818 ANNUAL, REPORT OF THE Off. Doc. 



FORMS OF PHOSPHATE OF LIME. 



In mauufactui'ing fertilizer from phosphate of lime the aim has 

 been to change it from the insoluble condition to a soluble condition 

 which would be more available to plants. In the natural and insolu- 

 ble state, the phosphate exists in what is chemically known as tri- 

 calcium phofiphate (three-lime phosphate), and in the conrse of manu- 

 facture this is changed chemically so that, at the end of the opera- 

 tion there exists four kinds or combinations of calcium (lime) and 

 phosphoric acid, which are as follows: 



(1.) Soluble phosphate of lime, or mono-calcium phosphate. 



(2.) Keverted phosphate of lime, or di-calcium phosphate. 



(3.) Insoluble phosphate of lime, or tri-calcium phosphate. 



(4.) Tetra-calcium phosphate, or four-lime phosphate. 



( 1 ) SOLUBLE PHOSPHATE OF LIME. 



This is popularly known under several other names as ''Super-phos- 

 phate," ''Super-phosphate of lime," "Acid phosphate," "Acid phos- 

 phate of lime," "Water-soluble phosphate," "Acid calcium phos- 

 phate," "Mono-calcium phosphate," and "One-lime phosphate." 

 Phosphoric acid does not occur naturally in the soluble state. 



Soluble phosphoric acid is made by treating bones or mineral phos- 

 phates with sulphuric acid (oil of vitriol.) The chemical change 

 Avhich occurs is practically as follows: Sulphuric acid and water 

 being applied to the materials containing insoluble phosphates (tri- 

 calcium phosphate), the sulphuric acid combines chemically with 

 two parts of lime and forms sulphate of lime or gypsum (land 

 plaster), while the Avater unites with the jihosphoric acid and one 

 part of lime, forming mono-calcium or soluble i^hosphate of lime. 

 The substances being mixed, it is a natural chemical action or re- 

 action that takes place with the result stated. The total conversion 

 of the insoluble to the soluble form, cannot be accomplished without 

 using such an excess of sulphuric acid as would be injurious to seeds 

 and roots of plants which would come in contact with the fertilizer, 

 and also would make the fertilizer of such a mechanical condition 

 as to be difficult to handle and apply. In practice, less acid is added 

 than is necessary to wholly convert all the phosphoric acid to the 

 soluble form, consequently more or less of all the forms of phos- 

 phoric acid are found to be present after the fertilizing materials 

 have been treated or dissolved. Leibig, in 1840, was the first person 

 to suggest the treatment of bones and mineral phosphates with sul- 

 phuric acid, for the purpose of rendering the phosphoric acid more 

 available for plants. This may be said to be the beginning of the 

 use of genuine artificial fertilizers. In the course of dissolving 

 phosphates some of the phosphoric acid is set entirely free and 



