1889.] PUBLIC DOCUMENT — No. 4. 29 



1868. South Carolina rock first mined, in all 6 tons. 



1878. " " " amount mined, 58,760 tons. 



1880. " " " " " 163,000 " 



1883. " " " « " 332,077 " 



1885. " " " " " 437,856 " 



1886. (P) Thomas-Gilchrist slag first used. 



To-day it is said that two-thirds of all the phosphoric acid 

 used in commercial fertilizers comes from South Carolina 

 rock. This rock is found about Charleston, S.C., although 

 it is not confined to one locality, but occurs ia both Caro- 

 linas and in Florida. 



Bone-black is a common and very desirable form of bone 

 which yields about thirty-four per cent of phosphoric acid. 

 It is simply bone charcoal. It is used. by the sugar refiners 

 for removing color and impurities from crude sugars ; in 

 time, however, this charcoal loses the power of purifying, 

 and is then sold to fertilizer manufacturers. Phosphoric 

 acid is present in fertilizers in three forms : one soluble in 

 water ; one insoluble in water, but soluble in vegetable acids, 

 and the third insoluble, except in strong acids. The phos- 

 phoric acid in bone, bone-black, South Carolina rock and 

 Spanish rock is insoluble, belonging to the third form ; that 

 in the Thomas-Gilchrist slag is soluble in citric acid. The 

 basis of Liebig's suggestion was the fact that bone, if treated 

 or "cut" with sulphuric acid, has its phosphate of lime 

 changed from the insoluble to the soluble form ; and his 

 reasoning was that a comparatively small amount of bone, if 

 rendered soluble, would supply as much available plant 

 food as a much larger quantity of raw bone. This was, in 

 practice, found to be true ; six or eight bushels of bone dis- 

 solved with sulphuric acid would yield as large crops for the 

 first year as would fifty bushels of crushed bone untreated. 

 It is now the general practice of all fertilizer manufacturers 

 to treat all phosphatic material with acid ; so that we have 

 dissolved South Carolina rock, dissolved bone-black, dis- 

 solved bone, each of which furnishes a good per cent of solu- 

 ble phosphoric acid. 



It is claimed, however, that the " slag phosphate," though 

 insoluble in water, is nevertheless available to plants. The 

 truth of this needs more demonstration than has yet been 

 given. 



