MANURES AND FERTILIZING MATERIALS. 65 



pally from phosphate rocks and bones. The phos- 

 phate rocks are classed as river rock and land rock. 

 The river rock is dredged from the bottom of rivers 

 and the land rock is mined. Phosphate rock is found 

 in Tennessee, Florida, North Carolina and South 

 Carolina. When phosphate rock is ground it is called 

 ground phosphate rock, or floats. 



CORN GROWN WITH FERTILIZER. 



The phosphoric acid in phosphate rock is in a form 

 that the plants cannot use readily. In order to make 

 this phosphoric acid available as a plant food, sul- 

 phuric acid is added to the ground phosphate rock. 

 This acid changes the phosphoric acid into a form 

 which the plant can readily take up. When sulphuric 

 acid is added to the ground phosphate rock, the re- 

 sultant product is called acid phosphate. The phos- 

 phoric acid in acid phosphate exists in three forms: 



