BACTERIA OF THE SOIL 



395 



265. Bacteria. Of the several forms of micro- 

 organisms found in the soil, bacteria are the most 

 important. In fact, the abundant and continued growth 

 of plants upon the soil is absolutely dependent upon 

 the presence of bacteria, as through their action chemi- 

 cal changes are brought about which result in making 

 soluble both organic and inorganic material necessary 



for the life of higher plants, and 

 which, in part at least, would 

 not otherwise occur. 



Bacteria are thus transform- 

 ers, and not producers, of fer- 

 tility in the soil, although, as 

 we shall see later, certain kinds 

 of bacteria take nitrogen from 

 the air and leave it in the soil. 

 With this exception, however, 

 they add no plant food to the 

 soil. It is their action in render- 

 ing available to the plant ma- 

 terial already present in the soil 

 that constitutes their greatest 

 present value in crop-produc- 

 tion. It is to their activity in conveying nitrogen from 

 the air to the soil that we are indebted for most of 

 our supply of nitrogen in virgin soils. 



It is not usually the entire absence of bacteria 

 from the soil that is to be avoided in practice, for all 

 arable soils contain bacteria, although sometimes not 

 all of the desirable forms; but, as great bacterial 

 activity is required for the large production of crops, 



"^ 



Fig. 109. Some types of 

 soil-bacteria, highly magnified. 

 a, Nitrate formers; b, nitrite- 

 formers; c. Bacteria graveolens; 

 d, B. fusiformis; e, B. nebtilis; 

 f, Closteridium pasteurianum. 



