NUMBERS OF MICROORGANISMS 15 



The first important modifications in the composition of the medium 

 for a quantitative determination of soil bacteria were made by the 

 introduction of the soil infusion agar, 23 and later by the elimination 

 even of the soil extract, 24 using a synthetic agar, with only 0.05 gram of 

 peptone per liter. The soil extract media do not, however, meet the 

 qualification of being "standard in composition," since the soil infusion 

 varies with the soil used for making the infusion. The synthetic 

 medium was further modified 25 by the substitution of egg-albumin and 

 casein for peptone. Among the other synthetic media suggested for the 

 quantitative estimation of soil bacteria and actinomyces, sodium 

 asparaginate agar, 26 asparaginate-mannite agar, 27 and urea nitrate 

 agar 28 should be mentioned. For the estimation of fungi, special acid 

 media have to be used. For the protozoa, the dilution method still 

 remains the most reliable, and nutrient agar or special liquid media 

 can be employed for the development of the organisms in the final 

 dilutions. 



Composition of synthetic media 



I. Fischer's soil extract agar: 



Soil extract 1000 cc. 



Agar 12 grams 



K 2 HPO« 2 grams 



The soil extract is prepared by heating soil for half an hour at 15 pounds pres- 

 sure with an equal weight of a 0.1 per cent solution of Na 2 C0 3 . 



23 Fischer, H. Bakteriologisch-chemische Untersuchungen. Bakteriologis- 

 cher Teil. Landw. Jahrb. 38: 355-364. 1909. 



!4 Lipman, J. G., and Brown, P. E. Media for the quantitative estimation 

 of soil bacteria. Centrbl. Bakt. II, 25: 447-454. 1910. 



25 Brown, P. E. Media for the quantitative determination of bacteria in soils. 

 Centrbl. Bakt. II, 38: 499-506. 1913; also Iowa Agr. Exp. Sta., Res. Bui. 11, 

 396-407. 1913. Waksman, S. A. Microbiological analysis of soils as an index 

 of soil fertility. II. Methods of the study of numbers of microorganisms in the 

 soil. Soil Sci. 14: 283-298. 1922; Waksman, S. A., and Fred, E. B. A tentative 

 outline of the plate method for determining the number of microorganisms in the 

 soil. Ibid. 14:27. 1922. 



26 Conn, H. J. Culture media for use in the plate method of counting soil 

 bacteria. N. Y. Agr. Exp. Sta. Tech. Bui. 38. 1914. 



27 Thornton, H. G. On the development of a standardized agar medium for 

 counting soil bacteria, with especial regard to the repression of spreading colonies. 

 Ann. Appl. Biol. 9: 241-274. 1922. 



28 Cook, R. C. Quantitative media for the estimation of bacteria in soils. 

 Soil Sci. 1: 153-161. 1916. 



