164 THE SOIL FLORA 



This results in greater aeration, and hence often 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 supplying of organic 

 matter to bacteria at a great depth. Lipman found the ammonify- 

 ing organisms at all depths to the tenth foot and at times the nitrify- 

 ing organisms to a depth of eight feet. The nitrogen-fixing organ- 

 isms seldom occurred below the third or fourth foot. I 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 : 



Number of colonies. 

 Depth. Irrigated soil. Dry-farm soil. 



Ifoot 6,240,000 4,372,000 



2 feet 1,760,000 1,267,000 



3 feet 1,147,000 1,174,000 



The larger number found in the irrigated soil is due to the pres- 

 ence of a better supply of organic matter and not to the moisture 

 supplied. 



Kinds of Microorganisms in Soil. The work so far done in this 

 field clearly establishes the fact that soils have a definite bacterial 

 flora as do water and cheese. The work done on the soil so far is 

 meager and has been carried on by a few investigators Hiltner and 

 Stormer in Germany, and Chester, Harding, and Conn in this 

 country. By far the best and most extensive piece of work is that 

 of Conn, and it is on his work that the main points of the following 

 are based. 



Hiltner and Stormer found that normally soil contains 5 per cent, 

 of liquefiers, 70 per cent, of non-liquefiers, and 20 per cent. Strepto- 

 thrix. The 5 per cent, liquefiers include the B.-subtilis and Ps. 

 fluorescens groups. Chester showed that the relative abundance of 

 these three groups is nearly constant in normal soil and that any 

 external influence which disturbed the equilibrium of the soil flora 

 would be indicated by a change in the relative abundance of these 

 three groups. This conception of the soil microorganisms as being 

 normally in a state of equilibrium has proved of considerable value 

 in interpreting soil phenomena. 



Conn divides soil bacteria into the following groups: 



1. Spore-producers 



2. Non-spore-producers 



Liquefaction rapid 

 Liquefaction slow or none 

 Rods 



Yellow chromogenic 



Non-chromogenic 

 Cocci 

 3. Actinomycetes Actinomycetes 



"Rapid liquefiers" 



"Slow growers" 



