SOIL AIR AND SOIL TEMPERATURE 



145 



The rate of diffusion depends on the total volume of the 

 pore spaces and not on their average size. A soil in good 

 tilth is therefore in suitable condition for permitting dif- 

 fusion of atmospheric and soil air. 



184. Composition of soil air. — The greater part of the 

 soil air, like atmospheric air, is composed of nitrogen and 

 oxygen. The principal difference between soil air and 

 atmospheric air, in respect to composition, is that the former 

 contains more moisture and more carbon dioxide. The 

 moisture comes from evaporation of water in the soil. 

 The carbon dioxide is produced for the most part by the 

 germs in the soil and by roots. The following table shows 

 how soils may vary in their content of carbon dioxide. 



Table 32. 



Percentage of Carbon Dioxide in Air of Differ- 

 ent Soils at Same Depth 



Character of Soil 



Percentage Composition 



Carbon 

 Dioxide 



Oxygen 



Nitrogen 



Forest soil . 



Clay soil 



Asparagus bed not manured for one 



year 



Asparagus bed freshly manured . . 

 Sandy soil six days after manuring . 

 Vegetable mold compost 



0.87 

 0.66 



0.74 

 1.54 

 2.21 

 3.64 



19.61 

 19.99 



19.02 



18.80 



16.45 



79.52 

 79.35 



80.24 

 79.66 



79.91 



Soils that are high in organic matter and in which decom- 

 position goes on readily, usually have a large quantity of 

 carbon dioxide. 



185. Production of carbon dioxide in soils. — It has 

 already been shown that plant roots give off a considerable 

 quantity of carbon dioxide throughout the growth of the 



