12 
BULLETIN 1120, U. S. DEPARTMENT OF AGRICULTURE. 
Table 8. — Daily maximum soil temperature in the heat-canker plat at Fargo, N. Dak.y 
in June and July, 1920 and 1921. 
[Data in degrees centigrade, as recorded by a soil thermograph with the bulb barely covered, which 
corresponds to the soil temperature at a depth of half an inch.] 
Day of month. 
1920 
1921 
Day of month. 
1920 
1921 
June. 
July. 
40.0 
44.0 
47.5 
37.5 
2$. 5 
37.5 
35.0 
42.5 
43.5 
44.0 
50.0 
32.5 
2S.5 
35.0 
39.5 
45.0 
June. 1 July. 
June. 
July. 
June. 
July. 
1 
17.5 
20.5 
24.5 
25.5 
20.0 
26.5 
20.0 
26.5 
49.5 
46.0 
20.0 
35.0 
19.5 
25.5 
3.5.0 
42.5 
17 
35.0 
4.3.5 
39.5 
47.0 
30. 5 
40.0 
42.0 
42.5 
42.5 
35. 5 
33.5 
40.0 
46.0 
46.0 
44.0 
44.5 
49.0 
48.0 
45.5 
50.5 
27.0 
2 
18 
40.0 
3 
19 
! 20 
21 
1 22 
i 23 
41.0 
4 

43. 5 45. 
45.0 
5 
43.5 
45.5 
46.0 
27.0 
34.0 
25.0 
21.5 
32.0 
40.0 
31.5 
42.5 
43.5 
45.5 
47.5 
41.5 
40.5 
45. 5 
49.0 
42.5 
44.0 
45.5 
41.0 
6 
48.0 
7 
41.5 
36.0 
37.5 
21.5 
33.0 
41.0 
49.0 
37.0 
37.5 
44.5 
44.5 
8 
24 
42.0 
9 
31.5 47.0 
33. 46. 
35. 43. 5 
25 
35.0 
10 
26 
34.5 
11 
27 
24.5 
12 
34.0 
29.0 
40.0 
35. 5 
28 
29.5 
13 
29 
35.0 
14 
29.5 ; 39.5 
29.0 39.5 
34.5 1 31.0 
1 30 
26.5 
15 
31 
16 
1 
Data obtained in 1921 from maximum thermometers having the 
bulbs painted green showed that the soil-surface temperatures on hot 
days were considerably higher than the soil temperatures at the depth 
of half an inch. On the hottest days, that is, on June 27, June 30, 
and July 9, the soil-surface temperatures recorded by the maximum 
thermometers were respectively, 7, 4.5, and 7 degrees higher than 
the temperatures at a depth of half an inch recorded by the soil 
thermograph. Unfortunately, checks on the soil-thermograph rec- 
ords like these were not taken during 1920. However, if the aver- 
ages of these corrections are transferred and added to the maximum 
soil temperatures as recorded by the soil thermograph on the days 
of high temperature in 1920, following which heat canker resulted, it 
is possible to approximate the temperatures that produced the 
canker. The four highest maximum soil temperatures in 1920, as will 
be noted from Table 9 and Figure 3, occurred on June 13, July 3, July 
11, and July 28. Following each of these dates heat canker occurred 
on young flax plants in plats G, H, I, and J, which plats were sown 
periodically in order that young flax plants in at least one of these 
plats would be exposed at high temperatures on the hot days at any 
given time during the season. No canker occurred in any of the 
plats except the four mentioned and following high soil temperatures. 
These data are presented in Table 8 and Figure 3 and summarized in 
Table 9. They indicate that under conditions favorable for produc- 
ing heat canker the critical temperature is about 54° C. 
As shown in Table 9, only a few plants cankered in plats H and J, 
while many plants cankered in plats G and I (Table 4) . In plat H 
the lack of severe injury probably was due to the fact that relatively 
old plants were subjected to comparatively low temperatures. Prob- 
