Legend: Heteromita
Cercomonas
Dimastigamoeba
Bacteria
Temperature
Moisture
Owing to the variations in the numbers of both protozoa and bacteria, little reliance can be placed on figures obtained from an isolated count, since on one day the total numbers of flagellates may be nearly 2,000,000 per gram and drop by more than half this figure in 24 days. It is certain, however, that the numbers recorded in the past are much too low, since the total flagellate and amœbæ species were lumped together in two groups. Some idea of the size of the soil population can be obtained, nevertheless, by using the fourteen-day averages mentioned above. In [Table VIII.] are tabulated the average total numbers of flagellates, and amœbæ for the two periods of the year when the population was at its maximum and minimum respectively. An endeavour has also been made to strike a rough balance sheet as to the amount of protoplasm represented by protozoa and bacteria in a ton of soil. For this purpose it has been assumed that the organisms have a specific gravity of 1·0 and are spheres of diameters, 6μ for the flagellates, 10μ for the amœbæ, and 1μ for the bacteria; and that they are uniformly distributed through the top nine inches of soil. The top nine inches of soil is taken as weighing 1000 tons.
TABLE VIII.
| Maximum Period. | Minimum Period. | |||||
|---|---|---|---|---|---|---|
| No. per Gram. | Weight in Gram per Gram. | Weight in Tons per Acre. | No. per Gram. | Weight in Gram per Gram. | Weight in Tons per Acre. | |
| Flagellates | 770,000 | 0·000087 | 0·087 | 350,000 | 0·000039 | 0·039 |
| Amœbæ | 280,000 | 0·000147 | 0·147 | 150,000 | 0·000078 | 0·078 |
| Bacteria | 40,000,000 | 0·000020 | 0·02 | 22,500,000 | 0·000012 | 0·012 |
It must be remembered that the above figures are minimum ones, as many species of bacteria and protozoa, known to occur in the soil, are not included in the statement owing to their not appearing on the media used for counting purposes.
Fig. 16.—Daily variations in the numbers of active individuals of a species of flagellate, Oicomonas termo (Ehrenb.) during March, 1921. (From Phil. Trans. Roy. Soc., vol. ccxi.)
X-axis: March
Y-axis: Active numbers per gramme of soil
Before leaving the discussion of daily variations in numbers of protozoa, reference must be made to the flagellate species. As already mentioned, their active numbers fluctuate rapidly, and for the most part entirely irregularly. One species, however, Oicomonas termo, is characterised by possessing a periodic change; high active numbers on one day being succeeded by low, which are again followed by high on the third day. This rhythm was maintained, with few exceptions, for 365 days ([Fig. 16]), and has been shown to take place in artificial culture kept under controlled laboratory conditions ([Fig. 17]).