322 
Journal of Agricultural Research 
Vol. XXXI, No. 4 
Temperatures obtained in soil treated by the steam-pan method 
have apparently received relatively little attention. A number of 
investigators, including Russell and his associates ( 15, 16, 17, 18) in 
England, and Johnson (10, 11) in this country, have studied the 
effect of heat on soil and on plant growth, but no schedule of steam-pan 
temperatures was found in the literature. So far as the writers have 
been able to discover, the most complete schedule of temperatures 
obtained under a steam pan are given by Beinhart (2), who says, 
“In sandy soils, after 30 minutes of steaming, the temperatures to 
be expected in the upper two inches of soil directly under the pan 
are approximately 208° to 212° F., at 3 to 4 inches 170° to 180°, and 
at 6 inches 120°. Two hours after the removal of the pan the temper¬ 
ature at 6 inches should be about 160° F.” 
The work herewith reported gives a much more comprehensive 
series of temperatures than those of Beinhart. 
APPARATUS AND METHODS 
The construction of the steam pans used in the potato-wart 
extermination experiments is explained on pages 303 and 304. 
The source of steam was a high-pressure 8-inch main which carried 
steam for use in the coal mines for pumping, hoisting, and other 
purposes. The pressure in the main was almost constant, usually 
varying only between 135 and 140 pounds, but often dropped appreci¬ 
ably for a few moments, particularly when hoisting outfits were 
being started. The quality of the steam varied considerably. The 
point at which the steam-pan supply line was attached to the main 
was said to be several miles from the boiler generating the steam. 
When rain fell on the exposed part of the pipe line there was more 
condensation than usual. The steam was also much poorer, i. e., 
contained more condensation water, when the mines were using 
little steam. 
During the early part of the season the steam pan was located 200 
feet from the large main, 24-inch pipe being used to carry the steam 
almost to the pan, 20 feet of steam hose being used at the pan. 
Later in the season the pan was used only 75 feet From the steam line. 
Steam at 140 pounds pressure is not readily managed and is not 
always available, hence it was thought best to reduce the pressure. 
Beinhart and others recommended the use of steam at 100 pounds 
boiler pressure with a minimum of 70 pounds. In the writers’ work 
a gauge was placed on the pipe line near the point of attachment of 
the steam hose and only enough steam turned on the closed line to 
give a reading of 90 to 95 pounds on this gauge. This method of 
regulating the steam was not entirely satisfactory when the steam 
pan was 200 feet from the main. When the pan was moved to 75 
feet from the main it was found practically impossible to have the 
gauge pressure at 90 to 95 pounds with the line closed and then have 
any steam flow at all when the steam was turned on the pan. It was 
found desirable to take the gauge reading with the steam running 
wide open into the air. Under these conditions a gauge reading of 
between 5 and 10 pounds, nearly 10 pounds, seemed to give a flow 
of steam equal to that formerly obtained with the 95 pounds pressure. 
The observations made led to the conclusion that the flow of steam 
was probably different following each separate setting of the valve 
by gauge reading with the line closed, even when the pan was 200 
feet away. In order to secure an accurate index of the steam flow a 
