BUYING PLANT FOOD FOR THE SOIL 237 



posure to rain results in the removal of the potassium, so 

 that when ashes are subject to this sort of treatment they 

 lose their fertilizing value. 



The chief source of potash materials, however, is the 

 Strassfurt mines in Germany, where they occur in great 

 abundance and variety. 



Kainit. This substance is largely used in the South as 

 a potassium carrier for cotton. It contains twelve and 

 one-half per cent, of potassium, or two hundred and fifty 

 pounds to the ton. It is a crude product of the Strass- 

 furt mines, the impurities present being common salt and 

 magnesium chlorid. 



Muriate of potash. It is a purified product of the 

 potash mines, and is one of the richest materials supply- 

 ing potassium. It contains about fifty per cent, of potas- 

 sium, or one thousand pounds to the ton. 



Sulphate of potash. This material contains from 

 forty-eight to fifty per cent, of potassium, an average of 

 one thousand pounds to the ton, which is in the form of 

 sulphate, and it possesses several advantages for such 

 crops as tobacco and Irish potatoes. 



The sulphate of potash is more expensive than is either 

 muriate of potash or kainit, but is less extensively used, 

 although its use is on the increase. 



