266

QUANTITATIVE AGRICULTURAL ANALYSIS

PH equals about 7.2, this being completed at about 7.9. It is
evident that with such a soil a large amount of limestone would
have to be applied to precipitate all of the manganese, and in
some instances this cost would be prohibitive.

Determination of Soil Acidity: Potassium Thiocyanate Method.1—Prepare
the following reagents:

(a) Potassium Thiocyanate Solution.—Prepare a 5-per cent solution in
95-per cent ethyl or methyl alcohol. This solution should become slightly
pink (Pn = 5.4) upon the addition of methyl red. If neces-
sary, add very dilute potassium hydroxide or hydrochloric
acid, drop by drop, until this color is obtained with a few
drops added to methyl red on a test plate.

(b) Alcoholic Solution of Potassium Hydroxide.—Prepare
a tenth-normal alcoholic solution of potassium hydroxide
by dissolving the base in 95-per cent ethyl or methyl alcohol.
Titrate against (c), using methyl red.

(c) Alcoholic Solution of Hydrochloric Acid.—Prepare a
tenth-normal alcoholic solution of hydrochloric acid by
diluting concentrated acid with 100 volumes of 95-per cent
ethyl or methyl alcohol. Standardize against sodium car-
bonate, first dissolving the weighed salt in a small amount
of water. See page 83.

Place 50 gm (25 gm of muck) of 10-mesh air-dried soil in
a 100-cc glass-stoppered cylinder or in the lower chamber of
the specially designed glass tube shown in Fig. 58. Add 30
cc (50 cc for muck) of potassium thiocyanate solution.
Stopper the cylinder and agitate for two minutes. Place
in an upright position, allow to settle for several minutes and
note the color of the supernatant liquid. If the solution is
pink or red, add from the upper burette a few tenths
of a cubic centimeter at a time (depending upon the
color) of tenth-normal alcoholic solution of potassium
hydroxide. Shake well after each addition and allow several
minutes to settle. Continue the addition until the red or
pink color has just disappeared. Let stand fifteen hours
and add more base, if necessary, to remove any pink which
may have developed. If too much base has been added
titrate back to a faint pink color using tenth-normal alco-
holic acid. Note the volume of tenth-normal base required
and calculate the pounds of calcium carbonate required
to correct the acidity of the soil, on a basis of 2,000,000 Ib. of
ordinary soil or 1,000,000 Ib. of muck soil per acre.
The time required for complete development of color in the thiocyanate
solution may be shortened by use of the mixing machine.

1CARB, J. Ind Eng. Chetn., 13, 931 (1921).

ISOcc

FIG. 58.—Soil
acidity
burette.