Fig. 10.—Daily numbers of active amœbæ (Dimastigamœba and Species α) and bacteria in 1 gram of field soil, from August 29 to October 8, 1920. (From Phil. Trans. Roy. Soc., vol. ccxi.)
X-axis: August September October
Y-axis (left): Amoebae Active numbers per gramme of soil
Y-axis (right): Bacteria in millions per gramme of soil
Legend: Dimastigamoeba
Species α
Bacteria
The number of all the organisms showed large fluctuations of two kinds, daily and seasonal. The size of the changes that took place within so short a period as 24 hours was, perhaps, the most surprising fact that the experiment revealed. Thus three consecutive samples gave 58·0, 14·25 and 26·25 millions of bacteria per gram respectively; and the changes exhibited by any of the species of protozoa were at times even larger. This fact is of extreme importance, since in the past it has always been assumed that the number of bacteria remained fairly constant from day to day, and investigators have not hesitated to separate the taking of soil samples by long periods. It is now obvious that such a procedure is of little use for comparative purposes ([Fig. 10]).
It has usually been assumed that the changes in the external conditions markedly affect the density of the soil population. To test this the environmental conditions—temperature, moisture content and rainfall were examined; but contrary to all expectation no connection could be traced between any of these and the daily changes in numbers of any of the organisms investigated, and moreover the species of protozoa appeared in the main to be living independently of one another.
It is difficult to believe that external conditions are as inoperative as appears from the above; and in view of the known complexity of the soil it is possible that further research will show that certain combinations of external conditions are important agents in effecting the changes.
Fig. 11.—Numbers of active amœbæ (Dimastigamœba and Species α) and bacteria to 1 gram of field soil for typical periods in February and April, 1921. (From Phil. Trans. Roy. Soc., vol. ccxi.)