SOIL STERILIZATION 91 



than through compact silts and clays. Again the time 

 required will depend upon the pressure and volume of 

 steam, and the volume of soil to be sterilized. In most 

 of the greenhouses using high pressure steam with 100 

 horse power boilers or more, sterilization goes on for an 

 hour. One large establishment with a 350 horse-power 

 boiler regards 45 minutes as ample time. Others with 

 high pressure steam sterilize for an hour and a quarter, 

 while occasionally an hour and a half is regarded as 

 necessary. The safe practice of one very careful grower 

 is to continue steaming for half an hour after the soil 

 reaches a temperature of 212 degrees. 



The shortest period of sterilization is used by a very 

 large establishment at Toledo. This firm uses a 350 

 horse-power boiler and sterilizes for only 10 minutes 

 with a pressure of 90 degrees at the boiler. In this case 

 the peg method is employed as described later in this 

 chapter. It is claimed that the plan has given entire 

 success. With low pressure steam a much longer time 

 is required to heat all the particles of soil to the re- 

 quired temperature. Four or five hours is not too much 

 time, and then the beds should be covered over night to 

 retain the heat. 



Boiler and pressure. Large boilers and high pressure 

 steam are advantageous in every respect. Less time is 

 required to raise the temperature of the soil to the re- 

 quired temperature than with small boilers and low 

 pressure steam. A large volume of steam under high 

 pressure makes it possible to sterilize a larger area at one 

 time, and this is usually a matter of great economy from 

 the labor standpoint. Boilers of 300 horse-power or more 

 are used for the steaming of soils, although much smaller 

 boilers are often employed. 



One of the largest and most successful greenhouse 

 plants maintains a boiler pressure of 90 to 100 pounds 

 for 45 minutes. Many establishments sterilize with a 



