IS PRINCIPLES OF SOIL MICROBIOLOGY 



or organic compounds, and of actinomyces, but give somewhat different 

 results for different soils, as shown by Conn. Egg-albumin agar and 

 especially soil extract agar were found by others 30 to give higher and 

 more uniform results than other media. 



In preparing the media, the constituents including the agar or gelatin 

 are dissolved in boiling water: after filtration through cotton, the more 

 decomposable constituents are added, as in the case of urea, ammonium 

 nitrate and glucose in the urea nitrate agar, albumin and casein solu- 

 tions and the sodium asparaginate as well as the glucose, in the cor- 

 responding media, so as to avoid any possible effect of preliminary 

 heating on these substances. 



The reaction of the media is an important factor. Its adjustment to 

 a definite point of titration is practically valueless in media, where the 

 buffer content is variable. 31 A slight acidity has usually been found to 

 be the most favorable reaction for the development of the majority of 

 soil bacteria. When determined by the hydrogen-ion concentration, 

 colorimetrically or electrometrically, this reaction was found to be pH 

 6.5 (albumin and casein media) to pH 7.0 (asparaginate agar media). 



The media are placed in 10-cc. portions in test tubes, or in 100-cc. to 

 200-cc. portions in Erlenmeyer flasks. These are plugged with cotton 

 and sterilized in the autoclave, at 15 pounds pressure, for 15 to 20 minutes. 

 A soil may contain 5 to 30 million bacteria (including actinomyces) 

 per gram and only 50,000 fungi or less. The final dilution used for the 

 determination of bacteria would be 100,000 or 200,000. Some of the 

 plates will have no fungi at all, while others may have one, two or more 

 colonies. In this case the proper dilution for the determination of 

 fungi would be 1000, but there will be so many bacteria on the plate 

 that most of the fungi may actually fail to develop or make only a 

 scant growth. A medium should be used which allows only the develop- 

 ment of fungi. Use is made of the fact that the majority of fungi can 

 stand greater degrees of acidity than the bacteria and actinomyces 

 For qualitative purposes and for pure culture study, any medium favor- 

 able for the development of fungi, such as raisin extract (60 grams of 

 raisins heated at 60° to 75°C. in 1000 cc. of tap water, filtered), which is 



30 Smith, N. R., and Worden, S. Plate counts of soil microorganisms. Jour. 

 Agr. Res. 31: 501-517. 1925. 



31 Clark, W. M. The "reaction" of bacteriologic culture media. Jour. Inf. 

 Dis., 17: 109. 1915. Clark, M. W. The determination of hydrogen ions. 2d 

 Ed. The Williams & Wilkins Co., Baltimore. 1922; Gillespie, L. J. Colori- 

 metric determination of hydrogen-ion concentration without buffer mixtures, 

 with especial reference to soils. Soil Sci. 9: 115-136. 1920. 



