EXPERIMENTS WITH PHOSPHORIC ACID. 21 



1. Insoluble phosphate of lime. This is the form in which 

 nearly all the phosphates exist in nature and from which the 

 other forms described below are derived. The bones of animals 

 are made up largely of this substance and are accordingly exten- 

 sively used in the preparation of commercial phosphates; but 

 the chief source of the insoluble phosphate now used in this 

 country is rock phosphate, large deposits of which are found in 

 South Carolina and Florida. It is insoluble in water and, 

 unless very finely ground, its phosphoric acid is given up very 

 slowly to the plant. 



2. Soluble phosphate of lime. When the insoluble phos- 

 phate is treated with dilute sulphuric acid, a large part is con- 

 verted into a form soluble in water, hence known as soluble 

 phosphate. It is then in a condition to be immediately used 

 by the plant. This is the most expensive of the three forms. 



3. Reverted or citrate soluble phosphate of lime. If a solu- 

 ble phosphate is allowed to stand for a long time it frequently 

 happens 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 soluble form, but experience has proved that this is 

 not the case. In fact, if a soluble phosphate is added to a 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 the citrate soluble are often classed together as available. 



There is another phosphate, not so generally used, in which 

 the phosphoric acid is combined with iron and alumina instead 

 of with lime. In its original condition it is not only insoluble 

 in water and but very slightly soluble in hot ammonium citrate, 

 but it is even less available to the plant than the corresponding 

 salt of lime. A phosphate of this description is quarried at 

 Redonda, a small island in the West Indies, and is known as 

 Redonda phosphate or Redondite. It is a characteristic of this 

 phosphate that at a high temperature it loses water, and at the 



