ARTIFICIAL AND CONCENTRATED MANURES. 71 



The different results obtained from the use of the 

 different forms of nitrogen determine what is called its 

 '^agricultural value/^ or the improvement it causes in 

 the growth of the plant. 



This agricultural value, which is true of any manure, 

 is, too, separate and distinct from the commercial value, 

 or cost in market ; which is determined by market and 

 trade conditions, as cost of production, transportation, 

 selling, and the demand for it in other industries. It 

 is for this reason that the best forms of plant-food may, 

 and frequently do, cost less per pound of the actual 

 ingredient than when furnished by other more slowly 

 available, and less directly useful, forms. 



Nitrate of Soda. — Nitrate of soda, also called Chile 

 saltpetre, is the chief source from which nitrogen as a 

 nitrate is secured for manurial purposes. It possesses 

 chemical and physical properties which distinguish it 

 from all other materials ; it is a salt with a definite 

 chemical composition. When pure it contains sixteen 

 and forty-seven hundredths per cent of nitrogen. 



Vast natural deposits of the crude nitrate salts occur 

 in the rainless districts of South America, though mainly 

 in Chile. The crude salts are relatively poor, and also 

 variable in their content of nitrogen ; hence before ship- 

 ment they are purified by dissolving in water and recrys- 

 tallizing, the impurities remaining being chiefly water 

 and ordinary salt. 



The commercial product is quite pure, containing on 

 the average sixteen per cent of nitrogen. It resembles 

 in appearance ordinary salt, though the use of salt as 

 an adulterant has not been practised to any extent in 



