31 



THE MICRO-ORGANISMS OF THE SOIL. 



Reference has Been made in previous Reports to the impor- 

 tant part played by the soil micro-organisms in determining the 

 productiveness of the soil. These organisms break down the 

 organic matter in the soil, the plant residues, farmyard manure 

 and other organic manures, converting them into useful plant 

 foods. They effect at least three kinds of action which are 

 directly beneficial to the plant : — 



1. The production of nitrates. 



2. The decomposition of plant material producing struc- 

 tureless compounds having valuable colloidal proper- 

 ties. 



3. The decomposition of intermediate products which 

 would be toxic to plants. 



The most striking result brought out by recent observations 

 has been the fluctuation of the micro-organisms in natural field con- 

 ditions. There are two-hourly fluctuations, recognised at present 

 only in the case of bacteria, which have been measured in the 

 Bacteriological Department. Superimposed upon these are daily 

 fluctuations which are known to affect not only bacteria but 

 protozoa also, the level of numbers for any species at 9 a.m. 

 varying from day to day. Further, there are seasonal fluctua- 

 tions; a great rise in spring, a fall in summer, a rise in autumn, 

 and a fall in winter ; bacteria, protozoa and apparently also algae 

 and fungi being affected. It is not known whether there are 

 annual fluctuations, though this would appear not improbable. 

 The phenomena are not confined to soil micro-organisms ; similar 

 fluctuations are recorded for plankton and pond algae, though the 

 data are not so complete. 



The cause of the daily and probably of the hourly fluctuations 

 of bacteria is fluctuation of the number of the amcebse which 

 feed upon them. Why the amcebae should fluctuate was for long 

 a mystery; Mr. Cutler and Miss Crump have thrown some light 

 upon it by showing that rate of reproduction of amoebae depends 

 upon the number of bacteria present; when the bacteria fall 

 below a certain level no division of the amcebae occurs ; it begins 

 only when they rise above this. 



The spring and autumn increases in number, however, affect 

 bacteria and protozoa alike, so that some other cause is apparently 

 operating. 



All this work has been possible through the elaboration by 

 Mr. Cutler of methods of counting protozoa in the soil, and the 

 development by Mr. Thornton of a plating medium in which 

 bacterial colonies would develop uniformly and without the 

 spreading which in the older technique suppressed some of the 

 slow growing forms. The medium has the further advantage of 

 being prepared from pure substances so that it can be reproduced 

 with precision whenever desired, and it has thus been possible 

 to apply a statistical formula whereby the degree of accuracy 

 of the plate counts can be estimated. 



