154 SOILS AND FERTILIZERS 



172. Leguminous Crops as Nitrogenous Manures. - 

 The frequent use of leguminous crops for manurial pur- 

 poses is the cheapest way of obtaining nitrogen. When 

 the crop is not removed from the land but is plowed 

 under while green, the practice is called green manur- 

 ing. This does not enrich the land with any mineral 

 material, but results in changing inert plant food to 

 humate forms. Green manuring with leguminous crops 

 should take the place of bare fallow, as its effects upon 

 the soil are more beneficial. With green manuring, 

 nitrogen is added to the soil, while with bare fallow there 

 is a loss of nitrogen. Leguminous crops, as clover, peas, 

 crimson clover, and cowpeas, should be made to serve 

 as the main source of the nitrogen for crop production. 

 A good crop of clover will return to the soil over 200 

 pounds of nitrogen per acre. 



173. Sodium Nitrate. The nitric nitrogen most 

 frequently met with in commercial forms is sodium 

 nitrate, commonly known as Chili saltpeter. It is a 

 natural deposit found extensively in Chili, Peru, and 

 the United States of Colombia. Various theories have 

 been proposed to account for these deposits, but it is 

 difficult to determine just how they have been formed. 10 

 The commercial value of nitrogen in fertilizers is regu- 

 lated mainly by the price of sodium nitrate, which, when 

 pure, contains 16.49 P er cent f nitrogen. Commer- 

 cial sodium nitrate is from 95 to 97 per cent pure. 

 An ordinary sample contains about 16 per cent of 



