PART IV 

 BACTERIA IN RELATION TO SOIL FERTILITY 



CHAPTER XV 



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 liquid 

 will be found to contain not merely a large number of 

 microorganisms, 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- 



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