166 DISEASES OF GLASSHOUSE PLANTS 
Each lot of soil is heated for 24 hours, the heat being 
evenly distributed throughout the mass by means of the 
moisture, and after some time a temperature of 180° F. 
is obtained throughout. It is important that the soil 
should not be too dry or the heat will be imperfectly 
distributed and the bottom layers overheated. 
In sterilization by the “tank” and “ boiler tray ” 
methods it is easy to over-sterilize. For this reason 
these methods should be performed in the summer, at 
least three months before using the soil. The soil should 
be mixed with clean stable manure and allowed to 
recover before using. 
The main principles governing the practice of steriliza- 
tion by heat have been given, but growers may have 
occasion to alter the small details to suit their own 
houses and convenience. Thus, in the tray method of 
steaming, the size of tray is optional and may be adjusted 
to conform to the structure and size of the house. Galvyan- 
ized iron is often useful for constructing small trays to 
fit into narrow places between pipes and walls, etc. 
Modifications in size and shape of grids and combs may | 
also be devised to suit individual requirements. 
Sterilization by Baking 
When sterilizing small quantities of soil for propaga- 
tion purposes it is often inconvenient to employ steam. 
In such cases the soil may be sterilized by baking. In 
this method damp soil is placed in a suitable oven (of 
which there are a number on the market) and heated 
until the temperature at the interior of the soil mass is 
between 205° F, and 210° F. This should be maintained 
from 60 to 120 minutes and the soil removed from the 
sterilizer to a suitably covered position, where there is no 
danger of reinfection. 
Our experience has indicated that for satisfactory 
work it is necessary to have a factory thermometer fitted 
through the side of the oven, with the bulb in the soil 
