SOURCES AND COMPOSITION OF FERTILIZERS 



295 



nitrates, or as easily washed out. It changes to nitric acid in the 

 soil, and nitric acid and sulphuric acid unite with lime to form 

 nitrates and sulphates. The use of ammonium sulphate tends to 

 decrease the carbonate of lime in the soil, or to render the soil 

 acid. 



Calcium Cyanamide. This substance is prepared by passing 

 atmospheric nitrogen over calcium carbide. It decomposes slowly 



Fig. 71. Crystallizing pans with nitrate of soda. 



in the soil, with the production of nitrates. Under the most 

 favorable conditions, it appears to have a value equal to sodium 

 nitrate, but if applied too soon before planting, or to acid humus 

 soils, it may have injurious effects. 



Organic Materials. The organic nitrogenous fertilizers cannot 

 be taken up directly by plants but must first be converted into am- 

 monia or nitrates. Their value depends upon their content of 

 nitrogen, and the readiness with which they undergo decomposi- 

 tion in the soil. 



