46 THE ELEMENTS OF PLANT FOOD 



The element phosphorus is not taken up in its pure 

 form by the plant, but in the form of dissolved mineral 

 matter, called phosphates, which are absorbed by the 

 roots. The most important of these used by the plant 

 is calcium phosphate, or, as it is commonly called, 

 phosphate of lime. 



Note. Phosphates are an essential part of the food for man and 

 other animals. They are not only necessary to bone formation, but 

 are found in all the tissues of the body, the nerve tissues and the brain 

 containing a large percentage. Phosphates are excreted by the kid- 

 neys, the quantity excreted seeming to bear a direct ratio to the amount 

 and intensity of brain action. 



Calcium Phosphate (Ca 3 (PO 4 ) 2 ). --There are two 

 principal sources of calcium phosphate, the bones of 

 animals and phosphatic rock. Phosphatic rock de- 

 posits are probably the accumulated remains of the 

 bones of prehistoric animals turned into stone; so 

 that it may be said there is but one great source 

 of calcium phosphate, namely, the bones of animals. 



The fresh or green bones of animals have much 

 organic matter in them, and because of this, though 

 the bones are ground fine, the powder docs not dis- 

 solve readily for the use of the plant. The bones are 

 therefore boiled and steamed to extract all of the ani- 

 mal matter. They are then ground fine and sold as 

 steamed bone meal. This contains about 30 per cent 

 of phosphoric acid. In the large packing establish- 

 ments, where many thousands of animals are killed 

 yearly, the bones accumulate in such quantities that 

 they are economically treated to make phosphate 

 fertilizers. 



Mineral phosphates, as they are mined, contain 

 varying amounts of phosphoric acid. That mined in 



