84 GENERAL CONSIDERATIONS 



bones, too, becomes available more easily than that 

 from rock phosphates. Sometimes the bone meal is 

 treated with sulphuric acid to make it all soluble. 

 It is then called dissolved bone. The rock phos- 

 phates are nearly always treated with sulphuric acid 

 before being used. They are then known as acid 

 phosphates or superphosphates. Bird and bat 

 guanos all contain considerable quantities of phos- 

 phoric acid. The bat guanos in particular usu- 

 ally owe most of their value to this substance. 

 Thomas slag, a by-product of certain iron furnaces, 

 is considerably used in England and Europe as a 

 source of phosphoric acid. Hard-wood ashes also 

 contain a considerable percentage. 



Potash for fertilizing purposes also comes from 

 hard-wood ashes, but it is usually derived from cer- 

 tain potash salts that are extensively mined in 

 Germany. Kainit, one of these salts in its crude 

 state, is very largely used in the manufacture of low- 

 grade fertilizers. It contains considerable quantities 

 of common salt and other impurities and so is ob- 

 jectionable for tobacco and some other crops. The 

 muriate of potash prepared from these crude salts is 

 usually the cheapest form in which to apply potash, 

 especially at interior points where high freight rates 

 increase the comparative cost of the crude salts. It 

 is useful for corn, cotton, potatoes, and all ordinary 

 crops; but it cannot be used for tobacco, since chlorine 

 in any form ruins the burning quality of the leaf, 

 and it is claimed by some investigators that it 

 should not be used for sugar cane as it is thought to 

 increase the per cent of glucose and other impurities. 



