SOIL AIR AND AERATION 311 



in soil air than in the atmosphere, while the oxygen varies inversely 

 with the amount of carbon dioxide, the nitrogen remaining prac- 

 tically the same. 



The above table shows the amount of carbon dioxide in soils 

 under different conditions with the comparative amount of oxygen. 



Aeration or Soil Ventilation. Aeration as spoken of in con- 

 nection with soils is an interchange between the atmosphere and 

 the soil air. It is necessary, first, to supply the oxygen needed by 

 roots and soil organisms; second, the supply the nitrogen needed by 

 nitrogen-fixing bacteria, and, third, to remove the carbon dioxide, 

 an excess of which becomes injurious because of the fact that it 

 excludes oxygen. Soil ventilation may be accomplished in a variety 

 of ways. 



(a) Diffusion is the mixing of gases of different composition 

 due to molecular movement. It may be well illustrated by filling 

 a bottle with carbon dioxide, and although this gas is heavier than 

 ordinary air. yet it' the bottle is left unstoppered for a short time 

 it will gradually diffuse into the surrounding atmosphere. As seen 

 in the preceding table, soil air contains a larger amount of carbon 

 dioxide than the atmosphere and it is constantly, but slowly, being 

 removed by diffusion. This process takes place more rapidly in 

 soils of large total pore space than in those with large individual 

 pores, so that for heavier soils with a high porosity and a high air 

 content, diffusion will take place more rapidly than in sandy soils 

 with larger pores and a smaller total pore space. This seems con- 

 trary to the fact that, sandy soils are better aerated than clay soils, 

 but it must be remembered that other agencies are at work that 

 bring about better aeration in sandy soils. Compacting a soil, by 

 any means, tends to lessen diffusion because it lessens the total 

 pore space. For this reason a soil in good tilth permits more rapid 

 diffusion than one in poor tilth. Temperature afTects diffusion in 

 that a higher temperature produces greater molecular activity. 

 which results in more rapid interchange of the gases. 



(b) Removal of Water. The removal of water from the soil 

 by any process permits air fo enter, thus bringing into the soil a 

 .new supply of pure air. As the water is carried out bv drainage 

 the air follows downward from the surface. The removal of water 

 by the roots of plants has the same effect, but the change is very 

 slow. 



(c) Changes in Atmospheric Pressure. Barometric pressure 

 is not constant. Regular changes take place in the region () f the 



