THE SOIL AS A CULTURE MEDIUM 641 



Growth of microorganisms in soil in pure and mixed culture. There 

 is no method available for sterilizing the soil without changing its 

 physical and chemical properties. The common method used at pres- 

 ent for sterilizing the soil consists in heating it in flowing steam for 

 thirty minutes on seven consecutive days, 64 or at high pressure (15 

 pounds) for two hours. Both of these treatments cause a decided 

 change in the physical and chemical condition of the soil which results 

 in an increase in the available organic matter in the soil. Sterile 

 soil forms an excellent medium for the development of various bacteria 

 and other microorganisms. 65 A number of soil organisms, such as 

 Azotobacter or Bac. amylobacter, will regain their vigor of fixing nitrogen, 

 when cultivated in sterile soil. 66 



However, from processes carried out by pure cultures of micro- 

 organisms grown in sterile soil we cannot determine what actually 

 occurs in normal soils. Not only is the nature and the composition of 

 the culture medium completely changed by sterilization of soil, but 

 the various antagonistic and associative influences which are active in 

 normal soils are eliminated. This can be readily illustrated by the 

 following instances. An organism belonging to the Bac. mesentericus 

 group was found 67 capable of dissolving and clarifying cultures of Bact. 

 coli, Staphylococcus, and other bacteria, a phenomenon which may not 

 take place in pure culture. The inhibitive effects of filamentous fungi, 

 especially of ascomycetes, on the growth of microorganisms has been 

 commonly observed; 68 this may be due to exhaustion of nutrients or 

 formation of some toxic products during growth. 69 Various symbiotic 

 processes take place in the soil, such as the symbiosis between the 

 chlorophyll-bearing algae and the nitrogen fixing Azotobacter, between 

 the anaerobic CI. pastorianum and the aerobic Azotobacter, between 

 Azotobacter and cellulose-decomposing bacteria. Then we have proc- 



64 Eckelmann, E. Uber Bakterien, welche die fraktionierte Sterilization 

 lebend uberdauern. Centrbl. Bakt. II, 48: 140-178. 1918. 



155 Barthel, C. Kulturen von Mikroorganismen in sterilisierter Erde. Centrbl. 

 Bakt, II, 48: 340-349. 1918. 



66 Bredemann, 1909 (p. 111). 



67 Kimmelstiel, P. tJber eine biologische Eigenschaft eines Wurzelbazillus. 

 Centrbl. Bakt. I, Orig. 89: 113-115. 1922. 



68 Porter, C. L. Concerning the characters of certain fungi as exhibited in 

 their growth in the presence of other fungi. Amer. Jour. Bot., 11: 168-188. 

 1924. 



69 Liesegang, R. Gegenseitige Wachstumshemmung bei Pilzkulturen. 

 Centrbl. Bakt. II, 51: 85-86. 1920. 



