THE GERM LIFE OF THE SOIL 137 



The importance of this process to soil productiveness may 

 be realized when it is considered that nitrogen is the most 

 expensive of all the ingredients of commercial fertilizers, 

 and that many pounds to the acre may be secured by en- 

 couraging the growth of the bacteria concerned in the op- 

 eration. 



176. Nitrogen fixation through symbiosis with higher 

 plants. — The value of certain plants as soil improvers has 

 long been recognized, and within the last half century their 

 ability to improve soil has been traced to their property of 

 taking nitrogen from the air and leaving it in the soil. The 

 plants that do this belong, with a few exceptions, to the 

 family of legumes. 



The method by which nitrogen is transferred from the air 

 to the soil is not perfectly understood, but it appears to 

 be somewhat as follows : 



On the roots of leguminous plants are found nodules or 

 tubercles, which are large enough to be seen with the naked 

 eye, and in which live the bacteria that remove the nitrogen 

 from the soil air and convert it into nitrogenous organic 

 matter, that then becomes a part of the host plant. As a 

 consequence legumes are very rich in nitrogen, and the 

 tubercles contain an especially large quantity. When the 

 roots and nodules decay and when the aboveground part of 

 the plant is plowed under, the nitrogenous matter they con- 

 tain becomes a part of the soil. 



If the nitrogen-fixing bacteria are not present in the soil 

 or other medium in which the legumes grow, no nodules 

 will be formed and no atmospheric nitrogen will be fixed. 

 The plant must then live on the combined nitrogen of the 

 soil just as other plants do and consequently it does not 

 serve to increase the store of soil nitrogen. In fact, the 

 reverse occurs, for on account of the high nitrogen content 

 of legumes, they withdraw, under these conditions, large 



