166 SOILS AND FERTILIZERS 



Phosphoric acid 

 per acre. 

 Lbs. 



Corn, 65 bu 18 



Stalks, 4,000 Ibs 4 



Total 22 



Peas, 3,500 Ibs 25 



Red Clover, 4,000 Ibs 28 



Potatoes, 150 bu . . . 20 



Flax, 15 bu 15 



Straw, i, 800 Ibs 3 



Total... 18 



212. Amount and Source of Phosphoric Acid in 

 Soils. To meet the demand of growing crops for 25 

 pounds of phosphoric acid per acre, there are present 

 in soils from 1,000, and less, to 8,000 pounds of phos- 

 phoric acid per acre, of which, however, only a frac- 

 tion is available as plant food at any one time. The 

 availability of phosphoric acid is a factor which has a 

 great deal to do in determining crop-producing power. 

 Many soils contain a large amount of total phosphoric 

 acid which has become unavailable, because of poor 

 cultivation and the absence of stable manure and lime 

 to combine with the phosphates and render them 

 available. 



The phosphates in soils are derived mainly from 

 the disintegration of phosphate rock and from the 

 remains of animal life. The phosphate deposits 

 found in various localities are supposed to have been 

 derived either from the remains of marine animals or 

 from sea-water highly charged with soluble phos- 



