282 SOILS AND MANURES 



By a further process of purification, a large proportion of 

 the magnesium sulphate can be eliminated and practi- 

 cally pure sulphate of potash produced. 



Kainite. The composition of the pure mineral is repre- 

 sented by the formula (K 2 S0 4 , MgS0 4 , MgCL, 6H 2 0) but 

 as obtained from the mines, it is always associated with 

 a considerable quantity of common salt and other im- 

 purities. The proportion of potash is usually about 

 10 or 12 per cent., which is equal to 18 or 20 per 

 cent, of sulphate of potash. By submitting it to a 

 process of lixiviation, a large proportion of the chlorides 

 of sodium and magnesium can be eliminated. This in- 

 creases the proportion of potash in the residual salt and 

 otherwise improves its quality as manure, but it is not 

 necessary for ordinary agricultural purposes. 



Polyhalite. This is a substance of similar composi- 

 tion, but contains no chloride. Tbe pure substance may be 

 represented by the formula (K 2 S0 4 , MgS0 4 , 2CaS0 15 H 2 0). 

 The commercial product contains about 25 per cent, 

 of sulphate of potash. 



Kieserite. The pure mineral consists simply of mag- 

 nesium sulphate and contains no potash at all. The 

 commercial product, however, is usually mixed with a 

 quantity of carnallite, and contains about 6 per cent, of 

 potash in the form of chloride. 



The tables on p. 283 show the average composition of 

 the more important potash salts produced at the mines. 



The total output of potash salts from the mines amounts 

 to nearly a million and a half tons annually, and of this 

 quantity, kainite and sylvinite together form about two- 

 thirds. The imports into the United Kingdom are esti- 

 mated at upwards of 40,000 tons annually, including 

 some 25,000 tons of kainite and sylvinite. 



Manurlal Value. Kainite is distinctly the most popular 

 source of potash for agricultural use, It is not, per- 



