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.