Many attempts have been made to correlate the temperature and moisture of field soils with the bacterial numbers and activities. These attempts have given very discordant results. It is generally agreed that a plentiful moisture supply is beneficial. Thus Greaves, in Utah, found the optimum water content for ammonia and nitrate production to be about 60 per cent. of the water-holding capacity. On the other hand, Prescott[56] found that the summer desiccation of soil in Egypt was followed by increased bacterial activities. Fabricius and Feilitzen,[18] using moor soil, found a direct relationship between soil temperature and bacterial numbers, showing that temperature can be a limiting factor under certain conditions. With normal arable soils, however, no such direct effect of temperature or moisture can be found[16] (see Fig. 8). It has even been found by Conn[11] that freezing of the soil may cause a marked increase in bacterial numbers. The erratic effects of temperature and moisture on the soil bacteria probably afford instances of a disturbance of the equilibrium between the bacteria and other components of the soil micro-population. Thus desiccation and freezing, though they harmfully affect the bacteria, may inhibit other micro-organisms to a greater degree, thus freeing the bacteria from competition. It is in the investigation of this equilibrium, and of the factors that can control it to our benefit, that the great advances in soil biology in the future are to be expected. [Illustration: FIG. 8.--Effect of frost on the bacterial numbers in the soil. (After CONN.) X-axis: Nov.-May Y-axis (bottom): Temperature--Degrees C. Y-axis (top): Bacteria--Millions per Gramme of Soil.] REFERENCES TO CHAPTERS II. AND III. [1] Ashby, S. F., Journ. Agric. Sci., 1907, vol. ii., p. 35. [2] Beijerinck, M. W., Centr. f. Bakt., 1900, Abt. II., Bd. 6, p. 1. [3] Beijerinck, M. W., and Van Delden, A., Centr. f. Bakt., 1902, Abt. II., Bd. 9, p. 3. [4] Bewley, W. F., and Hutchinson, H. B., Journ. Agric. Sci., 1920, vol. x., p. 144. [5] Bonazzi, E., Journ. Bact., 1921, vol. vi., p. 331. [6] Burrill, T. J., and Hansen, R., Illin. Exp. Sta., 1917, Bulletin 202. [7] Christensen, H. R., Centralblatt. f. Bakt., 1915, Abt. II., Bd. 43, p. 1. [8] Christensen, H. R., Centralblatt. f. Bakt., 1907, Abt. II., Bd. 17, pp. 109, 161. [9] Christensen, H. R., and Larsen, O. H., Centralblatt. f. Bakt., 1911, Abt. II., Bd. 29, p. 347. [10] Conn, H. J., Centralblatt. f. Bakt., 1910, Abt. II., Bd. 28, p. 422. [11] Conn, H. J., Centralblatt. f. Bakt., 1914, Abt. II., Bd. 42, p. 510. [12] Conn, H. J., Journ. Bact., 1916, vol. i., p. 187. [13] Conn, H. J., Journ. Bact., 1917, vol. ii., p. 137. [14] Conn, H. J., Journ. Bact., 1917, vol. ii., p. 35. [15] Cramer, E., Arch. f. Hyg., 1893, Bd. 16, p. 151. [16] Cutler, W., Crump, L. M., and Sandon, H., Phil. Trans. Roy. Soc., 1923, Series B, vol. ccxi., p. 317.