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. 9. THE MICRO-ORGANIC LIFE IN SOIL. [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. [Sidenote: Cycle of micro-organic activity during the year.] The picture of the cycle of micro-organic activity in the upper layers of the soil during the various seasons of the year is probably the following. In winter, especially during that period when frost and ice bind the earth, micro-organic life is apparently at its lowest ebb, and may in some very cold climates come to a standstill altogether, when micro-organisms may be said to hold their vegetative winter sleep. With the return of life and the awakening of nature in spring—especially with the approach of higher temperatures and the formation of moisture—micro-organic activity once more makes itself felt all round. During the summer months it is exposed to some injurious influences such as the heating and drying up of the upper layers of the soil, but, still gradually increasing, micro-organisms reach the climax of their activity during the autumnal rains, to remain in this state until with the advent of the cold season their activity gradually declines again. [Sidenote: Micro-organic life in layers from 3 ft. to 6 ft. in depth.]