Soil Atmosphere

By soil atmosphere is understood the air filling the pores between the particles of the soil mass. Although part of the ordinary atmosphere, its composition is influenced by two opposing forces. On the one hand the various organisms of the soil abstract the oxygen and evolve carbon dioxide; on the other hand gaseous interchange with the outside air, brought about by diffusion and other processes, tends to restore the normal oxygen content of the atmosphere. The net result, therefore, is determined by the difference in velocity between these two processes.

Analyses of the soil atmosphere show that it suffers greater fluctuations in composition than ordinary air. As a rule it contains less oxygen, but nearly ten times as much carbon dioxide as the air, as shown by the following data:

	Soil atmosphere	Ordinary air.
Carbon dioxide	0.25	0.03 by volume
Oxygen	20.60	21.00 by volume
	20.85	21.03 by volume

Usually the sum of these two gases in the soil atmosphere is only slightly less than that in the air although at periods when nitrates rapidly increase, and in water-logged soils, there is a perceptible reduction in oxygen.

In addition to this free atmosphere there is a second soil atmosphere, consisting mainly of carbon dioxide and nitrogen, with practically no oxygen, which is dissolved in the water and colloids of the soil. The existence of this second atmosphere is important, in that it renders possible the existence of anaerobic organisms in the soil.