FERTILITY AND SOIL LIFE 



105 



You have learned what is meant by "rotation of crops" 

 (see page 95). To grow corn or wheat for two years and 

 then clover for one is a common example of rotation. 

 Various benefits arise from such rotation, but probably the 

 principal one is the enrichment of soil in nitrogen in the 

 way indicated. Soy-beans are sometimes grown by 

 farmers and then "ploughed under," the sole value of this 

 crop being to enrich the soil in nitrogen for other crops. 



FIG. 46. One of the fungi. 



Besides the bacteria, which are the smallest of the 

 micro-organisms, there are hosts of other kinds of lowly 

 organisms which live in rich soil and affect its fertility. 

 There are, for example, many kinds of colorless plants 

 (fungi} which grow underground in the form of spreading 

 threads, more delicate even than the threads of cobweb (see 

 Fig. 46). Occasionally, when the conditions are just right, 

 a mass of such cobwebby threads will produce a structure 

 much larger than itself which grows up above the surface; 

 these structures contain millions of almost invisible, pow- 



