, owing to having received heavy doses of sewage, the application of lime has proved very beneficial by accelerating the process of nitrification, and in this respect interesting experiments have been made on the Berlin sewage farms. The action of lime is said to be a twofold one.
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.