PART IV 



Bacteeia in Relation to Soil Fertility 



CHAPTER XV 



1 



NUMBER AND DISTRIBUTION OF BACTERIA 



IN THE SOIL 



There are species of bacteria so common in cultivated 

 soils as to constitute a definite bacterial flora. This flora 

 may vary with climatic conditions, the composition of 

 the soil, and the methods of tillage and cropping. How- 

 ever, it shows fairly constant characteristics. By agi- 

 tating a small quantity of fresh soil with some sterile 

 water, a turbid liquid, in which the bacteria remain in 

 suspension for a considerable length of time, is obtained. 

 When placed under the microscope, a drop of this Hquid 

 will be found to contain not merely a large number of 

 micro6rg3,nisms, but, also, numerous species, as indicated 

 by differences in shape and size. Rod-shaped, spherical, 

 spiral and boat-shaped forms may be distinguished 

 among them without great difficulty. The rod-shaped 

 organisms will be present in by far the greatest pro- 

 portion. As in the case of water and sewage, some of 

 the organisms are endowed with the power of motion 

 and others are devoid of it. There will also be spore- 



(135) 



