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. | Weight | Weight| No. per | Weight | Weight| | | per |in Gram |in Tons| Gram. |in Gram |in Tons| | | Gram. | per | per | | per | per | | | | Gram. | Acre. | | Gram. | 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. [Illustration: 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]