THE SOIL 73 



other materials necessary for their life processes. In 

 return they absorb the nitrogen gas from the air, which 

 circulates through the soil, and turn it over to the plant 

 in a form that the plant can utilize. Without the pres- 

 ence of bacteria and without the presence of the nodules 

 this process would not go on. The legume plants would 

 behave like ordinary crops, using only the nitrate- 

 nitrogen found in the soil. 



This process is termed symbiosis, which means that 

 the two organisms, the bacteria and the plant, live to- 

 gether to their mutual benefit. Thus the bacteria are 

 provided with a home (the nodule) and food (sugar), 

 and in turn they supply the plant Avith an inexhaustible 

 nitrogen supply. Neither is injured by the union, but 

 on the other hand both are greatly benefited. 



If legumes are grown in soil which contains no nitro- 

 gen but are inoculated and given all the essentials for 

 growth except nitrogen, the plants Avill live and thrive 

 normally, extracting in this case, through the aid of the 

 bacteria, their entire nitrogen supply from the air. If, 

 on the other hand, the soil contains nitrogen, it will draw 

 in part upon this and in part from the air. According- 

 ly as the soil is richer in nitrogen, especially as nitrate, 

 the plants will draw more heavily upon the soil and less 

 upon the air for their nitrogen. Hopkins has figured 

 that upon normal soil of average fertility, red clover 

 when inoculated secures about 2/3 of its nitrogen from 

 the air and 1/3 from the soil. When cut for hay the 

 part removed contains 2/3 of the nitrogen present in the 

 entire plant, the roots and stubble containing 1/3. Thus 

 when the hay is removed, as much nitrogen is removed 

 as was obtained from the air. The stubble and roots 

 turned under do not produce any gain for the soil, the 



