_The Composition of the Free Air of the Soil_ usually differs from the air above, in that besides being saturated with moisture, its nitrogen-content is slightly increased (by one-half to over one per cent); the oxygen-content on the other hand, is diminished, being in part (sometimes nearly to the extent of one-half of its volume) replaced by carbonic gas, derived partly from its secretion by the roots, partly from the oxidation of organic substances. It naturally follows that the richer the soil in the latter, the more carbonic gas will be formed under favoring conditions; so that in freshly-manured land the amount of oxygen transformed into carbonic gas will be greatest, while in the surface-soil of ordinary fields, carbonic gas rarely reaches to as much as one per cent. In all cases, however, the content of carbonic gas in the air of the soil is materially higher than that of the air above it, and thus serves to intensify greatly the solvent and disintegrating effect of the soil water upon the soil materials (see chap. 2, p. 17). The soil-mass itself, however, retains carbonic dioxid with considerable tenacity, so that it is not possible to wash it out completely by filtering water through it. When water containing carbonic gas in solution is filtered through the soil, the gas is sometimes completely absorbed, the water passing off free from gas. The presence of free carbonic gas in soils is readily demonstrated by passing through the warmed soil a current of air, which is then made to bubble through lime water; a clouding of the latter, and the ultimate formation of a precipitate of calcic carbonate, proves the presence of the gas, and may also serve to measure its amount. From the fact that the free air in normal soils may contain as much as one-fortieth of its bulk of carbonic gas, besides what may be contained in the condensed form, we may conclude that this gas is formed within them with considerable rapidity; for otherwise, in view of the free communication and diffusion with the outer air, such large amounts could not be maintained in the surface-soil. Doubtless a considerable proportion of the carbonic gas normally contained in the atmosphere is thus supplied from within the soil itself.