INTRODUCTION 



By L. H. BAILEY 



The exposed surface of the crust of the earth tends 

 always to pass into a loose and disintegrated layer. 

 In this layer many organisms live, and out of it many of 

 them derive an essential part of their nourishment. 

 The organisms die and their remains return to the place 

 whence they came. In every successive epoch of the 

 earth's history, this layer has tended to become more 

 differentiated and complex in each epoch supporting 

 a higher type of plant, and in each succeeding age main- 

 taining a more advanced kind of activity. Thus the soil 

 has been formed, and the evolution of it and of the plant 

 tribes that grow out of it have been reciprocal, one con- 

 tributing to the other. If the soil is essential to the 

 growing of plants, so have the plants been essential to 

 the formation of soil. 



This marvelously thin layer of a few inches or a very 

 few feet that the farmer knows as "the soil," supports 

 all plants and all men, and makes it possible for the globe 

 to sustain a highly developed life. Beyond all calculation 

 and all comprehension are the powers and the mysteries 

 of this soft outer covering of tho earth. We do not know 



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