THE BRAIN OF THE EARTH 



if he is on the wrong track in his farming; 

 what element in the soil is lacking if the soil 

 is not up to average productiveness; how this 

 missing element may be supplied to the farmer 

 at a maximum of service and a minimum of 

 cost. It is estimated that the farmers of the 

 United States in recent years have paid out at 

 least seventy millions of dollars annually for 

 commercial fertilizers. In a generation, at this 

 rate, the tillers of the soil in this country are 

 expending over one billion, five hundred mil- 

 lions of dollars for fertilizers. The greater 

 portion of this vast sum will be saved to the 

 farmers, amateur and professional, as they come 

 to a closer study of their soils and as they 

 learn how to restore when depleted, how to 

 avoid such depletion in the future. 



While the soil, as physically composed, has 

 but comparatively few substances in its make- 

 up, being formed of ground-up rock and 

 decayed vegetable and animal matter, as chem- 

 ically constituted it has many substances, nearly 

 seventy elements being found in the earth's 

 crust. Only twelve of these, however, are pro- 

 nounced essential to agriculture, while only 

 four of the twelve — nitrogen, phosphorus, 



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