ORGANISMS IN THE SOIL 



433 



forms predominate in summer and others in winter (see 

 Fig. 60). 



Fig. 60. — Periodical enumeration of bacteria in soil of two plats during 

 two years, expressed in millions to a gram of dry soil. 



Brown and Smith l obtained results that in the main 

 confirmed Conn's work, and they advanced the theory 

 that the concentration of the soil solution immediately 

 surrounding the soil particles, together with the high 

 surface tension exerted by the soil particles, prevents the 

 freezing of the surface film and that this water forms a 

 suitable medium for the development of bacteria. 



354. Conditions affecting growth. — Many conditions 

 of the soil affect the growth of bacteria. Among the most 

 important of these are the supply of oxygen and moisture, 

 the temperature, the presence of organic matter, and the 

 acidity or the basicity of the soil. 



355. Oxygen. — All soil bacteria require for their 

 growth a certain amount of oxygen. Some bacteria, how- 

 ever, can continue their activities with much less oxygen 

 than can others. Those requiring an abundant supply 

 of oxygen have been called aerobic bacteria, while those 

 preferring little or no air are designated as anaerobic 



1 Brown, P. E., and Smith, R. E. Bacterial Activities in 

 Frozen Soil. Iowa Agr. Exp. Sta., Research Bui. 4. ?912, 



2f 



