PLANT LIFE. OSMOSIS. FERTILIZERS 413 



Potassium chloride occurs in the Stassfurt deposits as sylvine 

 KC1, but is chiefly associated with magnesium chloride as car- 

 nallite KCl ; MgCl 2 ,6H 2 0. When water is added to carnallite, a 

 large part of the potassium chloride, which is much less soluble, 

 separates out. Complete extraction and purification of the salt 

 involves a series of recrystallizations. The Alsace deposits con- 

 tain very little magnesium salts, and the separation of the potas- 

 sium chloride from the sodium chloride with which it is mixed is 

 comparatively simple. 



Potassium sulphate is also obtained from salt deposits, and is 

 substituted for potassium chloride as a fertilizer for certain crops, 

 such as tobacco. Chlorides, in general, melt at lower tempera- 

 tures than sulphates, and the presence of a chloride in tobacco 

 results in an ash that fuses on burning. This is, obviously, an 

 undesirable property, especially for cigars. 



Calcium. Calcium is naturally present in many soils as 

 calcium carbonate CaC0 3 . In contact with water containing car- 

 bonic acid (see p. 400), this gives a solution of the more soluble 

 bicarbonate Ca(HC0 3 )2. Other compounds of calcium which are 

 used as fertilizers include lime CaO or Ca(OH) 2 , calcium phos- 

 phate and acid phosphate, calcium sulphate or gypsum (p. 387), 

 and calcium cyanamide. 



Manure. One ton of farm manure contains about 10 pounds 

 of nitrogen (chiefly as urea CO(NH 2 )2 and proteins), 5 pounds of 

 phosphoric acid and 10 pounds of potash. The manure is mixed 

 with the soil just before planting seed, or is used as top dress- 

 ing. 



The bacteria in the air and in the soil assist materially in the 

 changes in the manure. Thus urea is hydrolyzed to ammonium 

 carbonate (NH 4 ) 2 C0 3 : 



CO(NH 2 ) 2 + 2H 2 -> (NH 4 ) 2 C0 3 . 



