150 



PRINCIPLES OF SOIL MICROBIOLOGY 



PRESENCE IN NUMBER 

 OF SOIL SAMPLES 



ORGANISM 



B. petasites 



B. cereus 



B. megatherium 



B. subtilis 



B. mesentericus 



B. vulgatus 



B. mycoides 



B. mesentericus var. flavus 

 B. cereus var. fluorescens . . 



B. fusiformis 



B. brevis 



B. simplex 



B. cohaerens 



B. agri 



Total isolations 



of spore-forming organisms. 15 When a long incubation period is used 

 they are found to form only about 5 per cent of the colonies. 1617 The 

 presence of spore-forming bacteria, which are destroyed with difficulty 

 by partial sterilization, varies with the soil. 18 Here belong Bac. 

 mycoides, Bac. mesentericus and Bac. mesentericus ruber. The simplest 

 way of destroying these is to heat the soil for 30 minutes at 100°C. 

 on seven consecutive days and then to incubate the soil, between suc- 

 cessive sterilizations, at room temperature. 



Non-spore forming bacteria. Although the heterotrophic non- spore 

 forming bacteria include the most predominant group of soil organisms 

 developing on the plate and even more so in the microscopic examination 

 of the soil, very little attention has been paid to their identification. 

 This has been due, probably, to the fact that these organisms grow 

 poorly and only very slowly on laboratory culture media. The inap- 

 preciable results obtained by the ordinary chemical tests led to the 

 general assumption that they are of little importance in the soil. The 

 fact must also be kept in mind that certain very important physiological 



15 Chester, F. D. Study of the predominating bacteria in a soil sample. 

 Agr. Exp. Sta., Rept. 14: 52-63. 1903. 



16 Hiltner and Stormer, 1903 (p. 12). 



17 Conn, 1917 (p. 142). 

 18 Eckelmann, 1918 (p. 641). 



Del. 



