BACTERIA, OR SOIL FLORA 19 



been calculated by more than one writer on agricultural affairs 

 that the surfaces of all the particles in a cubic foot of fine 

 soil would amount to, at least, one acre of superficial ex- 

 tent. A thin film of moisture lines all these internal caves, 

 cracks and fissures. In these minute cavities enormous 

 numbers of tiny bacteria are to be found. Some of these are 

 to be seen under the microscope in active motion, winding their 

 way through the watery films; others are motionless. Their 

 size is extremely minute, and some 1,800,000,000 of ordinary 



FIG. 2. 'Diagram of magnified section of Soil, showing 

 particles surrounded by Water-film, and Root-hairs 

 (modified from Sachs). 



bacteria might easily be contained in so small a space as one 

 cubic millimetre of water. The illustration given herewith 

 shows a small patch of ordinary garden soil slightly magnified 

 so as to give an idea of the cavities and spaces which exist 

 even in a fine soil. 



Ignoring the many other classes of bacteria, the beneficial 

 bacteria which add to the fertility of soil may be divided, so 

 far as present knowledge goes, into four principal classes : 



i. Those which act on the organic matter present in the 

 soil and convert its nitrogen into ammonium compounds. 



