CHAPTER VII 



FERTILIZERS 



The available mineral in the soil. The elements needed by 

 plants exist in the soil in very unequal proportions. Some are 

 so abundant as to be practically inexhaustible ; others occur in 

 such small quantities, or are so slowly weathered out of the 

 soil, that cropping for a few years may deplete the supply to 

 a point where more must be added before the land will again 

 be fully productive. It has been estimated that in the upper 

 seven inches of certain soils there is sufficient iron to produce 

 a hundred-bushel corn crop every year for two hundred thou- 

 sand years, and enough calcium, sulphur, and magnesium to 

 produce such crops for from two thousand to fifty thousand 

 years, but only enough nitrogen and phosphorus for from fifty 

 to seventy years. Other soils may differ as to the amounts of 

 each element they contain, but the proportions are likely to 

 be about as given here. It is thus seen that the soil is not an 

 indestructible asset, but that it may easily wear out by having 

 all its store of certain elements abstracted by growing crops. 

 Nor is it necessary that an element be entirely lacking in a 

 soil to render it unfertile. If the element be in a form that is 

 not available to plants, the effect is the same as if it were 

 entirely absent. 



Toxic substances in the soil. Occasionally an analysis of 

 the soil may show that it contains sufficient food materials for 

 good crops, yet the plants that grow upon it do not flourish 

 because of certain substances excreted by the roots of the 

 plants themselves, which seem to be toxic or poisonous to 

 that particular crop. When crops of one kind are grown for 



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