l6o AGRICULTURAL BACTERIOLOGY. 



The soil bacteria have important relations to agricultural proc- 

 esses in at least five different directions : 



1. They unquestionably aid in the decomposition of rocks. 

 While the so-called weathering of rocks is chiefly a physical 

 process, a part of it is due to the agency of microorganisms 

 that are able to grow upon the surface of rocks without the 

 necessity of organic foods. In the formation of sulphates and 

 iron salts of various kinds bacteria also contribute very largely. 

 Not a little of the slow oxidation which occurs in nature, af- 

 fecting the mineral substances in the earth's crust, is attributed 

 in a measure to the action of living bacteria, so that the forma- 

 tion of the so-called mineral ingredients in the soil is, in no 

 slight degree, dependent upon the action of these organisms. 



2. The bacteria certainly have the power, even when acting 

 alone, of fixing free atmospheric nitrogen in the soil in the 

 form of some chemical compounds which can later be utilized 

 by other organisms. To what extent this is possible we do 

 not yet know, nor do we know whether the species of organ- 

 isms that produce such fixation are abundant or few ; but, 

 that atmospheric nitrogen is fixed in this way has been abun- 

 dantly demonstrated. From this fact we must conclude that 

 the soil organisms may possibly have been important agents in 

 primitive times in fixing the atmospheric nitrogen in the soil 

 and thus furnishing the basis of the soil nitrates. In regard 

 to the primitive origin of nitrates we have, of course, only 

 speculation to guide us. But the bacteria certainly are able to 

 fix atmospheric nitrogen, and were, possibly, one of the forces 

 in primitive times for seizing this element and fixing the 

 organic nitrogen in the soil for vegetation. 



3. The bacteria decompose all organic bodies that have been 

 built up into highly complex compounds through the agency 

 of either plant or animal life. It is their duty to pull to pieces 

 everything that has ceased to live. In thus pulling to pieces 

 these organic bodies, they liberate their carbon, causing it, in 



