298 Disease-Producing Microorganisms 



can also be obtained. The sterility of the soil thus treated must be 

 carefully checked. 



Though we know little about transmission through the soil of 

 virus diseases of animals, we do know that various virus diseases 

 of plants may be thus transmitted; for example, mosaic virus of 

 wheat. To inactivate the virus, the soil, according to Johnson, must 

 be heated for 10 minutes at 50-60° C. 



Origina 

 100 



CSo added 1 



5 6 7 



Month 



Fig. 118. InHuence of CS2 upon the numbers of bacteria and actinomycetes in 

 soil (from Hiltner and Stormer). 



It often appears necessary to partly sterilize the soil to destroy 

 certain injurious insects or pathogenic fungi, but not to kill the whole 

 soil population. Such partial sterilization can be brought about by 

 use of heat, as by steam and dry heat, or by means of various vola- 

 tile and nonvolatile antiseptics, the first comprising carbon bisulfide, 

 toluol, formaldehyde, and hydrocyanic acid; the second including 

 phenol, cresol, and chloropicrin. These disinfectants do not accumu- 

 late in the soil, but are either lost by volatilization or destroyed by 

 soil organisms. The treatments have a selective eflPect upon the soil 

 microbiological population, affecting particularly the fungi, many of 

 the protozoa, and certain bacteria, such as the nitrifying organisms. 



