152 BOARD OF AGRICULTURE. [Pub. Doc. 



any of the materials just named and the reverted phosphoric 

 acid of ignited iron and aluminum phosphate, when taking into 

 account both the immediate and subsequent effects produced. 

 Strictly speaking, "reverted phosphoric acid" is such as 

 has irone back or reverted from the water-soluble condition 

 (monocalcic phosphate) to dicalcic phosphate, and which is 

 thus no longer soluble in water. This dicalcic phosphate 

 (popularly called two-lime })hos})hate), though essentially 

 insoluble in pure water, is readily dissolved by the carbonic 

 acid in the soil water and by the acid action of the plant 

 rootlets, so that plants can readily obtain it. Chemists, in 

 searching for an artificial measure of the readily assimilable 

 phosphoric acid, first leach the sample with pure water to 

 remove the soluble phosphoric acid, then the remainder is 

 treated for a given thne at a definite tem[)erature with a neu- 

 tral solution of ammonium citrate. This solution removes 

 all of the dicalcic phosphate, leaving behind the tricalcic 

 (popularly called tliree-lime phosphate) or insoluble por- 

 tions. A considerable i)ortion of the phosphoric acid of 

 steamed bone and tankage is also dissolved by the ammo- 

 nium citrate, notAvithstanding that it is not combined as 

 dicalcic but as tricalcic phosphate. It is not infrequent that 

 one-half of the phosphoric acid of tankage and considerable 

 of that of bone therefore appears upon analysis as reverted 

 phosphoric acid. Certain iron and aluminum phosphates 

 are likewise dissolved to a greater or less extent by the 

 ammonium citrate, so that the phos})horic acid contained in 

 them appears upon analysis as "reverted," though none of 

 it is combined either as dicalcic or as tricalcic phosphate. 

 One of the most important deposits of iron and aluminum 

 phosphate is that upon the island of Redonda, which has 

 been given the name of "redondite," This material in its 

 natural condition shows but traces of soluble phosphoric acid, 

 and but al)Out two to three per cent of reverted acid. Upon 

 subjection to heat it loses water, and becomes highly soluble 

 in the ammonium citrate solution, showing "reverted" phos- 

 phoric acid often in excess of thirty-two per cent. In 

 earlier experiments at the Rhode Island station both the raw 

 and ignited (heated) iron and aluminum phosphate were 



