SOIL TEXTURE AND COMPOSITION 



95 



most inch or two of soil is lower than in the layers of soil imme- 

 diately below. Beyond the depth of eight or nine inches, the 

 number diminish rapidly, and in humid regions the number below 

 two feet is extremely low. This is illustrated by results reported 

 by Chester on facing page. 



The conditions, however, are quite different in the arid regions 

 where we have the slow formation of clay substances in the soil, 

 and therefore the absence of the cementing substances. This 

 results in greater aeration, and hence offers ideal conditions 

 for bacterial growth to a great depth. Moreover, the roots in 

 search for water penetrate to a great depth in the soils of the 

 arid regions. This results in an aeration of the soil and the supply- 

 ing of organic matter to bacteria at a great depth. Lipman found 

 the ammonifying organism at all depths to the tenth foot and at 

 times the nitrifying organisms to a depth of eight feet. The 

 nitrogen-fixing organisms seldom occurred below the third or 

 fourth foot. We have found great numbers of bacteria in both 

 dry- farm and irrigated soils of the arid regions in the second and 

 third foot. The average of several hundred such determinations 

 is given below; 



Depth 



Number of Colonies 





Irrigated Soil 



Dry-farm Soil 



1 foot 



2 feet 



3 feet 



6,240,000 

 1,760,000 

 1,147,000 



4,372,000 

 1,267,000 

 1,174,000 



The larger number found in the irrigated' soil is due to the 

 presence of a better supply of organic matter and not to the mois- 

 ture supplied. 



Kinds of Microorganisms.^ — Many methods have been used 

 to classify bacteria of the soil. Some workers have classified them 

 as to morphology, form and structure, others as to physiology and 

 action upon the soil constituents. The last is the most important 



