III. 



BACTERIA IN THE SOIL. 



SURFACE soil, and especially that which is rich in organic matter, 

 contains very numerous bacteria of many different species. Some of 

 these are of special interest on account of their pathogenic power. 

 Thus the bacillus of malignant oedema and the bacillus of tetanus 

 have been shown to be widely distributed species, which have been 

 obtained by investigators in various parts of the world by inoculating 

 susceptible animals guinea-pigs or mice with a little rich surface 

 soil. Other species are interesting because of their action in nitrifi- 

 cation and in the destructive decomposition of organic material by 

 which it is fitted for assimilation by the higher plants. Many of the 

 bacteria present in the soil are strictly anaerobic, and in attempts to 

 estimate the number and kind of microorganisms present in a given 

 sample this fact must be kept in view. 



The simplest method of studying the bacteria in the soil consists 

 in introducing a small quantity into liquefied gelatin in test tubes, 

 and, after carefully crushing it with a sterilized glass rod and thor- 

 oughly mixing it with the gelatin, making roll tubes in the usual 

 way. Some of these should be put up for anaerobic cultures i.e., 

 the tube should be filled with an atmosphere of hydrogen according 

 to Frankel's method. If the object in view is to estimate the num- 

 ber of bacteria in a given sample of soil the difficulty is encountered 

 that, however finely crushed, the little masses of earth are likely to 

 contain numerous bacteria, and we cannot safely assume that each 

 colony originates from a single germ. Thoroughly washing a small 

 quantity of soil, by agitation, in a considerable quantity of distilled 

 water, and then adding a definite quantity of the water to nutrient 

 gelatin and making roll tubes or plates, as in water analysis, sug- 

 gests itself as a simple method ; but Frankel has shown that it is far 

 from being reliable when the object is to estimate the number of 

 bacteria. He obtained more uniform and accurate results by intro- 

 ducing the earth at once into liquefied gelatin and crushing it as 

 thoroughly as possible with a strong platinum wire, after which as 

 thorough a mixture as possible was effected by tilting the tube up 



