390 NATURE AND PROPERTIES OF SOIL 



214. Bacteria. — Of the several forms of micro-organisms 

 in the soil, bacteria are probably the most important. In fact, 

 the abundant and continued growth of higher plants on the 

 soil is absolutely dependent on the presence of bacteria. 

 Through their action chemical changes are brought about 

 which result in the solution of both organic and inorganic 

 material necessary for the life of higher plants, and which, 

 in part at least, would not otherwise be available. 



Bacteria are single cell organisms and are probably the 

 simplest forms of life with which we have to deal. They are 

 generally much smaller than yeasts, multiplying by elongat- 

 ing and dividing into half. They are, therefore, often called 

 fission fungi. Molds multiply by budding. The activities of 

 both groups are similar, in that they produce their effects 

 very largely by the production of enzymes.^ The importance 

 of enzymic influences must constantly be borne in mind in all 

 biological transformations in the soil. 



Bacteria are very small, the larger individuals seldom ex- 

 ceeding one or two microns (.001 to .002 m.m.) in diameter. 

 In the soil there is good reason to suppose that there are 

 many groups which are too small to be seen under the micro- 

 scope. Such organisms may, therefore, function as a part of 

 the colloidal matter of the soil. Many of the soil bacteria 

 are equipped with extremely delicate vibrating hairs called 

 flagella, which enable the organisms to swim through the 



^Bacteria, as weU as most fungi, bring about their important trans- 

 formations largely by means of enzymes. These enzymes are catalytic 

 agents and are generally considered as colloidal in nature. A number of 

 transformations may be accelerated by enzymes, the exact reaction de- 

 pending on the nature of the enzyme itself. The change in the soil of 

 ammonia (NHa) to the nitrate form (NO3) is an example of oxidation 

 and is spoken of as nitrification. The reversal of this action is desig- 

 nated as reduction and is probably not entirely enzymic. A splitting 

 action is very common. The breaking up of glucose into alcohol and 

 carbon dioxide is an example of this (CeHiaOg = BCaHsOH -f 2C0a). 

 A fourth reaction that may be hastened by enzymic infiuenee is hydrol- 

 ysis. Cane-sugar may thus quickly produce glucose and fructose 



