[51] Mockeridge, J., Biochem. Journ., 1915, vol. ix., p. 272. [52] Nabokich, A. J., and Lebedeff, A. F., Centr. f. Bakt., 1906, Abt. II., Bd. 17, p. 350. [53] Nagaoka, M., Bull. Coll. Agr., Tokyo, 1900, vol. vi., No. 3. [54] Omelianski, W. L., Comptes Rendus Acad. Sci., 1895, vol. cxxi., p. 653; 1897, vol. cxxv., pp. 907, 1131; Arch. Sci. Bio., (St. Petersburg), 1899, vol. vii., p. 411. [55] Omelianski, W. L., and Sohmskov, M., Arch. Sci. Biol., Publ. Inst. Imp. Med. Exp. (Petrograd), 1916, vol. xviii., pp. 327, 338, 459; vol. xix., p. 162. [56] Prescott, J. A., Journ. Agr. Sci., 1920, vol. x., p. 177. [57] Söhngen, N. L., Centr. f. Bakt., 1905, Abt. II., Bd. 15, p. 513. [58] Sen Gupta, N., Journ. Agr. Sci., 1921, vol. xi., p. 136. [59] Shearer, C., Journ. Hyg., 1919, vol. xviii., p. 337. [60] Tappeiner, Ber. Deut. Chem. Gesell., 1883, vol. xvi., p. 1734; Zeitsch. Biol., 1884, vol. xx., p. 52. [61] Thornton, H. G., Ann. Appl. Biol., 1922, vol. ix., p. 241. [62] Wallin, I. E., Journ. Bact., 1922, vol. vii., p. 471. [63] Waksman, S. A., and Joffe, J. S., Journ. Bact., 1922, vol. vii., p. 239. [64] Wilson, J. K., Cornell Agric. Exp. Sta., 1917, Bulletin 386. CHAPTER IV. PROTOZOA OF THE SOIL, I. That protozoa could be isolated from the soil was a matter of common knowledge to the biologists of the nineteenth century, but not until the early part of the present century was it suggested that these organisms might be playing some part in the general economy of the soil micro-population. Of recent years a great deal of our knowledge of the cytology of the different groups of protozoa, especially the Amœbæ, has been obtained from the study of representatives normally living in the soil; but unfortunately little or no knowledge has been gained of the biology of these animals in their natural habitat. The view that the presence of these organisms in excessive numbers may lead to “soil sickness” was first put forward by Russell and Hutchinson in 1909, and elaborated in their further papers dealing with “Partial Sterilisation of the Soil.” It is unnecessary to discuss in detail this important branch of agriculture, but to obtain a clear idea of the development of the study of soil protozoa it is necessary to give as briefly as possible the conclusions deduced by Russell and Hutchinson from their extensive experiments on soils treated with steam and various volatile antiseptics[21],[22]:-- “(1) Partial sterilisation of the soil causes first a fall, then a rise, in bacterial numbers, which goes on till the numbers considerably exceed those present in the original soil. “(2) Simultaneously there is a marked increase in the rate of accumulation of ammonia which is formed from organic nitrogen compounds. “(3) The increase in bacterial numbers is the result of improvement in the soil as a medium for bacterial growth, and not an improvement in the bacterial flora.