428 SOILS: PROPERTIES AND MANAGEMENT 



the roots of many forest trees and known as mycorrhizal 

 fungi have the ability to fix atmospheric nitrogen, and that 

 in some .way the trees obtain a part, at least, of the nitro- 

 gen so fixed. The growth of forests on poor, sandy soil 

 containing practically no nitrogen has been urged as an 

 example of this process. 



350. Bacteria. — Of the several forms of microorgan- 

 isms found in the soil, bacteria are the most important. 

 In fact, the abundant and continued growth of plants on 

 the soil is absolutely dependent on the presence of bacteria, 

 for through their action chemical 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 trans- 

 formers, not producers, of 

 fertility in the soil, although, 

 as will be seen later, certain 

 kinds of bacteria take nitro- 

 gen from the air and leave it 

 in the soil. With this excep- 

 tion, however, they add no 

 plant-food to the soil. It is 

 their action in rendering 

 available to the plant ma- 

 terial already present in the 

 soil that constitutes their 

 greatest present value in crop 

 production. It is to their 

 activity in conveying nitro- 

 gen from the air to the soil 

 that we are indebted for most of our supply of nitrogen 

 in virgin soils (see Fig. 59). 



Fig. 59. — Some types of soil mi- 

 croorganisms highly magnified, 

 (a) , nitrate formers ; (6) , ni- 

 trite formers; (c), B. graveo- 

 lens ; (d) , B. fusiformis ; (e) , B. 

 subtilis ; (/), Clostridium pas- 

 teurianum. 



