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 
