Aug. 15, 1925 
Soil Disinfection for Potato Wart 
333 
filtrate was then tested for the presence of the chemical with which 
the soil had been treated. After some experience it was often pos¬ 
sible to estimate the depth of penetration, so that only a few tests 
for each tube were necessary. 
The Chemical Tests Used 
TEST FOR FORMALDEHYDE 
The first test made was one for formaldehyde. A number of tests, 
particularly Schiffs, were tried without success. Schryver’s test as 
given by Haas and Hill (5 p. 59) was found to give good results in 
general. The method is as follows: 
To 10 c. c. of the liquid to be tested add 2 c. c. of a 1 per cent solution of 
phenylhydrazine hydrochloride freshly made up and filtered; then add 1 c. c. 
of a 5 per cent solution of sodium ferricyanide, also freshly made up, and 5 c. c. 
of hydrochloric acid; a brilliant magenta color is produced. The test is a very 
delicate one and will detect quantities of formaldehyde varying from 1 part in 
1,000,000 to 1 part in 100,000. Acetic aldehyde gives no color with this reagent 
In preliminary work 10 with soil solutions to which known quantities 
of formaldehyde had been added, successful tests were obtained in 
dilutions as great as 1 part in 1,250,000. Yet in some cases positive 
tests were not obtained in dilutions greater than 1 part in 250,000. 
Slight differences in the weighing or measuring of chemicals, in their 
uniformity, or in the age of the solutions, may have been responsible 
for the variations. The color produced is brilliant only when there 
is sufficient formaldehyde present. When nearing the limit of dilu- 
lution for positive tests the color produced is very faint pink. In 
the case of a few soil samples a deep green color was produced, com¬ 
pletely screening the red color if such were produced. This green 
color was not encountered in soil except where foreign material was 
f >resent. It seemed to be due to the presence of iron in certain 
orms. 
TEST FOR MERCURY AND COPPER 
Hydrogen sulphide was used as a test for mercury bichloride and 
copper sulphate. For convenience water was charged with hydrogen 
sulphide gas and some of this added to the clear filtrate obtained 
from the soil solution. When the amount of metal present was 
small the precipitate would cause only a faint browning of the solu¬ 
tion. It had been feared that this test would be unusable owing to 
possible action of soil chemicals. There was no visible evidence 
that the hydrogen-sulphide tests were affected by soil chemicals at 
any time. 
Manufacturers are now testing a number of mercury compounds 
as possible disinfectants and fungicides. Some of these compounds 
do not form Hg-ions in water solutions and therefore are not precipi¬ 
tated by H 2 S. One of the manufacturing companies suggested that 
we ignite the soil, boil gently in acetic acid, filter, and precipitate 
with ammonium sulphide. The presence of iron in the soils caused 
an excessively heavy precipitate which obscured any mercury precipi¬ 
tate that may have been present. It had been suggested that these 
non-ionizing compounds should penetrate better than mercury 
bichloride. Without a usable test for their presence in soil solutions 
the writers have been unable to test this theory. 
io l. O. Overholts of Pennsylvania State College, performed many of the preliminary experiments with 
the formaldehyde tests. 
61911—25f-3 
