No. 6. DEPARTMENT OF AGR1CUL,TURK. 819 



will be found as free phosphoric acid in freshly made goods, but 

 this will remain so for only a comparatively short time. It wull, in 

 time, act on the insoluble phosphates contained in the fertilizer. 

 The water-soluble phosphoric acid is readily distributed in the 

 «oil, and is in a form that can be immediately absorbed by the roots 

 and used by the plants as food, but, unfortunately, the water- 

 soluble phosphoric acid will not remain long in the soil in this 

 condition, for, coming in contact with the lime, magnesia, etc., in 

 the soil, it reverts to a condition insoluble in water. In reverting, 

 this water-soluble phosphoric acid is precipitated in such a way as 

 to form a fine powder or coat over the particles of soil, and is thus 

 in a finely divided state and presents a considerable surface which 

 makes it easily dissolved by the acid soil waters, and the acid exuda- 

 tions of rootlets and thus still possesses a greater availability than 

 any other form of phosphoric acid. 



( 2 ) REVERTED PHOSPHATE OP LIME. 



Keverted phosphate of lime, also spoken of as "Reverted phos- 

 phoric acid," "Reverted calcium phosphate," "Precipitated phos- 

 phate of lime," "Citrate soluble phosphate," "Neutral phosphate of 

 lime," and "Di-calcium phosphate," is quite insoluble in pure water, 

 but is easily dissolved in water containing carbonic acid or salts 

 of ammonia and in weak acids. The term reverted was originally 

 intended to imply that this phosphoric acid had once been soluble, 

 but for some cause had "gone back" to a form insoluble in water. 

 This probably does take place to a limited extent, but as a matter 

 of fact, in the course of manufacture, there is not sufficient acid 

 used to make all the phosphoric acid soluble, and some of the tri- 

 calcium phosphate loses only one part of lime, and thus leaves di- 

 calcium phosphate remaining. As has been stated, some phos- 

 phoric acid is set entirely free, which will unite with insoluble 

 phosphoric acid, and bring some of it to the so-called reverted form. 

 This form of phosphoric acid is readily assimilated by plants, be- 

 cause the soil and solutions from the plant roots usually contain 

 acids strong enough to dissolve it. Reverted phosphoric acid is, 

 therefore, considered nearly as valuable as a plant food as the water- 

 soluble form. Reverted phosphoric acid is often met with in small 

 quantities in nature in connection with some insoluble phosphates, 

 guanos, limes and other organic matter. 



AVAILABLE PHOSPHORIC ACID. 



In the commercial world and in stating the results of an analysis, 

 the percentage of soluble phosphoric acid and the reverted or citrate 

 soluble phosphoric acid are added together and the sum called avail- 

 able phosphoric acid. 



