PERMANENT SOIL FERTILITY 



the soil. He used chemical analysis as a means of taking in- 

 voice, as he was wont to say, of these substances within the 

 soil just as the merchant takes an invoice of the goods upon 

 his shelves. Whether or not the proper use is made of this 

 plant food depends largely upon the kind of farming the 

 farmer carries on, just as it depends upon the business ability 

 of the merchant whether or not his business is successful. 

 Again and again Doctor Hopkins urged that an accurate in- 

 voice of stock with which to work is as absolute a necessity 

 for the farmer as it is for the merchant. 



The chemical analysis of the soils of Illinois made with 

 this idea in mind at once showed a marked variation in the 

 amount of the various essential plant foods present in the soil. 

 Phosphorus, for instance, as measured in a typical area of 

 brown silt loam in the corn belt was found to be the most 

 limited element, not only as measured by the absolute amounts 

 present but also as measured by crop requirements. While 

 the supply of calcium present was found to be sufficient, as 

 Doctor Hopkins expressed it, for the production of a 100- 

 bushel crop of corn annually for ninety centuries, the supply 

 of magnesium sufficient for thirteen centuries, and the supply 

 of potassium sufficient for eighteen centuries, the supply of 

 phosphorus was found to be sufficient for only sixty-two years. 

 Such illustrations are very significant in their emphasis of 

 the importance of providing for prosphorus in a system that is 

 to insure permanent and maximum crop production. 



The problem connected with the fifth element, nitrogen, is, 

 in the words of Doctor Hopkins, "the most important prac- 

 tical problem confronting the American farmer." Nitrogen is 

 required by crops in large quantities, while the supply in the 

 soil is markedly limited, and this element, if purchased on the 

 markets of the world, is the highest priced of all the plant- 

 food materials. A hundred pounds of nitrogen is required 

 for every 100 bushels of corn, and nitrogen at present sells for 

 thirty cents a pound. At such a prohibitive price the farmer 



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