FERTILIZERS AND PLANT FOOD. 225 



ing a hundred pounds or more, and forms a closely packed layer, 

 covering a considerable area near Charleston, S. C, and in other of 

 the coast States. It is intermingled with shark's teeth and the bones 

 of various animals, which have changed to stone. The "Rock" 

 contains from twenty-five to thirty-two per cent of phosphoric acid. 

 This rock was first put on the market in 1868, six tons being the 

 total output. In 1885 437,850 tons were mined, and in 1887 

 480,558 tons. 



The phosphoric acid in these rock phosphates is insoluble, but 

 when ground and treated with sulphuric acid they become converted 

 into superphosphates, or soluble phosphates. 



Another valuable source of phosphoric acid is in boneblack. This 

 substance is simpl}^ bone charcoal ; it is used by sugar refiners, the 

 raw sugar being filtered through it. In this process the impurities 

 of the sugar are held b^' the boneblack and from the sj'rup the gran- 

 ulated sugar is crystalized. After a time the boneblack loses its 

 power of removing the impurities, it is then sold to fertilizer manu- 

 facturers, for it contains the phosphate of lime originally in the bone. 



Crude boneblack contains about thirty-four per cent of insoluble 

 phosphoric acid. 



At the present time there is a comparatively new source of phos- 

 phoric acid is the so-called Thomas-Gilchrist Slag. This comes 

 from the manufacture of iron or steel from certain ores of iron, 

 which contain phosphoric acid. The slag has about twenty percent 

 of phosphoric acid in a form not soluble in water. 



POTASH. 



Until 1868 the chief source of potash was ivood ashes. It is true 

 that nitrate ot potash, or saltpetre, has been used from a very early 

 time, records dating back to 1625, but it was more for the nitrogen 

 which this substance contained, than for the potash ; hence we may 

 regard the discovery and use of .the "German Potash Salts," as the 

 first rival of ashes. Unleached wood ashes vary very much in their 

 composition, containing from two and five-tenths to eight and one- 

 half per cent of actual potash. In "Canada" ashes* the average is 

 not far from six per cent of actual potash (KjO). While leached 

 ashes may contain anywhere from one-hall to two and one-half per 

 cent., according to the thoroughness of leaching. 



*Massachusetts Experimeut Station Report 1887, average of seveoty-one analyses. 



