Aug. 15, 1925 
Soil Disinfection for Potato Wart 
349 
Table XV. — Lateral penetration, under an obstacle in the soil, of mercuric chloride 
plus sodium chloride solution as shown by hydrogen sulphide and silver nitrate 
tests 
Depth 
to 
obstacle 
Lateral penetration under obstacles at 1-inch and 
6-inch depths on successive days 
After 2 days 
After 3 days 
After 4 days 
H 2 S test for HgCh__ 
AgN03 test for NaCl.... 
Inches 
1 1 
l 6 
{ 6 
Inches 
1 to 2 (4) .. 
Otol (l),none (3) 
1 to 2 (4).. 
1 to 2 (4)_ 
Inches 
0 to 1 (2), 1 to 2 
(2). 
None (4)_ 
1 to 2 (4). 
1 to 2 (4)_ 
Inches 
0 to 1 (3), 1 to 2 
(1). 
None (4). 
1 to 2 (4). 
1 to 2 (4). 
• Numbers in parentheses show the number of pots, with the lateral penetration indicated. 
Pots, 8-inch; petri dishes, 3!Ni-mch; soil, Leonardtown silt loam containing 13.5 per cent moisture. 
A second set of 24 pots was prepared in the same manner as the 
above but with the use of 8-inch instead of 6-inch pots and by placing 
the petri-dish bases at 1-inch and at 6-inch depths. As shown in 
Table XV, there was some penetration of mercury under all of the 
petri dishes at the 1-inch depth, while only one positive test for 
mercury was obtained under the petri dishes at the 6-inch depth. 
The diminishing number of positive tests under the center of the 
petri dishes at 1 inch would seem to indicate the gradual change of 
the mercury to compounds not precipitated by the H 2 0 + H 2 S test 
used. Positive tests with AgN0 3 throughout the soil under the petri 
dishes at both the 1-inch and the 6-inch depths indicated a thorough 
penetration by the NaCl. 
Just what is responsible for the apparently poor penetration of the 
mercury is not known. The percentage of HgCl 2 was made as low 
as seemed safe to insure penetration in soil free from obstacles. 
It is well known that lateral penetration in soil is poor. It seems 
S ’ J e likely that most of the mercury reaching the top of the petri- 
base remains there. Even if it all worked over the edge of the 
petri dish, much of it would probably go down rather than laterally, 
under the obstacle. It would seem probable that the amount of 
mercury moving back under the petn dish would be so small that 
reactions with soil chemicals would soon change its form so that it 
would not react to the H 2 0 + H 2 S test used. Much the same thing 
may occur in tubes of dry soil treated and allowed to stand until 
there is no longer a sharp line between the moist treated soil and the 
dry untreated soil but a very gradual transition from one to the other. 
Soil from the. transition zone is likely to give only negative results 
when tested for mercury. 
Upward Penetration of Mercury 
A single experiment was run to determine the approximate upward 
penetration of mercury as compared to downward penetration. Of 
24 tubes filled with Leonardtown silt loam containing 4.26 per cent 
moisture, 12 tubes were treated in the usual way, 1 + 1, and the other 
12 were treated by standing them in beakers and pouring the solu¬ 
tion and the water into the beakers. The water was not added until 
the solution had been taken up by the soil. Three tubes from each 
lot were tested on the second, third, fourth, and seventh days. The 
