_Relation of Carbonic Gas to Bacterial and Fungous Activity._—It has been fully demonstrated by the researches of Koch, Miquel, Adametz, Fuelles, Wollny and others, that the formation of carbonic gas in the soil is not a purely chemical oxidation process, but is essentially dependent upon the presence and life-activity of numerous kinds of organisms, bacterial as well as fungous. The crucial proof of this fact is that the presence of any antiseptic diminishes, and if exceeding certain proportions completely suppresses, the formation of carbonic gas; while on the other hand all conditions known to be favorable to the life of such organisms, viz., the proper conditions of temperature and moisture (varying with different kinds), increase the formation of the gas. Such formation is of course, however, conditioned upon the presence of oxygen. In the case of most bacteria, there is a certain limit beyond which the presence of their own product exerts an injurious or repressive effect upon their activity; so that if the gas accumulates beyond that limit, the rate of its formation decreases despite of otherwise favorable conditions. It follows that the best life-conditions of these organisms (even when anerobic) cannot be fulfilled below a certain limited depth in the soil; and all observations show that their number decreases very rapidly with increasing depth (see chap. 9, p. 142), varying with the perviousness of the soil, but rarely exceeding four or five feet in the humid regions; though doubtless found at greater depths in the arid climates. It is also obvious that the use of any antiseptic or poisonous materials on the field or in the manure pile will tend to disturb and restrain the useful activity of these organisms. _Putrefactive Processes._—Carbonic gas is formed also, but to a much more limited extent, in _putrefactive processes_, occurring in the absence, or with only limited access, of air or oxygen. These processes likewise are conditioned upon the presence or activity of (largely anerobic) bacteria; but they should not occur in normally constituted, and especially in tilled soils, being as a rule inimical to the growth of cultivated plants (see chap. 9, p. 145). CHAPTER XV. THE COLORS OF SOILS. The natural coloration of soils forms a prominent part of the characters upon which farmers are wont to base their judgment of land quality; hence the origin and value of soil-colors deserve consideration.