82 



The term "reverted" is so applied because tlie soluble 

 phosphoric acid of the fertilizer "turns back" toward its 

 original insoluble form, and a similar behavior is noticed in 

 soils containing good proportions of iron and alumina or 



lime. 



The "reverted" phosphate of lime is the form previously 

 referred to as insoluble in pure water, but soluble in a neu- 

 tral solution of citrate of ammonia, this solution being 

 assumed to approximate in solvent power the soil water with 

 which the fertilizer will be brought in contact. 



"Reverted" or "citrate soluble phosphoric acid" together 

 with the water soluble, constitute what is termed" available 

 phosphoric acid," and though usually assigned the same 

 value, there is considerable diversity of opinion among sci- 

 entific investigators as to the relative values of these two 

 forms of phosphoric acid. 



While the soluble phosphoric acid is soluble in water 

 only so long as its free acid remains uncombined with some 

 such base as lime, oxide of iron, alumina, etc., and while it 

 speedily reverts in soils containing any considerable propor- 

 tions of these substances, nevertheless it is believed to have 

 an initial diffusive property not possessed by the reverted 

 form; that is, it is disseminated more rapidly and fertilizes 

 more soil in a given time than the same amount of reverted 

 would do. 



While the soluble phosphoric acid is seldom found in ap- 

 preciable quantities in natural phosphates, the reverted form 

 is frequently found in small quantities along with the in- 

 soluble in phosphate rock, and in still larger quantities in 

 some tropical phosphates, such as orchilla guano, etc.; it is 

 also found in bones to a considerable extent, and the phos- 

 phoric acid in many of our organic fertilizers, such as cotton 

 seed meal, is readily soluble in ammonium citrate. 



The superphosphates prepared from bones are believed by 

 many to have a higher value than those from phosphate 

 rock; this superiority, however, if it exists, extends only so 

 far as the availability of its insoluble phosphoric acid is 



