THE NITROGEN CYCLE 461 



leguminous crops on soils not having previously grown 

 such crops successfully. Extensive experiments showed 

 the practicability of inoculating land for a certain legumi- 

 nous crop by spreading on its surface soil from a field 

 on which the same crop is successfully growing. It is 

 manifestly much better to apply the organisms from a 

 certain species of legumes from a field having grown the 

 same species, than to attempt to use organisms from an- 

 other species of legumes. The fact that soil inoculation 

 by means of soil from other fields may possibly transmit 

 weed seeds and fungous diseases, and also necessitates 

 the transportation of a great bulk and weight of material, 

 has led to numerous efforts to inoculate soil by means 

 of pure cultures. The pure culture may also make it 

 possible to bring to the soil bacteria of greater physio- 

 logical efficiency than those already there. 



The first attempt at inoculation by pure cultures 

 was made in Germany, the cultures being sold under the 

 name of " nitragin." Careful experiments made with 

 this material previous to the year 1900 did not show 

 it to be very efficient ; but in recent years improvements 

 in the method of manipulating the cultures have resulted 

 in much greater success. In " nitragin " the medium 

 used for growing the organisms is gelatin, and before use 

 this was formerly dissolved in water; but now a solution 

 of greater density is used in order to prevent a change of 

 osmotic pressure, which may cause plasmolysis and result 

 in the destruction of the bacteria. 



Within recent years a number of cultures for soil 

 inoculation have been offered to the public. The first 

 of these utilized absorbent cotton to transmit the bac- 

 teria in a dry state from the pure culture in the laboratory 

 to the user of the culture, who was to prepare therefrom 



