_Takes place in water-logged Soils._ The exclusion of oxygen from a soil may be effected by saturating the soil with water; and Warington has found in experiments carried out in an arable soil, by no means rich in organic matter, that complete reduction of nitrates may be effected in this way. It would thus seem that the process of denitrification will take place in water-logged soils, or in the putrefaction of sewage matter in the presence of large quantities of water. Whether this reduction will result in the production of nitrites, nitrous oxide, or free nitrogen, depends on different conditions. This process is one of great importance from an economic point of view, as it reveals to us a source of loss which may take place in the fermentation of manures. In the rotting of our farmyard manure it is possible that the denitrifying organisms may be more active than we have hitherto suspected, and that a considerable loss of nitrogen may in this way be effected. _Distribution of the Nitrifying Organisms in the Soil._ The nitrifying organisms are probably chiefly confined to the soil, and do not usually occur in rain or in the atmosphere. That, however, they are found in spots which we might be inclined to think extremely unlikely, is shown by some recent interesting researches carried out by Muentz, who discovered that the bare surfaces of felspathic, calcareous, schistose, and other rocks at the summit of mountains in the Pyrenees, Alps, and Vosges, yielded large numbers of them, and that they occurred to a considerable depth in the cracks and fissures of the rocks. The nitrifying organisms are also found in river-water, in sewage, and well-waters. _Depth down at which they occur._ In Warington's earlier experiments, the conclusion he arrived at was that the occurrence of the nitrifying organisms was almost entirely limited to the superficial layers of the soil, and that they were seldom to be met with much below a depth of 18 inches. His subsequent experiments, however, considerably modified this conclusion, and showed that nitrification may take place to a depth of at least 6 feet.[118] But although it may take place at this depth, it probably, as a general rule, is limited to the surface-soil, as it is only there the conditions for obtaining circulation of air are sufficiently favourable. A great deal, of course, will depend on the nature of the soil--_i.e._, as to its texture. In a clayey subsoil the principal hindrance to nitrification will be the difficulty of obtaining sufficient aeration. In clay soils it is probable, therefore, that nearly all the nitrification goes on in the surface layer; in sandy soils it may take place to a greater depth.[119] _Action of Plant-roots in promoting Nitrification._