22 DEPARTMENT OF AGRICULTURE GEORGIA. [308] 



easily worked. The rocks and bones from them are 

 crushed by machinery, ground to a powder, and subjected 

 to the action cf sulphuric acid, by which the bone phos- 

 phate is converted into superphosphate of lime and sul- 

 phate of lime, and made available as plant food. 



There is from 24 to 30 per cent, of phosphoric acid in 

 the phosphate rock, from 10 to 15 per cent, of which is 

 made available by treatment with sulphuric acid. 



Bone Ash is obtained by burning bones with access to 

 oxygen, by which the gelatinous material contained in 

 them is burned out, leaving a friable mass which is readily 

 acted upon by acids. In the absence of mills for crushing 

 the bones, this is the cheapest and most effectual method 

 of reducing them. 



In the analysis of fertilizers, phosphoric acid is deter- 

 mined in three forms, viz: "soluble," "' precipitated or 

 reduced," and insoluble." The first is soluble in pure 

 water, the second^ in what is known as " soil water," or 

 water having absorbed in it vegetable acids, ammonia, and 

 carbonic acid, and is, therefore, available to plants. It is 

 so-called from having been once in a soluble condition, but 

 by time and other causes, has been changed, and its union 

 with the lime made stronger. 



The insoluble phosphoric acid is that which is still in its 

 original condition, not having been acted upon by sulphu- 

 ric acid, and soluble only in strong acids. It is not imme- 

 diately available to plants. 



In common insoluble phosphate of lime, there is a chem- 

 ical union of three equivalents of lime to one of phospho- 

 ric acid. If, by the aid of sulphuric acid, two parts of this 

 lime be taken up and converted into sulphate of lime 

 (gypsum), there is left a soluble phosphate, in which the 

 phosphoric acid is available as plant food. 



Such a compound, in which there is phosphoric acid 

 in a soluble or available form, is called a superphosphate* 



The terms, monobasic, bibasic and tribasic phosphate of 



