PHOSPHATE FERTILIZERS 2O3 



by this solvent. Citrate-soluble phosphoric acid may be 

 present in an old fertilizer in two forms, dicalcium 

 phosphate and hydrated phosphates of iron and alumi- 

 num. 



230. Available Phosphoric Acid. As applied to fer- 

 tilizers, the term 'available phosphoric acid ' includes the 

 water-soluble and citrate-soluble phosphoric acid. These 

 solvents do not, under all conditions, make a sharp dis- 

 tinction as to the available and unavailable phosphoric 

 acid when it comes to plant growth. Some forms of 

 bone which are insoluble in an ammonium citrate solu- 

 tion are available as plant food, while some forms of 

 aluminum phosphate which are soluble are of but little 

 value. The fineness of division of the fertilizer particles 

 also greatly influences the availability of the phosphoric 

 acid. The terms 'available' and ' unavailable phosphoric 

 acid,' as applied to commercial fertilizers, refer to the 

 solubility of the phosphates, and, as a rule, the value of 

 the phosphates as plant food is in accord with their sol- 

 ubility the more insoluble the less valuable. 



231. Phosphate Rock. - - Phosphate rock is found in 

 many parts of the United States, particularly in South 

 Carolina, North Carolina, Florida, Virginia, and Tennes- 

 see. The deposits occur in stratified veins, as well 

 as in beds and pockets. There are different types 

 of phosphates, as hard rock, soft rock, land pebble, 

 and river pebble. The pebble phosphates are found 



