146 SOILS AND FERTILIZERS 



plant (§ 126). This, however, does not account for the 

 gas that is formed in soils on which no plants grow. For 

 this the germ life of the soil is responsible. These organisms 

 consume fresh air and give off carbon dioxide in the process 

 of their growth. In soils that contain a large and active pop- 

 ulation of microorganisms there is more carbon dioxide formed 

 than in a more nearly sterile soil. 



It has been estimated that in an acre of ordinary soil to a 

 depth of four feet the germs produce between sixty-five and 

 seventy pounds of carbon dioxide a day for two hundred 

 days in the year, and that, during the growing period, the 

 roots of oats or wheat would give off nearly as much in an 

 acre. 



186. Conditions that affect the quantity of carbon dioxide 

 in soils. — As carbon dioxide is heavier than air, the quantity 

 increases with depth. In warm weather more carbon dioxide 

 is formed than in cold because the germs are more active. 

 The soil moisture exerts an influence by furnishing the 

 necessary moisture for the germs. A very dry or a very 

 wet soil is not favorable to the production of the gas. More 

 carbon dioxide is given off by roots during the blossoming 

 period than at other stages of plant growth, consequently 

 the carbon dioxide content of soil air is highest about the 

 time the plants are in blossom. 



187. Usefulness of air in soils. — The three gases, oxygen, 

 nitrogen and carbon dioxide, that go to make up practically 

 all of the soil air are useful in bringing about those processes 

 that make soils fertile. Each one of these gases has its 

 function in contributing to plant growth either directly, or 

 by taking part in processes that render the soil more habitable 

 to plants. The functions of each gas will be discussed sep- 

 arately. 



188. Oxygen. — This constituent of soil air serves the 

 following uses : (1) As a direct food material for plants, 



