90 



The amount of carbonic acid in the atmosphere is by weight about 

 4.5 parts in 10,000. The amount in the air is, as Van Hise says, "in- 

 significant in comparison with the amount in soils in regions of luxu- 

 riant vegetation. In such regions the carbon dioxide is from thirty to 

 more than one hundred times more abundant than in the atmosphere." 

 This carbonic acid in the presence of bases, sodium, potassium, cal- 

 cium, and magnesium compounds, forms carbonates c'nd bicarbonates. 

 This is the process of carbonation — one of the most important proc- 

 esses of change in surface soils. 



In view of the dominance of CO2 in soils we may anticipate that 

 many of the animals living in them possess some of the characteristics 

 of the plants, bacteria, fungi, etc., which are active in such soils. The 

 anaerobic forms live without free oxygen ; others live only where oxy- 

 gen is present. The animals which thrive in the soil are likely to be 

 those which tolerate a large amount of COo and are able to use a rela- 

 tively small amount of oxygen, at least for considerable intervals, as 

 when the soil is wet during prolonged rains. This is a subject to 

 which reference will be made later. 



The air is the main source of oxygen, and from the air it diffuses 

 into the soil ; thus the process of ecjuilibration is constantly in progress. 

 Carbonic acid, also present in the air, is washed down by rain and 

 concentrated in the soil, where it is increased by the decay of organic 

 debris and by respiring animals to such an extent that it exists under 

 pressure and diffuses into the air, thus contributing to the air. In the 

 soil, then, the process of decarbotiisation is of great importance to 

 animal life, and must not be neglected. The optimum soil habitat is 

 therefore determined, to a very important degree, by the proper ratio 

 or balance between the amount of available oxygen and the amount of 

 carbon dioxide which can be endured without injury. The excessive 

 accumulation of carbon dioxide, an animal waste product, is compar- 

 able to the accumvilation of plant toxins which may increase in the 

 soil to such a degree as to inhibit plant growth. Such substances 

 must be removed from the soil, or changed in it to harmless com- 

 pounds, or plants and animals can not continue to live in certain 

 places. I have used the term ventilation to cover both the oxvgena- 

 tion and decarbonization of land habitats, and the same principles 

 are applicable to life in fresh-water habitats. 



We have just seen how atmospheric ventilation favors the removal 

 of certain injurious waste products from the air and soil. In addition 

 to gaseous waste products there are also liquids and solid kinds which 

 may be equally harmful in a habitat. These are known to exist in con- 

 fined liquids, as in acjuaria (Colton, '08; Woodruff, '12), where they 



