328 
Journal of Agricultural Research 
Vol. XXXI, No. 4 
for the 7-inch depth to reach its maximum temperature. In several 
cases the temperature at the 7-inch depth fluctuated, seeming to 
indicate that for a time the soil received heat from above faster than 
it was transmitted to the soil below and that later the heat was trans¬ 
mitted much faster than received so that the temperature became 
lower. The only indication of such a condition in the table is at the 
7-inch depth of the first set of temperatures, 95 pounds for 25 min¬ 
utes. In this case the maximum reached was 29° C. Ten minutes 
later the temperature was still 29°, but at the end of 20 minutes and 
of 30 minutes it was 28°, returning to 29° at the end of 45 minutes. 
So far as could be determined, the fluctuations though slight were 
not due to errors in the readings, which were made to tenths of a 
degree. The data for all four of the pan areas used in making these 
averages show such fluctuations. 
The table shows that in most cases the temperature began to 
fall at the 1-inch depth almost as soon as the steam was shut off. 
At the 4-inch level the temperature continued to rise for several 
minutes after steam was shut off, except where the 100° C. was 
reached before the steam was shut off. At the 7-inch level the tem¬ 
perature continued to rise for a considerable time after steam was 
shut off, except where a temperature of 100° had previously been 
reached. The temperatures at all depths usually became somewhat 
uniform within 4 hours after steam was turned on, regardless of 
whether steamed 25 or 40 minutes, the temperatures being somewhat 
higher than the initial temperatures. 
A comparison of the temperatures obtained with 10 pounds and 
with 20 pounds of steam flowing for 40 minutes shows a closer ap¬ 
proach to the 1:2 ratio than might have been expected. With 10 
pounds of steam the maximum temperatures at 1 inch, 4 inches, and 
7 inches averaged 52° C. higher than the initial temperature, while 
with 20 pounds of steam they averaged 86.5° higher. With 10 
pounds of steam the maxima were reached in 29 minutes, 62.5 min¬ 
utes, and 155 minutes, as compared to 15 minutes, 32 minutes, and 
38 minutes at 20 pounds steam pressure. The rates of fall in tem¬ 
perature after the maxima were reached, in minutes per degree, were 
3.3 minutes, 6.0 minutes, and 58.0 minutes at 10 pounds as compared 
to 2.5 minutes, 3.4 minutes, and 3.4 minutes at 20 pounds of steam. 
Ten pounds of steam for 75 minutes gives approximately the same 
“maximum sustained temperature” as does 20 pounds of steam for 
40 minutes, viz., 100°, 99°, and 97° C. as compared to 99.5°, 99°, 
and 96°. It will be noticed, however, that the rates of rise and 
fall are approximately twice as rapid in soil treated at 20 pounds. 
Where a high temperature for a relatively short period would be 
effective the higher pressure would seem desirable on account of 
the saving in time. 
SUMMARY 
During the period of steaming, heat rapidly penetrates soil under 
a steam pan. After steam is turned off, heat penetrates slowly. 
Hence the shorter the steaming period the longer the time required 
for the deeper soil to reach maximum temperatures. Steam-pan 
treatments necessary to exterminate the potato-wart organism heat 
the topmost 8 inches of soil to approximately 100° C. 
Doubling the steam pressure almost doubles the rate of penetra¬ 
tion of heat in the soil under the steam pan. The rate of increase 
