206 



is as good inoculating material as any other. The whole farm 

 can then be inoculated with very little cost to the farmer. 



Piirc-culturc Method. 



In order to make the inoculation more simple and to meet the 

 various objections against the soil method, investigators have 

 devised the pure-culture method. The nodule-forming bacteria 

 are carefully removed from the nodules and are made pure. In 

 making the nodule bacteria pure we separate them from all kinds 

 of molds and other undesirable bacteria. When they are puri- 

 fied they are planted on some sterilized food in which they can 

 multiply. In such food an extremely large number of the bac- 

 teria may develop in a few days. When the nodule bacteria are 

 propagated in this manner the preparation is called a pure 

 culture. 



In some cultures the bacteria are propagated in liquid, in 

 others on vegetable gelatin, and in still others unknown mix- 

 tures are employed. After considerable investigation this de- 

 partment decided to employ sterilized soil as a medium in which 

 to grow the bacteria in pure culture. 



In using pure cultures for inoculation, the object to be attained 

 is to distribute the bacteria evenly over the entire field. Two 

 methods may be employed in order to accomplish this : ( 1 ) 

 The pure cultures may be mixed with a certain quantity of water 

 and then poured on the seed. The seed is stirred until each one 

 is moistened and is then ready for planting. It is assumed that 

 some bacteria will adhere to every seed and will be carried with 

 it into the soil. (2) When it is not convenient to treat the seed 

 as above described, the pure cultures may be mixed thoroughly 

 with loamy soil, allowing about two hundred pounds of soil for 

 each acre. The soil is then broadcasted over the field and 

 harrowed in. 



The pure-culture method of inoculation has some advantages. 

 Being pure, ther are no weed seeds, no insects, no diseases nor 

 undesirable bacteria, provided the culture is prepared properly. 

 It is easily obtained, easily handled, and should be cheap. 



In general, a new discovery of this kind does not at first always 

 give good results. This was true in the case of pure cultures. 

 The reason for these failures is very simple. At first not enough 

 was known about the nature and the habits of these nodule- 

 forming bacteria, and consequently they were not treated jirop- 

 erly. The result was that often, by the time the farmer pro- 

 cured the culture, the bacteria in it were all dead or some wrong 

 kind of bacteria had entered into it. Investigators have learned, 

 however, by the failures. The writer believes that at j^resent 

 enough is known about these bacteria to enable workers to ])rc- 

 ])are pure cultures that will give good results. 



