170 DISEASES OF GLASSHOUSE PLANTS 
therefore, is stimulated in much the same way as by the 
addition of nitrogenous fertilizers. The extent to which 
the plant is stimulated depends upon the method of 
sterilization employed. 
Heat is the most active in this respect, and liberates 
large quantities of plant foods, while chemical sterilizers, 
like cresylic acid, liberate considerably less. Again, the 
amount of food liberated and ammonia produced varies 
with soil type, being small in light sandy soils and large 
in heavy soils rich in organic matter. When soils have 
been sterilized by heat it is not advisable to give a base 
dressing containing nitrogen, but to wait until the plants 
develop, when nitrogen may be given as a top-dressing 
if necessary. Many a grower has regretted disregarding 
this principle when his crop has developed a soft, sappy 
growth which could only be corrected by excessive 
dressings of potash and phosphates. On some soils, 
therefore, sterilization encourages a soft, sappy growth 
if accompanied by normal base treatment, and thus the 
safest procedure is to give the normal quantity of potash 
in the base manure and to omit all nitrogenous manures. 
Phosphates may be added at the grower’s discretion. 
This applies especially to steam sterilization on all but 
the poorest soils, and to cresylic acid sterilization on 
heavy, rich soils. 
The presence of large quantities of ammonia in the 
soil has an inhibiting effect upon plant growth, which 
is especially noticeable in young seedlings. Generally, 
seeds sown in sterilized soil exhibit a retarded germination 
in comparison with those sown in virgin soil. After a 
time, however, those grown in sterilized soil forge ahead 
and become much better plants than the controls. The 
amount of ammonia produced is dependent upon a num- 
ber of factors, such as the richness of the soil, method of 
sterilizing, and duration of the sterilizing processes. It 
is greater in rich soil than in poor soil, in heated soil than 
in chemically treated soils, and in soils heated for a long 
time than in those heated for a short time. The growth 
