PHOSPHATE FERTILIZERS 1 77 



impurities in the phosphate rock. This process is a 

 fixation change, as described in Chapter V. In an old 

 fertilizer there may be present citrate-soluble phos- 

 phoric acid in two forms, as dicalcium phosphate and 

 as hydrated phosphates of iron and aluminum. The 

 citrate-soluble phosphoric acid in fertilizers is not all 

 equally valuable as plant food because of the different 

 phosphate compounds that may be dissolved. 



215. Available Phosphoric Acid. — As applied to 

 fertilizers, the term available phosphoric acid includes 

 the water-soluble and citrate-soluble phosphoric acid. 

 These solvents do not, under all conditions, make a 

 sharp distinction as to the available and unavailable 

 phosphoric acid when it comes to plant growth. Some 

 forms of bones, which are insoluble in an ammonium 

 citrate solution are available as plant food, and then 

 again some forms of aluminum phosphate which are 

 soluble are of but little value as plant food. The 

 terms available and unavailable phosphoric acid, as 

 applied to commercial fertilizers, refer to the solubility 

 of the phosphates, and as a general rule their value as 

 plant food is in accord with their solubilities. The 

 more insoluble the less valuable the material. 



216. Phosphate Rock. — Phosphate rock is found 

 in many parts of the United States, particularly in 

 South Carolina, North Carolina, Florida, Virginia, 

 and Tennessee. The deposits occur in stratified 

 veins, as well as in beds and pockets. There are dif- 



