Andrew Cunningham 71 



the soil in a mortar and allowed to act in a glass bottle with close-fitting 

 stopper for six days. A sterile suspension of 3 grams freshly slaked 

 lime in 50 c.c. water was added to combine with the formalin and render 

 it harmless. The bottle was placed in the 38° C. incubator for one day. 

 After some weeks at room temperature the soil was thoroughly broken 

 up with a large, sterile, metal spatula and weighed out in 20 gram 

 quantities into sterile Erlenmeyer flasks. The water content was not 

 determined but it probably amounted to about 10 % . 



In order to try to minimise, as much as possible, the effects of the 

 nutrient matter in the protozoa culture solution, soil extract + -05 % 

 K2HPO4 was selected as medium. This was inoculated with soil and 

 after the protozoa had developed a protozoa-free culture was prepared 

 from it. Both soil extract cultures were kept for about two months 

 before being used for inoculation purposes. Two of the flasks con- 

 taining sterilised soil received each 1 c.c. of the protozoa culture, the 

 other two 1 c.c. of protozoa-free culture. All four received 1 c.c. of 

 sterile water each, in addition, in order to bring up the water content 

 of the soil to about 20 % (roughly 70 % of the water-holding capacity). 

 In order to represent, more or less, the conditions obtaining in Russell 

 and Hutchinson's experiments a second series of four flasks was inocu- 

 lated, two with protozoa + bacteria and two with bacteria alone as in 

 the last case. The sterile water was replaced in this instance by an 

 equal quantity of a sterile 2 % filtered fleshmeal solution. Of the 

 controls two received 2 c.c. sterile water each, the remaining two each 

 1 c.c. sterile water and 1 c.c. sterile fleshmeal solution. 



The bacterial content of the protozoa-free culture was 121 luilKons 

 per c.c. : that of the protozoa culture 12 millions per c.c. (agar at 22° C. 

 was used as medium for the counts in this section). The numbers of 

 bacteria in the soil samples used in the experiment were determined 

 after 20 days at 22° C. The water contents were adjusted once more 

 to roughly 20 % with sterile water and the flasks were allowed to remain 

 for a further period of 20 days at 22° C. The bacterial contents of the 

 soil samples were again determined (Table 19). 



The results of the bacterial counts are rather irregular. This is 

 probably due to the fact that the soil samples used were only watered 

 once during the experiment. The inoculation of bacteria, therefore, 

 probably did not get thoroughly distributed in the soil. The only 

 cultures which have shown a decided depression in bacterial numbers 

 after 40 days (as compared with 20) are Nos. 7 and 8. Here the lowering 

 in numbers is quite marked and considerably larger than in any other 



