SOIL STERILIZATION 97 
extends 12 feet or more behind it. Four such devices are 
in operation at the same time in a large range at Toledo, 
and they are moved every 10 minutes. Two men can 
easily take care of this number and also rake down the 
beds as rapidly as the sterilizers are moved. It is a much 
less laborious system than when perforated pipes are 
used, as explained on another page in this chapter. 
Tile—The tile method has some advocates, although 
it has seldom met with favor in large houses. In prin- 
ciple and practice the system is similar to the perforated 
pipe plan of sterilization, except that the tiles are some- 
times laid permanently and not disturbed from year to 
year. When tiles are employed they may also be service- 
Fig. 32.—Peg steam sterilizer in operation at Toledo, Ohio. 
able in sub-irrigation, and be used to raise soil tempera- 
tures by the admission of steam whenever this is con- 
sidered desirable. When laid permanently the initial cost 
is rather heavy, but there would be a great saving in 
labor when a long term of years is considered. 
Frequency of sterilization —When sterilization is once 
started, nearly all growers seem to favor attending to it 
every summer. One successful grower has found every 
two or three years sufficient. Sterilization, however, is 
universally regarded in the same light as fire insurance, 
