94 THE PRINCIPLES OF AGRICULTURE 



with lime. The animals obtained the phosphoric 

 acid from the plants they ate, which in their 

 turn secured it from the soil. Another great 

 source are the deposits of phosphatic rocks in 

 the Carolinas, Florida and Tennessee. In these 

 rocks the phosphoric acid and lime are com- 

 bined in the same way as in bones. 



143. Bones and phosphoric rocks do not dis- 

 solve in water, and consequently the phosphoric 

 acid they contain is not easily absorbed by 

 roots. These materials, therefore, are com- 

 monly treated with acid, to make the phos- 

 phoric acid soluble ; and the material is then 

 known as an acid phosphate. 



144. In bones, one part of phosphoric acid 

 (PoO-,) is combined with three parts of lime 

 (CaO), and can be expressed as follows: 



Lime ^ CaO ^ 



Lime V Phosphoric acid ; or, CaO v PzOs 

 Lime j CaO J 



This substance is tri- (or three) calcic phos- 

 phate, and is insoluble. When sulfuric acid (or 

 oil of vitriol) and water are brought in con- 

 tact with the bones, part of the lime leaves 

 the phosphoric acid, and its place is taken by 

 water. If one part of the lime is united with 

 the sulfuric acid, then there results a sub- 

 stance which can be written thus : 



