AIR IN SOILS 65 



volume of air contained in different soils is quite variable, 

 and is affected by the soil structure, texture, organic matter 

 and moisture content. 



The greatest changes in the composition of soil air 

 are found in the air of clayey soils when the particles are 

 extremely small. Clay particles may be flocculated into 

 masses, or flocculated clay may be granulated, thus con- 

 siderably changing the pore space in the soil and the volume 

 of air that it will contain. 



The size of the soil particles, which determines texture, 

 also affects the pore space and, consequently, the air con- 

 tent of the soil. Soil particles are of varying sizes, and 

 under field conditions, the soils of fine texture generally 

 possess large air space. 



Organic matter is quite porous, and its effect in the 

 soil is always to increase the volume of air. It is necessary 

 to have a sufficient supply of air to promote the decay 

 of organic matter. The main benefits of organic matter 

 are gained through its decay. 



The more completely the pore spaces in soils are filled 

 with water, the smaller the amount of air that will be pres- 

 ent. Water is held by capillary attraction more securely 

 when the particles are small than when large, the capillarity 

 being greater in a soil of comparatively fine texture. The 

 volume of air increases and the capillary water diminishes 

 when larger particles or granules are present. Because 

 of this the flocculation of the clay soils of bottom lands 

 by the use of lime permits of better drainage and more 

 complete aeration, thus greatly improving this type of 

 soil. 



54. Effect of Carbon Dioxide on Decay. The very rapid 

 decay of organic matter and the liberation of carbon dioxide 

 in large volume might serve to decrease the rapidity of 

 decay on account of the harmful effect of large percentages 

 of carbon dioxide to certain of the organisms producing 

 decay. The percentage of carbon dioxide liberated in 



