[Sidenote: Process of decomposition of organic matters stored in soil during periods of rest.] The polluting substances of an organic nature thus stored in the pores undergo here—and that probably chiefly during periods of rest—a process of decomposition or disintegration, which goes on until the whole of the organic matter has been converted into stable mineral forms. [Sidenote: Explanation of the term “self-purifying power of soil.”] This process of retention, absorption and decomposition of organic impurities is called “the self-purifying power of soil.” [Sidenote: 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. [Sidenote: 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. [Sidenote: 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. [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.]