24 PRINCIPLES OF FARM PRACTICE 



be forced out by the water. In a similar way the amount of 

 air in a certain soil mass may be determined by measuring 

 the volume of water needed to crowd out or replace the air. 

 Since the water takes the place of the air the volume of water 

 used represents the volume of air present in the soil before 

 the water was added. 



The amount of air in the soil not always the same. Since 

 air is in the spaces between the soil particles, the amount of 

 it will depend upon the size and number of these spaces and 

 upon the degree to which they may be filled with water. 

 The latter point is important, because the air in the upper 

 part of the soil, where it is most needed, can be con- 

 trolled to a certain extent by regulating the amount of free 

 water. 



Why there is a need for soil air. Attention has been 

 called to the fact that the roots need oxygen all the time just 

 as much as other parts of the plant. In order to meet this 

 need of oxygen the soil should be well aerated. 



Soil aeration is important for other reasons. First, there 

 are certain useful bacteria that help to make the soil more 

 fertile. These bacteria need oxygen, and when the roots of 

 plants are provided with oxygen, the bacteria are served at 

 the same time. There are other bacteria that need nitrogen 

 as well as oxygen. As the air is about four-fifths nitrogen 

 which is thoroughly mixed with oxygen, there will always be 

 a good supply of nitrogen in well-aerated soils. 



Second, carbon dioxide always present in free air and more 

 abundant in soil air is absorbed by the soil water, thus giving 

 the water a greater solvent power than when pure. It is 

 then able to dissolve more soil material which is useful to 

 plants than it otherwise would. It is possible that too much 

 carbon dioxide may prove injurious to the roots of the plants, 



