Soil Sterilization and Partial Sterilization 



297 



factor in controlling the clc\elopnient of potato scab in the soil. 

 Moisture was found to be directly or indirectly the main factor, a 

 high moisture content controlling the disease, whereas abundant 



20 40 60 

 Time, days 



40 60 



Fig. ir 



Influence of toluol and heat upon the numbers and acti\ ities of micro- 

 organisms in soil (from Waksman and Starkey). 



scab was formed in dry soils. The development of scab is influenced 

 also by the temperature of the soil, the optimum for scab being 

 22°C. 



Soil Sterilization and Partial Sterilization 



The soil may be sterilized completely or only partially, whereby 

 not all the organisms are destroyed, but only certain groups. Com- 

 plete sterilization is difficult to accomplish in the field or in the green- 

 house, since the soil readily becomes reinfected again; it is not even 

 desirable. In the laboratory, it often becomes necessary to sterilize 

 a soil for growing pure cultures of organisms, for testing the purity 

 of certain strains, and for invigorating laboratory-kept cultures. To 

 sterilize a soil, it is placed in glass or clay containers and heated 

 under pressure, at 15-20 pounds, for 2-3 hours. By using flowing 

 steam for 1-2 hours, on 6 consecutive days, complete sterilization 



