PHOSPHORIC ACID FERTILIZERS 173 



originally present that is rendered soluble depends on the 

 quantity of sulfuric acid added. In practice there is usually 

 part of the phosphoric acid that is left in an insoluble form. 



224. Composition of, acid phosphate. — Acid phosphate 

 made from animal bone is called dissolved bone and contains 

 about 12 percent of available and from 3 to 4 percent of 

 insoluble phosphoric acid. It also contains some nitrogen. 

 When made from South Carolina rock, acid phosphate con- 

 tains from 12 to 14 percent of available phosphoric acid, 

 including from 1 to 3 percent of what is called reverted 

 phosphoric acid. The best Florida acid phosphate contains 

 as high as 17 percent, and the Tennessee acid phosphate 14 

 to 18 percent of available phosphoric acid. 



225. Reverted phosphoric acid. — A change sometimes 

 occurs in acid phosphate on standing, by which some of the 

 phosphoric acid becomes less easily soluble, and to that extent 

 the value of the fertilizer is lessened. This change is known 

 as reversion. It is much more likely to occur in acid phos- 

 phate made from rock than in that made from bone. The 

 quality of the material affects this change. The presence of 

 iron and aluminum is supposed to increase reversion. Re- 

 verted phosphoric acid is probably not so available as the 

 original acid phosphate. 



226. Absorption of acid phosphate by soil. — Like many 

 soluble substances acid phosphate, when applied to soil, 

 is in part absorbed and held in a form in which it will not be 

 leached out by the drainage water, but on the other hand, 

 remains in a condition in which it is available to plants. 

 Part of the soluble phosphoric acid may unite with iron or" 

 aluminum in the soil to form insoluble combination s. The 

 richer a soil is in lime the less is the danger of forming these 

 insoluble combinations. The availability of acid phosphate 

 may continue for a second year, or even longer, after being 

 applied to the soil. 



