56 AGRICULTURAL BACTERIOLOGY. 



never found. The number of species of bacteria in the soil is 

 also very great, although we have as yet no idea how great a 

 variety of microorganisms do here exist. 



The actual number of bacteria, or even the number of 

 species, is a matter of no special interest or significance. With 

 their wonderful powers of multiplication the question of their 

 abundance is only one of proper conditions for growth. Our 

 interest in soil organisms comes not from their number or 

 species, but from their functions. The agriculturist's concern 

 in them is wholly centered around the question of what they 

 are doing in the soil. The fact that the soil is full of living 

 bacteria, ready to grow and multiply if proper conditions are 

 furnished, is the only fact we need to notice concerning their 

 distribution. To the activities of these organisms in modify- 

 ing the constituents of the soil we must therefore turn our 

 whole attention. 



ORIGIN OF SOIL. 



Inasmuch as vegetation is dependent upon the soil, the fun- 

 damental question to consider is the origin of the soil. This 

 question resolves itself into two topics, with one of which bac- 

 teriology has very little to do, while with the other it is very 

 intimately associated. These two problems are connected 

 with two somewhat different classes of ingredients present in 

 ordinary earth. Soil contains large numbers of chemical com- 

 pounds, but these may be conveniently and, for our purpose, 

 satisfactorily, divided into two classes. The first includes the 

 so-called mineral ingredients, consisting chiefly of the ground 

 up rocks, and chemically composed of salts, bases and acids 

 of the various elements. The second consists of the organic 

 ingredients of the soil, frequently known as the Jin inns, the 

 origin of which, as we shall sec, must be traced to life. For 

 the continuation of plant life both mineral and organic ingredi- 

 ent^ are m <led, since plants feeding upon the soil consume 

 both mineral and organic substances. It is, however, the or- 



