412 



calcium ^nd-T^ ns r^ nfp (" superphosphate ") CaH 4 (PO 4 ) 2 , which 

 is made by heating pulverized natural calcium phosphate with 

 sulphuric acid, containing the requisite proportion of water: 



Ca 3 (P0 4 ) 2 + 2H 2 S0 4 + 4H 2 O -> CaH 4 (P0 4 ) 2 + 2CaS0 4 ,2H 2 0. 



The whole turns into a dry mixture, consisting of the superphos- 

 phate and gypsum (hydrated calcium sulphate). The latter does 

 not interfere with the fertilizing power of the soluble superphos- 

 phate, so the mixture is placed directly in sacks and sold as " super- 

 phosphate of lime." 



In slaughter houses the bones, after being deprived of fat and 

 gelatin, give a residue containing much calcium phosphate. This 

 residue is treated with sulphuric acid and made into fertilizer. 



(.Potassium^ Wood ashes contain much potassium carbonate, 

 andare used as fertilizers for this reason. The giant sea-weeds 

 (Kelp) of the Pacific coast have also been found to contain an 

 unusually large proportion of salts of potassium. By far the 

 most important source of this element, however, is potassium 

 chloride, obtained from natural salt deposits. 



The average production of the German deposits at Stassfurt 

 in the ten years preceding the war exceeded 1,000,000 tons (cal- 

 culated as K 2 0). These deposits constituted practically a world 

 monopoly, and the shortage of potassium salts for fertilizer pur- 

 poses during the war was consequently extreme. The most 

 strenuous efforts to develop the potassium resources of the United 

 States (natural brines, Kelp, recoverable by-products from the 

 cement and molasses industries, etc.) culminated in a production 

 of only 50,000 tons K 2 O in 1918. Fortunately, the cession of 

 Alsace to France has destroyed the German monopoly, since very 

 extensive deposits exist near Mulhouse. The working of these 

 deposits has been greatly hampered by the damage done to shafts 

 and machinery by the Germans before their evacuation, but 

 production in 1920 already exceeded 200,000 tons K 2 0. 



