CHAPTER XI 



SOIL ORGANISMS IN RELATION TO SOIL FERTILITY 



IN Chapter II it was stated that millions of organisms live in 

 the soil, and that many of them bring about changes that are 

 fundamentally important in determining fertility. These many 

 organisms may be classed as bacteria, fungi, yeasts (Fig. 113), 

 algae (al'je), worms, insects, and rodents. The first three groups 

 are classed as "microorganisms" because they are of microscopic 

 dimensions. The majority of the bacteria are not more than 



A 



FIG. 113. Other soil organisms. A, a common mold; B, yeast plants. 



0.0000197 of an inch in diameter, and it is believed that some are 

 too small to be seen with the aid of the most powerful microscope. 

 In one-third of a thimbleful (one gram) of normal field soil have 

 been counted from 140,000,000 to 400,000,000 microorganisms; 

 and in manured soil, as high as 750,000,000. The most tiny ones 

 (bacteria) are in greatest abundance, and it is they which play a 

 large part in nature's plans, and stand in close relationship to 

 the practices which make possible successful crop production. 



In this chapter will be considered in particular three groups of 

 the helpful soil organisms, viz.: (a) Those which cause decomposi- 

 tion or decay; (6) those which cause nitrification, and (c) those 

 which gather nitrogen from the air. 



ORGANISMS OF DECOMPOSITION 



Microorganisms Clear the World of Trash (Fig. 114-A). 

 What would this world be were it not for the fact that all plants 



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