TESTS FOR ACIDITY
_The Litmus Paper Test._ A method of testing soils for acidity, which
has been in use for many years, is the simple litmus paper method.
Because of its simplicity and fair degree of accuracy, the litmus paper
test is still used to a considerable extent in estimating the degree of
acidity of certain soils. The best manner of using litmus is to place a
strip of the blue paper in the bottom of a glass saucer, covering it
with filter paper or other paper which is neutral--that is, paper which
is neither acid nor alkaline.
A small quantity of the soil to be tested is moistened with rain or
distilled water and placed on this paper. If the acid is present the
blue paper will be changed to a reddish color, varying in intensity
according to the degree of acidity in the soil. Two objections to the
use of litmus paper are to be noted: One of these is that the red color
may be produced by carbonic acid gas without a trace of more powerful
acids being present, and this may give a wrong impression to the
operator. Another objection to the use of litmus is that the degree of
acidity is not accurately indicated, and therefore the farmer is
sometimes at a loss to know just how much lime should be applied to make
soil conditions favorable for growing crops.
_A More Accurate Method._ Within the last few years improved methods for
determining the presence of acidity in soil have been developed. Some of
these are suitable only for the chemist with his complete laboratory
equipment, while others are more simple and can be used by anyone
willing to exercise reasonable care.
One of the simplest and most accurate tests to date is that devised by
Professor E. Truog of the agricultural experiment station of the
University of Wisconsin. This test not only detects positively the
presence of soil acidity, but also gives definite information as to the
degree of acidity. The test is based upon the principle that when zinc
sulfid comes in contact with the acid, hydrogen sulfid gas is formed,
and when this gas comes in contact with lead acetate, lead sulfid, a
black chemical, is formed.
The method of making this test is simple, and consists in placing a
measured quantity of soil in a flask, to which is added a solution
composed of 20% calcium chlorid and 2% zinc sulfid. The mixture of soil
and chemical solution is heated to the boiling point by means of an
alcohol lamp, and the boiling continued for a minute for the purpose of
driving off the carbonic acid gas, which is liberated first. The boiling
is continued and a piece of moistened paper, previously impregnated with
lead acetate, is placed over the mouth of the flask. If the soil
contains acid, a chemical reaction occurs between it and zinc sulfid,
and hydrogen sulfid gas is liberated. The quantity of acidity in the
soil determines the quantity of gas which comes in contact with the lead
acetate paper, and this determines the depth of color produced on the
paper. A slight brownish color indicates the presence of very little
acidity, while an intense black signifies the presence of injurious
amounts of acidity. There are various degrees of coloration between
these two extremes, and each gives an accurate indication as to the
quantity of lime required to correct the acidity.
This test is simple and inexpensive, and at the present time most county
agent offices are equipped with this apparatus or a similar one for
testing soils for farmers. Some newer methods are being devised, and
doubtless this method will be improved upon as time passes, but the
Truog test has qualities of accuracy and simplicity which will always
make it valuable.