On such a plot it is possible to investigate the rapidity with which the numbers of the soil micro-organisms alter in point of time. For example, on the dunged plot of Barnfield, Rothamsted, which has been cropped with mangolds for forty-seven successive years, the area distribution of bacteria has been found to be so uniform that if a number of samples of soil are taken from the plot at the same time, the difference in bacterial numbers between the samples cannot be detected by means of the counting technique (see Table V.). The work of Cutler, Crump, and Sandon[16] on this plot showed that the bacterial numbers vary very greatly from one day to the next, and that these fluctuations took place over the whole plot, since two series of samples, taken in two rows 6 feet apart, showed similar fluctuations (see Fig. 7). The discovery of these big daily fluctuations in numbers led to an inquiry as to how quickly bacterial numbers change, and samples from Barnfield, taken at two-hourly intervals, showed that significant changes in numbers took place even at such short intervals. 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. [Illustration: 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.)]