34 



tivate a particular legume, and where that legume or a closely related 

 species has not been successfully cultivated and well supplied 

 with nodules, it is desirable that the proper bacteria be supplied. 

 Experiments and practical tests made by this Department show 

 that there is a great variation in the nitrogen-fixing power of these 

 bacteria, dependent in part upon the conditions under which they 

 have been growing. If they are in a soil which provides them 

 with an abundant supply of combined nitrogen they ultimately 

 lose, to a large extent, the power of fixing atmospheric nitrogen. 

 Under such conditions they are of little or no benefit to the crop. 

 It is an easy matter to select strains of high nitrogen-fixing power 

 as indicated by the effect of the bacteria on the crop when grow- 

 ing in comparatively nitrogen-free soils. The cultivation of these 

 selected strains on nitrogen-free media for a few generations 

 greatly increases their nitrogen-fixing power, and therefore their 

 value for inoculation purposes. Exactly the same principles ap- 

 ply to these bacteria as apply to other plants. There are some 

 strains of beets that will make 15 to 20 per cent, of sugar. 

 These are worth cultivating for their sugar : but the ordinary 

 strains from which these have been developed by selection con- 

 tained only 5 or 6 per cent, of sugar. A man who wants to make 

 a success of growing sugar beets plants seeds of high sugar-pro- 

 ducing strains. The importance of using selected seed for all 

 crops has been so amply demonstrated that no argument in fa- 

 vour of the practice is needed. It is the very foundation of pro- 

 gress in plant culture. Soil bacteria are no exceptions to the 

 rule, and pure-bred bacteria for specific work are as clearly an 

 economic necessity as pure-bred cattle or pure-bred sugar beets. 



DISTRIBUTION OF INOCULATED SOIL, 



We are often told that these bacteria are widely distributed 

 and are present in most soils. This is true as applied to the older 

 cultivated areas, where various legumes have been cultivated, but 

 it does not follow that soils containing a few or even a consi- 

 derable number of these bacteria would not be benefited by ino- 

 culation with suitable kinds of bacteria. The varieties present 

 may not be adapted to the legume which it is desired to plant, 

 and may have little or no beneficial effect on it, or if the proper 

 strain is present in small numbers it may, and usually does, take 

 several years to bring the bacterial content of the soil up to an 

 efficient basis. This, of course, is too slow a process. It would 

 be just as reasonable to depend on getting a crop of clover or 

 blue-grass in this way. It can be done in some places, but it is 

 at least a makeshift method and does not appeal to a practical 

 up-to-date farmer. 



In newer regions, where legumes have not been cultivated, very 

 few soils have enough of these bacteria in them to be of any 

 practical value, and inoculation is essential. This may be accom- 

 plished either by the use of soil from a field where the crop is 

 making a vigorous growth by the roots well supplied with nodules, 

 indicating that the soil contains the right bacteria, or by the use 



