154 Principles of Plant Culture. 



are nitrogen, phosphorus and potassium. These are all 

 liberated in greater or less quantities, when vegetable 

 or animal material (organic matter) decays in the soil; 

 hence all such material has more or less value as fertil- 

 izers. But we need not wholly depend upOn refuse 

 organic matter for fertilizers, since the leguminous plants 

 add nitrogen to the soil (259), and compounds of nitro- 

 gen, phosphorus and potassium may often be purchased 

 in artificial fertilizers at prices that place them within 

 the reach of the. cultivator. 



254. Nitrogen is the Most Important Fertilizing 

 Element because it is liberated in smallest amount by 

 rock decay and is most expensive in the market. Nitro- 

 gen is chiefly used by plants in the form of nitrates, 

 i. e., in combination with certain other substances as 

 soda, potash, lime, magnesia and iron. Ammonia, 

 which is a gaseous compound of nitrogen and hydrogen, 

 is also used to some extent by plants. Free nitrogen, 

 the most abundant constituent of the air, plays no direct 

 part in plant nutrition (259). 



255. The Sources of Nitrates in the Soil are 



a.— Nitrification (nit-ri-fi-ca-tion) , by which the nitro- 

 gen contained by organic matter and ammonium sulfate 

 in the soil is changed to nitric acid through the agency 

 of microscopic plants (bacteria). The nitric acid thus 

 formed combines with certain substances {hoses) in the 

 soil, as potash and lime, forming nitrates (254). 



h— Symbiosis (sym-bi-o-sis) on the roots of legumin- 

 ous crops, through which atmospheric nitrogen is 

 changed to nitric acid (259, 112) . 



