4 CIKCULAE XO. 113, BUREAU OF PLANT INDUSTRY. 



are not more than one one-luuidred-tliousandth of an incli in diameter, 

 and it is believed that some bacteria exist which are too small to be 

 seen even with the aid of the most powerful microscope. In spite of 

 their small size, however, they are concerned with every phase of our 

 daily life and by their incredible numbers and ceaseless activity over- 

 come their apparent insignificance. Bacteria cause diseases, make 

 milk sour, and in many ways are most troublesome. In spite of the 

 evil that some species of this group of plants cause, however, other 

 species, and even some of the troublesome species under different 

 conditions, are beneficial. 



CLASSES OF SOIL BACTERIA. 



The bacteria of the soil are chiefly of the beneficial types. They 

 occur in almost infinite numbers, a fertile soil having from 15,000,000 

 to 300,000,000 to the ounce. Their functions and value are variable, 

 both because the kinds of bacteria differ in soils and because any 

 given species may vary physiologically within certain limits according 

 to environmental conditions. The moisture, the temperature, the 

 degree of pulverization, the rock formation, or the geological history 

 of the soil, the aeration, the drainage, etc., are all factors which 

 partly determine the arction of soil bacteria; and perhaps more 

 important than any of these limiting conditions is the effect of one 

 kind of organism upon those with which it is closely associated, or, 

 more broadly speaking, the effect of the associative or competitive 

 action of the various groups of micro-organisms which act and react 

 upon each other. 



In outlining briefly the relation of soil bacteriology to crop pro- 

 duction it is simpler to consider the subject from the standpoint of the 

 bacteriologist and refer to groups of organisms more or less well known 

 that perform different functions in maintaining the fertility of farm 

 soils. These groups taken in their entirety include practically all of 

 the known types of decomposition and synthesis which take place in 

 the soil. They include organisms which produce hydrogen-sulphid 

 gas, indol, mercaptan, and other substances of this character whose 

 importance in agriculture has yet to be demonstrated and need not be 

 considered at this time. The chief groups and the order in which they 

 will be discussed are the carbohydrate fermenters, which change 

 starch, sugar, cellulose, etc.; the ammonifiers, which decompose com- 

 plex nitrogenous compounds and form ammonia; the nitrifiers, which 

 oxidize ammonia to nitrite and nitrite to nitrate; the denitrifiers, 

 which reduce nitrate to nitrite and sometimes to free nitrogen gas; 

 and the nitrogen fixers, which ''fix" or combine the free nitrogen gas 

 of the air with other substances to form proteids. 



[Cir. 113] 



