23 



were fairly constant so long- as the conditions of temperature, 

 water supply, etc., remained the same. Mr. Cutler's work 

 showed that this is not the case ; the protozoa and bacteria vary in 

 numbers from day to day (p. 38) , while Mr. Thornton has shown 

 that the bacteria may vary from hour to hour. Careful experi- 

 ments are beings made to see if the production of plant food by 

 the organisms varies in the same way. The changes in numbers 

 of bacteria seem to be brought about by changes in numbers of 

 active amoebte, but it is not clear why the amoebae should fluctuate 

 as they do. It does not appear that their variations in numbers 

 are determined primarily by variations in rrioisture supply or 

 temperature ; there seems to be some deep seated biological cause 

 at work. 



Besides these hour to hour and day to day variations, there 

 seems to be a seasonal variation in numbers ; bacteria, protozoa 

 and, apparently, fungi and algae, are uplifted in number in Spring 

 and Autumn, but depressed during Summer and Winter. 

 Laboratory experiments have been begun to find an explanation, 

 but the problem is clearly very complex. The depressing effect of 

 protozoa on bacteria in the soil was directly demonstrated by 

 inoculating protozoa and bacteria into sterilised soil ; the numbers 

 of the latter were greatly reduced (p. 38). This experiment has 

 often been attempted before, but without success, the experimental 

 difficulties having proved too great. The Bacteriological Depart- 

 ment, under Mr. H, G. Thornton, has successfully worked out 

 methods by which the bacteria in the soil can be counted, and their 

 changes in number followed, to a degree of refinement and accuracy 

 that satisfies statistical tests of far greater stringency than had 

 been previously applied (p. 37). 



THE CONTROL OF THE SOIL POPULATION. 



This, work was seriously checked in March, 1921, by the death 

 of Mr. W. B. Randall, who had provided funds for the main- 

 tenance of a special assistant. It is, however, being slowly 

 continued. The disappointing results given by certain organic 

 agents which promised well have been traced to their decomposition 

 in the soil. This is in the main bacterial, and a special study has 

 been made by Messrs. Thornton and Gray of the bacteria which 

 break down phenol, cresol and naphthalene. The introduction of 

 certain groups into the molecule retards decomposition and intensi- 

 fies activity ; thus nursery experiments indicate that dichlorcresol 

 is some 25 times as potent for sterilising purposes as ordinary 

 commercial cresol. The large scale experiments are recorded in 

 the report of the Cheshunt Experimental Station. 



The effect on the micro-organisms of treating soil with phenol 

 is being studied in the Bacteriological and Protozoological De- 

 partments. Three groups of bacteria are found capable of 

 decomposing this substance, belonging respectively to the 

 Mycobacterium, Pseudomonas and Clostridium types; the Myco- 

 bacteria are interesting among soil bacteria in that they appear to 

 have a definitely discontinuous geographical distribution ; the 

 Pseudomonas organisms are apparently of chief importance in 

 phenol decomposition, as they greatly increase in numbers 



