METABOLISM 51 



must be added. The plant obtains ni- 

 trogen from the soil and in some cases 

 from the air. It has been said that the 

 air above each acre of soil contains ten 

 million dollars worth of nitrogen. Many 

 plants of the pea family have tubercles 

 on their roots which contain bacteria 

 capable of taking nitrogen from the air; 

 many kinds of very simple plants living 

 independently in the soil can also fix ni- 

 trogen. The attempt has been made to 

 inoculate crop plants such as wheat with 

 organisms capable of fixing atmospheric 

 nitrogen. It has been stated that the 

 wheat plant has this power, independ- 

 ently of the presence of bacteria or other 

 organisms. 



Sulphur and phosphorus enter into 

 the composition of some proteins and 

 these substances are often added to the 

 soil. Lime salts are also very necessary 

 and in many cases are used as fertilizers 

 but their fimction is not clear. Lime . 

 salts have the power of acting as anti- 

 dotes to the poisonous action of other 



