164 DISEASES OF GLASSHOUSE PLANTS 
(e) Drain Pipe Method.—Where trays are unobtain- 
able or too expensive to use, steam may be passed 
through a buried system of pipes (Fig. 47). For this 
purpose 2-inch agricultural tiles may be used. These are 
laid at a depth of about 9 inches in rows 15 to 18 inches 
apart, each row consisting of 12 tiles, the far end of each 
row being stopped up. It is convenient to steam four 
rows at a time, the steam being introduced by a hollow 
four-branched tube of steam barrel, the branches of 
which fit into the four rows to be steamed. It is advisable 
to dig the soil first, and in covering the pipes the soil 
must be compacted uniformly to enable the proper 
diffusion of steam. The surface of the soil should be 
covered with a tarpaulin or steam-proof sail-cloth, and 
then the steam may be turned on. Four rows of 12 tiles 
with 18 inches between the rows will steam 72 square feet 
of soil surface. Steam should be passed for 1 hour in 
order to heat the soil sufficiently to a depth of 18 inches. 
It is advisable not to disturb the soil before it is quite 
cold in order to obtain the maximum benefit from the 
steam, and so an extra supply of tiles should be obtained 
to allow the work to proceed continuously. Two men 
can be constantly employed in laying and removing tiles, 
making up beds, and attending to the steam boiler for 
each set of tiles. It is a saving to employ several sets 
of tiles simultaneously. 
Surface Sterilization with Hot Water 
The ground is first well dug and broken up; and in 
this respect a mechanical cultivator is helpful. It is 
sufficient to run the cultivator over the ground once, as 
by repeating the process the soil is broken up to such a 
fine state that it pans badly. Boiling water is applied 
freely, and temperatures of 180° F. at 3 inches depth, 
140° F. at 6 inches depth, and 120° F. at 9 inches depth 
are usually reached. 
A special hot water boiler can be obtained to deliver 
