156 DISEASES OF GLASSHOUSE PLANTS 
and small animals. It is achieved by steaming or baking 
the soil, or treating it with some special chemical com- 
pound. 
Steam Sterilization 
This is effected by continuously passing into the soil 
large quantities of steam at a high pressure, until the soil 
temperature becomes sufficiently high, when it is main- 
tained there for a definite time. 
(a) “ Box and Grid” Method.—A trench about six 
feet wide and one foot deep is made across one end 
of the house, and the soil thus removed is taken to the 
other end. The subsoil at the bottom of the trench is 
forked up and if necessary a light dressing of lime may 
be given. Four boxes, six feet square, or of some size 
to suit the position of pipes in the house, and without 
bottom or top, are placed in position over the trench, 
the house being 27 feet wide. One box fits between the 
wall flowpipe and the return on each side of the house, 
and two in the middle of the house. The boxes average 
5 feet 6 inches square at the top and 6 feet square at the 
bottom, the sloping side being about 18 inches in depth. 
A “ grid” for distributing the steam is passed under one 
side of each box and laid on the subsoil. The grid 
resembles a large digging fork, and is made of perforated 
hollow pipes (Fig. 41). Two grids are connected, by 
means of a steam barrel T-piece fitted with two control 
valves, to a length of best flexible hose, which in turn is 
connected to the steam barrel leading to the steam 
boiler. When the boxes and grids are in position 
another trench 6 feet wide and 1 foot deep, adjacent to 
the line of boxes, is dug and the soil thrown into the 
boxes. It is important here to fill the boxes evenly, and 
in no case must the soil be firmer in one part than another, 
for steam always takes the line of least resistance, and 
the result will be that it will rush through the loose parts 
