BOX EXPERIMENTS WITH ITIOSPIIORIC ACID. 65 



unites with the acid to form sulphate of lime. The remaining 

 phosphate, containing much less lime than the original, is soluble 

 in water and is hence known as solu?jle phosphate, or, on account 

 of the excess of phosphoric acid, as "superphosphate." The 

 soluble phosphate is in a condition to be immediately used by 

 the plant. It possesses the additional advantages that by dis- 

 solving in the soil waters is becomes quickly and uniformly dis- 

 tributed through the soil, where the plant roots must everywhere 

 come into contact with it. It is the most expensive of the three 

 forms. This is also known as the monocalcic phosphate. 



3. Reverted or citrate soluble phosphoric acid. If a soluble 

 phosphate is allowed to stand for a long time it frequently hap- 

 pens that much of the soluble phosphate undergoes a change, 

 passing into a form insoluble in water but much more available 

 to the plant than the original insoluble phosphate from which 

 it was derived. This is the reverted or citrate soluble phosphate. 

 It was formerly supposed to be of much less value than the solu- 

 ble form, but experience has showm that there is but little differ- 

 ence as regards actual availability. In fact, if a soluble phos- 

 phate is added to the soil, a large part of it "reverts" before the 

 crops have had time to take it up. It is known as the citrate 

 soluble phosphate because, unlike the insoluble form, it is readily 

 soluble in a hot solution of ammonium citrate. This reagent is 

 therefore employed in the laboratory to distinguish the form in 

 question. The soluble and citrate phosphates are often classed 

 together as available. 



The reverted or citrate soluble phosphate may or may not 

 consist of dicalcic phosphate. While the dicalcic phosphate 

 possesses the characters ascribed to the third form mentioned, 

 yet it is found that in some cases the ammonium citrate will 

 dissolve a considerable quantity of the tricalcic phosphate, the 

 amount standing in intimate relation to the degree of fineness to 

 which the phosphate has been reduced. The citrate, then, does 

 not afford us the means of distinguishing sharply between the 

 dicalcic and tricalcic phosphates. This fact, however, in no way 

 affects the assumption that the citrate soluble phosphate is avail- 

 able to the plant. 



It should be added also that the so-called insoluble does not 

 strictly correspond to the tricalcic phosphate, for the reason just 



