The agent causing the bacterial fluctuations is mainly the active amoebae for during the year's count an inverse relationship was established between the active numbers of amoebae and the num bers of bacteria. Thus a rise from one day to the next in the amoebic population was correlated with a fall in the numbers of bacteria and vice versa. This conclusion has been substantiated by inoculation experiments in which sterile soil was inoculated in one case with bacteria alone and in the second case with the same bacteria together with amoebae. Since bacteria constitute the food of amoebae the above inverse relationship between the two groups is to be expected. Superimposed on the daily variations in num bers there are seasonal changes, in all the soil micro-organisms. In the spring and autumn the population rises to a maximum with corresponding falls in the summer and winter. It is interesting that these changes are very similar to those recorded for many aquatic organisms such as the algae and the plankton of the sea.
The action of protozoa on such soil reactions as ammonification, carbohydrate decomposition and carbon dioxide evolution has not been greatly tested; the small amount of evidence available, how ever, tends to the conclusion that in fertile soils amoebae act as inhibiting agents causing by their removal of bacteria a lessened production of ammonia from organic compounds and of carbon dioxide from carbohydrates. The fixation of atmospheric nitrogen
by the bacterium Azotobacter is on the other hand increased when protozoa are associated with it although the protozoa use Azoto bacter as their food supply. The probable explanation of the apparent anomaly is that the nitrogen in the bodies of the con sumed bacteria is not lost, but conserved in the protozoan proto plasm; and also that the continued feeding action of the protozoa keeps the bacteria at a high rate of reproduction necessitating high metabolism with consequent increased nitrogen fixation.
Considerably more research is required along these lines, but the information so far obtained shows that the protozoa are important members of the soil population. (D. W. C.)