294 



PRINCIPLES OF AGRICULTURAL CHEMISTRY 



about 25 per cent, nitrate of soda. The best quality of caliche con- 

 tains approximately 50 per cent, nitrate of soda, NaNO 3 , 26 per 

 cent, common salt, 6 per cent, sulphate of soda, 14 per cent, dirt, 

 etc., insoluble in water, and about 4 per cent, magnesium sulphate 

 and chloride, with small amounts of sodium iodide. The caliche 

 is treated with hot water and the solution run into crystallizing 

 vats. The crude nitrate of soda crystallizes out on cooling. 

 Nitrate of soda is readily soluble in water, easily taken up by 



Fig. 70. Nitrate of soda, partly blasted up. 



plants, and, unless taken up by plants, will be washed from the 

 soil. It contains about 15 per cent, of nitrogen. It is often 

 called chile saltpeter. 



Sulphate of ammonia is a by-product obtained in the manu- 

 facture from coal, of illuminating gas, and of coke. A part of the 

 nitrogen of the coal passes off as ammonia, and is removed by 

 passing the gas through sulphuric acid, forming sulphate of am- 

 monia (NH 4 ) 2 SO 4 . It contains about 20 per cent, of nitrogen. 

 Ammonia is fixed by the soil, and is not as available to plants a$ 



