MANURES 113 



(C) Potash Manures. — Potash in England is less frequently 

 deficient in soils than nitrogen and phosphorus. Certain 

 crops, however, require more potash than they can readily 

 obtain from some soils, and are much helped by the application 

 of potash manures. 



Plant ashes, which contain potjissium carbonate, were 

 formerly largely and still are, to some extent, used as a potash 

 manure, but the chief source of potash compounds has been the 

 immense deposit at Stassf urt, overlying huge beds of rock-salt.* 

 This is worked on an enormous scale, some of the products — 

 e.g., kainite — being sent into the market with no preparation 

 other than crushing, while others — e.g.., potassium sulphate — 

 are first purified by recrystallisation. The commercial products 

 contain their potassium either as chloride, KCl, or sulphate, 

 K2SO4. The former is the more soluble and diffusible, but 

 appears in the case of certain plants — e.g., tobacco, potatoes — 

 to produce ill-effects on the quality of the crops. The fol- 

 lowing are the chief forms in which potash is purchased for 

 manurial purposes : 



Kainite, the most widely used form, is a mixture of several 

 salts, including potassium chloride and sulphate, magnesium 

 chloride and sulphate, sodium chloride and calcium sulphate. 

 It usually contains about 12 to 13 per cent, potash, KgO, and 

 from 25 to 45 per cent, common salt. 



Muriate of Potash. — Under this, the old name for potassium 

 chloride, are sent out various qualities containing 70 to 95 per 

 cent. KCl (corresponding to from 44 to 58 per cent. K2O), the ' 

 chief impurity being common salt. 



Sulphate of potash is supplied generally either as of 90 or 

 96 per cent, purity — that is, containing about 49*6 or 52*5 per 

 cent, of K2O. 



Double sulphate of potash ami magnesia is obtained by 

 calcining the crystallised salt, MgSO4.K2SO4.6H2O. It usually 



 During the war several sources of potash have been proposed, the 

 most promising being the flue dust from the smelting of certain iron 

 ores. 



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