492 



CONTRIBUTIONS TO OUR KNOWLEDGE OF SOIL- 

 FERTILITY. 



ii.The Determination of Rhizobia in the Soil. 



By R. Greig-Smitii, D.Sc, Maclkay Bacteriolooist to the 



Society. 



It has been recognised by bacteriologists that the actual number 

 of bacteria in any particular soil is no criterion of the fertility, 

 for the kinds of bacteria that are inchided in the total number 

 may be of little use in bringing about the multifarious changes 

 that o-o to determine the value of the soil. The bacteria that 

 bring about the comparatively rapid transformation of the con- 

 stituents, or which have some specific activity, are alone of imme- 

 diate importance; and, as it is these only which determine the 

 economic bacterial potential, the number of the others is of little 

 or no interest. 



Selective methods of ascertaining the extent of these kinds 

 are, therefore, important, and various methods have been devised 

 with this object in view. These take into account the physio- 

 logical activity, and are really chemical tests, inasmuch as the 

 soil is seeded into solutions of various forms of saline and organic 

 substances, and the chemical changes determined. By using 

 graduated dilutions of soil suspension, one obtains a point where 

 no change occurs, and this mai'ks the limit of the pi-esence of 

 certain active organisms. Such is the method of Hiltner and 

 Sttirmer. There are certain objections to this method, the chief 

 of which is that, under the unnatural conditions which obtain, 

 the active bacteria may possibly be suppressed by inactive forms. 



At the present time, the micro-organism to which the chief 

 role of the non-symbiotic fixation of nitrogen in the soil is ascribed, 

 is Azotohacter chroococcum, discovered by Beijerinck in 1901. It 



