Protozoa in Relation to Soil Bacteria. 



311 



their shape. In all cases we get an early rise in bacterial numbers to a very 

 high level, followed by a rapid drop. This drop shows the coming into 

 action of some factor limiting bacterial activity, and the explanation which I 

 have advanced is that this is a nutrient rather than a protozoal factor. The 

 food added in the form of hay-infusion serves as a suitable medium for the 

 attainment of high bacterial numbers in the soil, and on the exhaustion of 

 this food the bacteria naturally fall in numbers. 



The protozoal counts in the present investigation show that there is little 

 or no multiplication of the protozoa added by mass inoculation, whereas the 

 counts in the previous work did not allow of such an interpretation, for the 

 initial and the later counts were not made by strictly comparable methods, 

 as was pointed out . in the earlier paper (p. 442). I have no doubt now, 

 however, in view of the results from the mass inoculations in the present 

 investigation, that there was no activity and multiplication of the protozoa 

 added by mass inoculation in the earlier investigation. 



The drop in the curves from the experiments in the addition of 5 per cent, 

 of untreated to treated soil in the present investigation shows the effect of 

 protozoal activity on bacterial numbers, and the curves must therefore be 

 interpreted as showing a protozoal rather than a nutrient limiting factor. In 

 the same way the curves for the (T + 5 per cent. U) and (1846 + 10 per cent. 

 1870) soil of the previous investigation, which showed evidence of some 

 factor checking bacterial increase, should probably be interpreted as showing 

 the effect of a protozoal limiting factor. The evidence at hand at the time 

 of writing the earlier paper did not warrant this conclusion, but it was fully 

 recognised that some factor had been introduced with the untreated soil 

 which tended to depress the numbers of bacteria. As a consequence, how- 

 ever, of the results obtained in the present research, by the addition of 

 untreated to treated soil, it seems reasonable to brin" the earlier results into 

 line with those of the present investigation, though one cannot show the 

 marked correlation between protozoal increase and bacterial decrease in 

 numbers in the earlier work as one can in the later. 



Three facts emerge clearly from the present investigation : — 



1. Where a treated soil has comparatively few protozoa added to it along 

 with a small amount of untreated soil, these protozoa can and do multiply 

 in the mixture, and after a time exert a depressing effect upon bacterial 

 number's. 



2. When large numbers of protozoa are added in cultures to a treated soil, 

 whether with or without bacteria, little or no multiplication of the protozoa 

 takes place, and there is no evidence of their exerting a limiting action on 

 bacterial activity. 



