SOIL MOISTURE AND ATMOSPHERE 17 



These various results are attempts to amplify and put 

 into mathematical form the familiar fact that a sandy soil 

 is dry and a clay or humus soil wet, and to enable us to 

 make finer distinctions. 



The Atmosphere of the Soil. — If the presence of fine 

 particles in the soil is chiefly of importance in retaining 

 water, the cnniiby structure provides for a free circulation 

 of air and for ready penetration of roots. The amount of 

 air in a given volume will tend to be greater in light dry 

 soils and, in a particular soil, an increase of the water is 

 accompanied by a decrease in the amount of air. Not only 

 is the amount of air in a given volume of soil limited and 

 subject to variation, its composition is different from that 

 of the open atmosphere and is influenced by various factors. 

 Russell and Appleyard (1915) found that in the top 6 in. 

 the percentage of oxygen is about 20"6 and of carbon dioxide 

 0*25, while in the lower layers the former gas tends to 

 decrease and the latter to increase. The cause of this is, of 

 course, the respiratory activity of roots and soil organisms, 

 and the decreased mobility of the gases. In grassland the 

 carbon dioxide content tends to be higher ; in one water- 

 logged soil it rose to 9*1 per cent., while the oxygen fell to 

 2"6 per cent. Now, as the roots of higher plants and 

 many soil organisms require an adequate supply of oxygen 

 for respiration, a condition which lowers the oxygen content 

 is potentially deleterious. Moreover carbon dioxide has a 

 narcotic action and may slow down root growth, or inhibit 

 the germination of seeds. Certain soil bacteria are anaerobes 

 (living only in absence of oxygen) ; these probably find a 

 normally suitable medium in the film water, which, in a 

 garden soil examined by Russell and Appleyard, contained 

 only 0*2 per cent, oxygen and as much as 99 per cent, 

 carbon dioxide. Water-logging and an abundance of 

 organic matter, which favours an excess of micro-organisms 

 and so a great consumption of oxygen, are the two factors 

 most concerned in reducing the supply of oxygen available 

 to the roots of the higher plants. 



Soil Organisms. — The organisms of the soil are numerous 



c 



