304 
Journal of Agricultural Research 
Vol. XXXI, No. 4 
strip projects to insure a steam-tight contact with the soil. The 
top consists of two layers of thin tongue-and-groove boards with a 
layer of tar paper between. Handles are bolted to the sides. The 
steam inlet is centrally located in the top of the pan, and a metal 
plate fastened just below the opening serves to deflect the steam in 
all directions parallel to the soil surface. A steam-pressure gauge 
was attached to the steam supply pipe about 40 feet from the pan. 
The pressure was read with the steam to the pan shut off. The dura¬ 
tion of steaming is expressed as the time elapsing between the 
opening and closing of the valve at the gauge. The pan was left 
in place for 30 minutes after steam was shut off, thus prolonging the 
heating effect. By using electric resistance bulbs the rise and fall 
of temperatures at different depths in the soil under the steam pan 
was followed. A summary of these data and of the conditions under 
which the treatments were carried out is presented later. It seems 
likely from later tests that the amount of steam supplied to the 
steam pan was not accurately measured by the gauge pressure. The 
temperatures obtained were not always uniform. This may explain 
the lack of complete disinfection in some of the plots which were 
steamed for long periods. 
When the pans were moved they were set down so as to overlap 
by at least 6 inches the area previously steamed. Usually two pan 
areas which were similarly treated constituted a plot. In a few 
cases, one or three pan areas made up a plot. As soon as the steam¬ 
ing of a plot was completed, a trench a foot wide and 3 inches deep 
was carefully dug around it. Stakes were then driven into the soil 
at each corner. Boards were nailed to these to form sides which 
projected above the soil level and slightly below the bottom of the 
trench. The trench and board frames were provided to prevent 
contamination of the plots by surface water. A 2-foot wire-netting 
fence was attached above these base boards and a wire-netting top 
was fitted on in order to exclude birds and quadrupeds which might 
otherwise contaminate the plot. Great care was taken to have boots, 
tools, lumber, and other supplies used about the plots sterile so that 
no infected material reached the steamed soil during the progress 
of the work. The wind blew outside soil across the steam plots 
on one or two occasions. It was found necessary to sprinkle the 
surrounding area lightly with water to prevent this. These experi¬ 
ments in 1920 were planned to retest the steam and formaldehyde 
treatments which the results obtained in 1919 indicated to be of 
greatest promise. In view of the striking results obtained in 1919 
by the application of dilute solutions of mercuric chloride, certain 
plots received a combination of mercury drench and steam treatment. 
The steam-pan treatments in 1920, tising steam alone, varied from 
70 to 115 minutes, with the gauge showing 90 to 95 pounds 
pressure, and from 12 to 25 minutes at 140 pounds pressure. The 
steam-pan treatments which followed the application of formal¬ 
dehyde at the rate of pint per square foot ranged from 25 to 40 
minutes of steaming at 90 to 95 pounds pressure. The steam-pan 
treatments which followed the application of mercuric chloride each 
lasted for 25 minutes with steam at 90 to 95 pounds gauge pressure. 
Mercuric chloride was applied at the rate of 80 c. c. of a 1 to 20,000 
solution to 80 c. c. of a 1 to 500 solution per square foot, and in one 
instance at the rate of 160 c. c. of a 1 to 400 solution per square foot. 
