The determination of the number of bacteria in soil. (Preliminary communication)

Abstract

1. The usual shaking methods have been examined and found unsatisfactory.2. A method has been evolved for the estimation of the number of soil organisms by a direct count, and the numbers found to be very much higher than any previously reported.3. A method for the disintegration and dispersion of the soil particles and bacteria has been devised, by means of which plate counts were made which were comparable with direct counts.4. The dispersive actions of shaking and of vibrating have been compared. The former was found to give results which depended on the moisture content of the soil, whereas the trembling motion gave results in which the effect due to the moisture content was not apparent.5. High counts made by previous observers can usually be correlated with one or more of the following factors:(i) Greater dispersion of the soil as a result of the action of frost, etc.(ii) An alkaline reaction,e.g.presence of ammonia, magnesia, etc.(iii) Presence of a protective colloid,e.g.the effect of the addition of large quantities of straw, or traces of gelatine.(iv) Absence of a flocculating agent, especially calcium compounds.In conclusion the author wishes to express his gratitude to Dr G. S. Graham-Smith, F.R.S. at whose suggestion the investigation was started and under whose guidance and direction the bacteriological work was carried out in the Pathological Laboratory, Medical Schools, Cambridge; to Professor T. B. Wood, C.B.E., M.A., F.I.C., F.R.S., for facilities for carrying out the physico-chemical investigation at the School of Agriculture; to Messrs L. F. Newman, M.A., F.I.C., W. B. Hardy, M.A., F.R.S., W. H. Harvey, M.D., E. K. Rideal, M.B.E., M.A., G. Udny Yule, M.A., F.R.S., F. W. Foreman, M.A., F.I.C., and to the late Professor Sir German Sims Woodhead for their kindly advice and encouragement; to Messrs E. G. Staples, B.A., W. M. Davies, B.A., B.Sc, A.I.C. and A. W. R. Joachim, B.Sc. for assistance in the execution of the physico-chemical experiment; and to Mr W. A. Mitchell of the Medical School for his assistance in fitting up apparatus.