$8 AGRICULTURAL BACTERIOLOGY, 



tors in the disintegration of the rocks. We have, indeed, in 

 past years, regarded these physical and chemical forces as the 

 sole agencies in the breaking up of the crystalline and, later, 

 of the stratified, rocks to form soil. 



So far as this soil formation is due to physical agencies, the 

 matter does not concern our general subject, and, if they ex- 

 plained the whole, the question of the origin of soil minerals 

 would not concern bacteriology. But there is beginning to be 

 evidence for believing that in some respects the microorgan- 

 isms may be directly concerned in the problem. Doubtless, 

 the part they play may be subordinate, but it is none the less 

 of some considerable significance. 



Rock Disintegration. Although this weathering of the rocks, 

 which results in their being broken into a powder and some- 

 what changed in chemical nature, is to be attributed chiefly 

 to chemical and physical agencies, we are fast learning that 

 vital agencies play an important part even in these processes, 

 and that in the slow chemical changes which occur in the 

 soil, microorganisms have a considerable significance. Even 

 in regard to the primary process of rock disintegration it is 

 becoming evident that the action of bacteria has had its in- 

 fluence. We shall learn on a later page that there are some 

 species of bacteria which require no organic food for their sus- 

 tenance, being able to live wholly upon mineral matter, together 

 with ammonia salts as a source of nitrogen. Such organisms 

 are capable of living and growing upon the surface of bare 

 rocks. Even before this fact was known it had been suggested 

 that microorganisms contributed to the phenomena of rock dis- 

 integration, since bacteria have been found ifi abundance upon 

 the surface of disintegrating rocks. If the organisms do grow 

 in such localities it is certain that the chemical changes result- 

 ing from their life will have an important effect upon the process 

 of weathering. These bacteria are active oxidizing agencies 

 and they produce a variety of by-products which cannot fail to 



