DISTRIBUTION OF MICBOOBOAN8M8 63 



applied phases of our subject. The cycle of each element 

 is as important in the economy of nature as is that of any 

 other element. The interest in decomposition processes is 

 chiefly limited, however, to the cycles of carbon, nitrogen, 

 and sulphur. 



Distribution of microorganisms. It is evident, from 

 what has been said, that microorganisms will abound wher- 

 ever organic matter is present and where other conditions 

 will permit them to grow. The annual crop of organic mat- 

 ter finds its way to the soil, there to be destroyed by the 

 microorganisms and by the lower forms of animal life. 

 Virtually every soil yields sufficient vegetation to support 

 an abundant crop of microorganisms. The soil is the most 

 important of the habitats of the bacteria. In it are found 

 the most varied kinds, both from the standpoint of mor- 

 phology and physiology. The number, as determined by 

 the plate-culture method, ranges from a few thousand per 

 gram to several million ; but it must be remembered that 

 the number determined by the plate culture represents only 

 a portion of the total bacterial flora of the soil. It is cer- 

 tain that each gram of cultivated soil contains hundreds of 

 millions of organisms. 



The number of bacteria in the soil decreases rapidly with 

 the depth, so that at varying depths the soil ultimately be- 

 comes sterile. The decrease in bacteria with increasing 

 depth is probably due to a number of factors, such as ab- 

 sence of free oxygen and lack of food. In the denser soils 

 the filtering action is undoubtedly important. Liquids can 

 be sterilized by passing them through a filter of unglazed 

 porcelain. The soil acts in a similar manner, removing 

 mechanically the bacteria from the water percolating 

 through it. The soil also contains yeasts, molds, and proto- 

 zoa in varying numbers. 



The bacterial content of the ground water will depend on 



