130 GROWTH AND WORK OF PLANTS 



This inoculation of the soil has been put into practice in a number 

 of different ways. One of the more recent methods is by prepar- 

 ing in the laboratory "pure" cultures of the bacteria on nitrogen- 

 poor culture media in small tubes which can be sold and sent by 

 mail to persons who wish to inoculate their soils.* The object in 

 growing them on nitrogen-poor substances is to create in them 

 "nitrogen hunger," for they will then more readily attack the 

 roots of the clover, etc., in order to put them in a condition to 

 "fix" the free nitrogen. In soil rich in combined nitrogen they 

 do not readily attack the roots of the legumes. It is, therefore, 

 not good policy to inoculate soils rich in combined nitrogen, for 

 the clover, etc., will find a sufficient amount in the soil already. 

 There have been some successes and many failures in inoculating 

 soils with the root tubercle bacteria. Some of the failures are to 

 be ascribed to a poor condition of the inoculating material. 

 Other failures are probably due to the fact that the soil is already 

 rich in combined nitrogen, and still others are probably due to the 

 fact that there are a sufficient number of organisms already in the 

 soil. Some of the conditions under which one might hope for 

 good results are, first, when the soil is poor in combined nitrogen 

 and there are few root tubercle bacteria already in the soil; 

 second, when the soil is poor in combined nitrogen and the root 

 tubercle bacteria may be plentiful, but of a "race" different from 

 that which readily attacks the kind of legume it is desired to grow. 

 For example, the race of bacteria which attack the roots of clover 

 will not readily attack the roots of the soja bean. Those which 

 attack peas will not readily attack the locust, etc. 



MYCORHIZA, SYMBIOSIS. 



206. Mycorhiza. The structure which results from the union 

 of bacteria or fungi with the roots of plants, similar to that de- 

 scribed in the preceding paragraphs, is called a mycorhim.\ 



* See Bull. No. 71 Bureau PI. Ind. U. S. Dept. Agr., Soil Inoculation fo'r 

 Legumes: Bull. No. 72, Pt. IV, Inoculation of Soil with Nitrogen-fixing 

 Bacteria. 



j" From jutfKi7s = mold, and pta = root. 



