TABLE V.—BACTERIAL COUNTS OF FOUR SOIL SAMPLES.
From Barnfield, Taken Simultaneously.
| Counts of Colonies on each Plate. | ||||
|---|---|---|---|---|
| Plate. | Sample I. | Sample II. | Sample III. | Sample IV. |
| 1 | 38 | 45 | 43 | 27 |
| 2 | 32 | 40 | 34 | 41 |
| 3 | 52 | 45 | 52 | 35 |
| 4 | 32 | 31 | 55 | 36 |
| 5 | 40 | 43 | 38 | 45 |
| Mean | 38·8 | 40·8 | 44·4 | 36·8 |
| Standard deviation between the four samples = 7·25. | ||||
| Standard deviation between parallel plates within the sets = 7·55. | ||||
Fig. 7.
X-axis (top): Days.
Y-axis (left): (Series A) Bacteria—millions per gramme of soil.
Y-axis (right): (Series B) Bacteria—millions per gramme.
Caption: Daily changes in bacterial numbers in field soil.
Counts from two series of soil samples taken 6 feet apart.
(After Cutler.)
Since the bacteria involved in this fluctuation are of great importance to the crops, being for the most part ammonia producing types, further knowledge as to the cause of this fluctuation and of its effect on the ammonia and nitrate in the soil is of fundamental importance. There is evidence, which will be discussed later, that the cause is connected with the changing activities of certain soil protozoa, since the daily changes in the numbers of active amœbæ in the soil have been found to be in the reverse direction to those of the bacterial numbers. It appears, therefore, that we are dealing with an equilibrium between the various members of the soil population, the point of equilibrium changing at frequent intervals.
In addition to daily changes, it is possible to detect changes in the numbers and activity of the soil population related to the season. There is a well-marked increase in the spring and autumn (see [Figs. 15], [16], pp. 89, 90). This is well seen when the fortnightly averages of the daily bacterial and protozoal counts from Barnfield soil are plotted. These spring and autumn increases comprise both the bacterial and the protozoal population, and therefore differ from the short time fluctuations in being due, not to a disturbance of the bacteria-protozoa equilibrium, but to a general rise in activity of both groups of organisms.