172 DISEASES OF GLASSHOUSE PLANTS 
inhibiting factors may be removed by thoroughly soaking 
the soil with water and allowing it to drain, or by leaving 
the soil to weather under cover for three or four weeks 
before using. Of all methods employed for sterilizing 
the propagating soil that of formaldehyde has proved to 
produce the best plants. Occasionally tomato seedlings 
raised in baked soil have turned a dark blue-green colour 
and refuse to grow. This is generally due to the soil 
having been over-sterilized, and a slight top-dressing of 
virgin soil is usually sufficient to restore normal con- 
ditions in a short time. 
The table on p. 171 indicates the effect of different 
methods of sterilization upon crop yield, and is drawn up 
from the results of experiments conducted on a com- 
mercial tomato nursery in the Lea Valley. 
The Effect of Sterilization upon Plant Disease 
One of the most important functions of the steriliza- 
tion of soil is the eradication of the disease organisms 
which inhabit it, and numerous investigations have shown 
that diseases may be prevented by sterilization by 
steaming, baking, or with formaldehyde and other 
chemical compounds. 
It is necessary to bear in mind, however, that steriliza- 
tion to be completely satisfactory must be very 
thoroughly done. If patches of untreated soil remain in 
corners, under pipes, etc., infection rapidly spreads into 
the adjacent sterilized parts, and after a year or so the 
whole of the soil may be reinfected. It has been shown 
that disease organisms spread more rapidly through 
sterilized soil than through virgin soil—a fact which is 
easily understood, as the enemies and competitors of the 
parasitic fungi, which restrict their growth in normal 
soils, have also been destroyed by sterilization. 
An experiment performed by the author is instru ctive 
in relation to the fact. Two troughs 8 feet long, 18 inches 
wide and 2 feet deep were filled, one with steam sterilized 
