88 VEGETABLE FORCING 
at a temperature of 212 degrees, that there is considerable difference 
in the yield of the crop. It has been found that the same quantity 
of soil, after the action of heat, yields a crop many times in excess 
of the former or untreated soil. In other words, by heating we con- 
vert the humus compounds in the soil into a more available form 
for the utilization of the plant. That heating of the soil gives rise 
to some changes is shown by its darker color and more porous con- 
dition, and it is undoubtedly due to these changes which have taken 
place in the humus compounds, which account for the accelerated 
and vigorous growth of the plants. 
“Another feature which is characteristic of sterilized soils is the 
unusual occurrence of humus-loving plants, or saprophytes, that grow 
upon it, which is a good indication that the organic matter contained 
in the soil has undergone changes through the action of the heat. 
We have ourselves observed more than once certain species of 
saprophytic fungi growing upon our steamed beds which have never 
shown any tendency to grow in unheated soil, although with the 
exception of being steamed the soil was exactly the same as that 
upon which they never appeared.” 
Evil results sometimes follow the use of steam, prob- 
ably because of injurious effects upon the physical 
properties of the soil, especially when the soil has not 
been properly handled after sterilization. All things 
considered, steaming is the most complete, effectual 
and practical method of soil sterilization. 
Formalin, however, has a useful place in the manage- 
ment of many greenhouses. While the usual strengths 
have little effect upon the animal life of the soil and do 
not destroy nematode eggs, many of the diseases may be 
controlled by the use of this disinfectant. Small areas 
of soil sometimes show infestation at midwinter, and 
they may be drenched with formalin when it would not 
be practicable to use steam. Again, there are hundreds 
of small houses heated by flues or hot water where steam 
is not available and formalin can be used to advantage. 
Its use is not so harmful to silty and clay soils, the 
structure of which is often injured by steaming. 
