40 CHEMICAL FERTILIZERS 



most agricultural purposes it is found advisable to treat 

 rock phosphates with sulphuric acid for the manufacture of 

 superphosphate, as described on p. 128. 



Potash Mines 



German Deposits. In prospecting for sources of common 

 salt, some mines near Stassfurt were found, about 1850, 

 to contain deposits of potash suitable for use as fertilizers. 

 At first the mines were chiefly worked for the production 

 of common salt, but at a later date the value of the potash 

 produced exceeded that of the sodium chloride raised. 

 The amount of potash contained in the different deposits 

 varies considerably from place to place. The richest crude 

 salt is sylvinite, which consists of magnesium and potassium 

 chlorides, but the deposits in Northern Germany yield 

 little sylvinite. Pure sylvinite contains 26-3 % of potassium 

 chloride. The next in order of importance among these 

 minerals is pure kainite, a mixed salt containing potassium 

 chloride and magnesium sulphate, with 26*6 % of potassium 

 chloride. Hartsaltz contains 21*2 % of potassium chloride. 

 Carnallite, composed of the chlorides of potassium and 

 magnesium, has 15-5 % of potassium chloride. Poly halite 

 contains potassium, magnesium and calcium sulphates. 

 Owing to the large proportion of magnesium compounds 

 which most of the deposits contain, considerable purification 

 must be carried out before pure potassium salts can be 

 obtained, but much of the crude salt has been graded so as 

 to contain 12 J % of K 2 O, the other ingredients being mostly 

 common salt and magnesium compounds. 



French Deposits. Mines in Alsace are richer than those 

 in Stassfurt, Alsace mines averaging 22 % potash, against 

 12 % at Stassfurt. German control has, however, checked 

 the development of the former, to prevent competition with 

 the older Stassfurt workings. The Alsace potash beds, 

 which are only now being fully exploited for the first time, 

 show that they extend over a fairly wide area. The centre 

 of these is in the Miihlhausen district, and the deposits 

 extend to a depth of 1000 metres, but, being nearly level, 



