172 SOILS AND FERTILIZERS 



most extensive being in the United States. When ground 

 these are often called " floats." South Carolina phosphate 

 contains from 26 to 28 percent of phosphoric acid. Florida 

 phosphate exists in the forms of soft phosphate, pebble phos- 

 phate and boulder phosphate. Soft phosphate contains from 

 18 to 30 percent phosphoric acid, and because of its being 

 more easily ground than most of these rocks it is often applied 

 to the land without being first converted into an acid phos- 

 phate. The other two forms, pebble phosphate and boulder 

 phosphate, are highly variable in composition, varying from 

 20 to 40 percent in phosphoric acid content. 



Tennessee phosphate contains from 30 to 35 percent of phos- 

 phoric acid. In addition to these deposits, which have been 

 extensively mined since their discovery, there have been found 

 much larger deposits in the states of Idaho, Wyoming and 

 Montana, but these have not yet been worked. 



Apatite and coprolites are other forms of natural phosphate 

 that are used as fertilizers. The former is found in Canada 

 and the latter in England and France. They are not of much 

 importance in the fertilizer business of this country. 



222. Basic slag. — This is also called Thomas phosphate. 

 It is a by-product in the manufacture of steel from pig iron 

 rich in phosphorus. The phosphoric acid in this material is 

 more readily available than that in the mineral phosphates, 

 and when used as a fertilizer it does not require treatment 

 with acid. It should be finely ground. It is not extensively 

 used in the United States. 



223. Acid phosphate. — The very difficultly soluble phos- 

 phates may be rendered more easily soluble by treatment 

 with sulfuric acid. The product is called acid phosphate. 

 When applied to soils it is much more available to plants 

 than are any of the natural phosphates. Acid phosphates 

 contain gypsum or land plaster as well as phosphoric acid. 

 The proportion of the total quantity of phosphoric acid 



