Florida Agricultural Experiment Station


desirable during July, August and September to list the soil at
weekly intervals, or as nearly so as the weather permits.
 3. Heat.-Direct readings of soil temperature at a depth of
1 inch showed that the soil frequently became hot enough to kill
all larvae in about 2 hours (1040 F.) and occasionally in only 8
minutes (1090 F.). Some larvae are killed by shorter exposures
than those mentioned (3).
 During August and September, 1938, soil temperatures at a
depth of 1 inch reached 1040 or above on 12 days, including 2
consecutive days with readings of 1100. During the same period
temperatures at a depth of 4 inches reached 990 on several days
and 100 once, but these temperatures would not likely kill nema-
todes.
 4. Drying.-A certain layer of surface soil may become dry
enough to kill nematodes (6, 7). This surface layer is changed
at each cultivation. Listing exposes more soil surface for dry-
ing (Fig. 6) than flat cultivation with a turning plow or harrow,
and incidentally reduces the amount of erosion. Listing of a
field may be done in the direction which best conforms to the
contour.
 These factors of heat and drying operate most effectively dur-
ing the hot weather from July to the first or middle of October.
Plots sowed in oats in October showed almost as good root-knot
control and maintained their yield and quality better than those
where clean fallow was continued until January. Therefore,
following clean fallow a cover crop of oats sowed about the mid-
dle of October is recommended for shades which are to be
planted in tobacco the following year. The oat crop should be
plowed under and completely destroyed by February 1 to avoid
excessive trouble with cutworms (1).
 An incidental benefit from fall plowing was mentioned by
 Chamberlin and Madden (1). They reported that 1 fall plowing
 of the soil to a depth of about 7 inches killed about 1/2 of the
 hornworm pupae which would otherwise have passed the winter
 successfully, and stated that a still greater mortality would be
 expected to result from several cultivations of soil infested with
 pupae.
 Clean fallow has been recognized by the Agricultural Adjust-
 ment Administration as an alternate qualifying practice for
 tobacco shades in 1942.4

 "Maintenance of clean fallow by complete listing at least 6 times
 during the period from the completion of harvest to October 1, 1942. The