REFLECTIONS ON DISEASE TREATMENT — 155 
to survive. The result is a specially clear field of action 
for those organisms which remain because of the destruc- 
tion of enemies which in normal soils restrict their 
development. Among the spore producing bacteria 
which remain after partial sterilization are the ammonia 
producing organisms, which are thus enabled to spread 
rapidly and carry out unrestricted operations. After 
partial sterilization there is an immediate drop in the 
number of bacteria present in the soils, when there is a 
rapid increase which continues for several days, after 
which the number decreases slightly and then remains 
constant for a considerable period. Similar effects are 
shown in the amount of ammonia produced. This ceases 
immediately after heating, but increases suddenly, drops 
slightly, and remains approximately constant following a 
curve similar to that of the bacterial numbers. As 
might be expected this increased ammonia production has 
a beneficial effect upon plants, as is shown by a com- 
parison of those grown in sterilized and unsterilized soil. 
Partial sterilization effects a reduction in the extent 
of disease, but this varies with the nature of the disease 
organisms. The vegetative hyphe and _ thin-walled 
spores are readily destroyed, but not so the thick-walled 
resistant spores and other bodies which, like the bacterial 
spores, are able to withstand such conditions. For the 
purpose of eliminating disease the soil must be heated to 
a greater extent than is required for increasing fertility. 
Generally, however, a temperature of 200-205° F., main- 
tained for an hour, is sufficient to eliminate most fungus 
diseases. Complete sterilization or over-sterilization is a 
possible contingency which must be guarded against, for 
when this takes place the soil is rendered unable to main- 
tain plant growth until reinoculated with fresh soil and 
sufficient time allowed for the introduced organisms to 
permeate the original soil and restore fertility. 
In practice, the aim of sterilization is to restore soil 
fertility and to effect the complete eradication of harmful 
soil organisms, including fungi, bacteria, amcebe, insects, 
