54 AGRICULTURAL BACTERIOLOGY. 



phere, but a considerable portion, and an extremely important 

 part of it, is derived directly from the soil. While the atmos- 

 phere is practically inexhaustible in its supply of plant food, 

 this is not true of the soil. When the soil is chemically an- 

 alyzed it is found to contain a certain amount of material that 

 can serve as plant food, but this material is limited in quantity, 

 and whenever a plant grows upon soil it extracts a certain 

 amount of this limited food supply. This being the case, it 

 would seem that the soil, in the course of time, would become 

 exhausted of all possible supply of plant food, and if such a 

 condition were reached the earth would become absolutely 

 barren. But nothing is more evident than that this has not 

 been the history of nature. Plants have been growing on the 

 surface of the earth for countless centuries, and, so far as can 

 be determined, they grow at the present time with no less vigor 

 and no less luxuriance than they did in earlier times. The 

 soil certainly has not become exhausted by the growth of 

 vegetation and probably contains as much food material for 

 plants now as it ever did in the history of the world. 



How is it that nature thus remains ever fertile and that 

 vegetation has continued through the ages with undiminished 

 vigor ? This is, evidently, simply a question of the food sup- 

 ply of plants in the soil. In brief, the answer to the question 

 is as follows : The processes of nature are such that the same 

 material can be used over and over again as food, passing from 

 plant to animal and from animal to plant in an endless cycle, 

 and as long as the energy of sunlight falls upon the surface of 

 the earth to keep food supply in motion through this cycle, so 

 long is it possible for the fertility of the soil to continue undi- 

 minished. It is upon the continuance of this food circulation 

 that agriculture is dependent. We have been using the same 

 soil year after year, and, in the longer settled countries, cen- 

 tury' after century. Plant foods are being drained from the soil. 

 The great wheat fields of the new western lands arc yielding, 



