204 QUANTITATIVE AGRICULTURAL ANALYSIS
Veitch Method.1—In the Veitch method for the determination
of soil acidity a measured quantity of lime water solution of
known concentration is evaporated to dryness with a definite
amount of soil. The mass is then extracted with distilled water,
phenolphthalein is added and the solution is concentrated by
boiling. If the quantity of calcium hydroxide added was more
than sufficient to neutralize soil acids, an indication of this will
be given by a pink color from the phenolphthalein. By this
method there is probably some error due to a combination of
calcium hydroxide with organic matter and possibly with carbon
dioxide from the air.
Determination of Lime Requirement of Soil: Veitch Method.—Weigh five
portions of 10 gm each of the soil into 8-cm porcelain evaporating dishes.
Add fiftieth-normal calcium hydroxide solution in such amounts that it will
range from 2 cc below to 2 cc above the probable amount of calcium
hydroxide needed, making a difference of 1 cc in the volume of calcium
hydroxide for each pair of consecutive members of the series. A series
extending over 5 to 10 cc of solution may be used as a beginning. Evapo-
rate all to dryness over the steam bath and immediately take up the residues
with distilled water and transfer to 300-cc flasks, using 150 cc of water,
previously freed from carbon dioxide by boiling for several minutes in an
open beaker or dish. Shake well, stopper and let stand over night, then
pipette 50 cc of the clear liquid from each flask into Pyrex beakers. Add a
drop of phenolphthalein and heat to boiling, continuing the boiling until
two-thirds of the liquid has been boiled away. Note in what beakers, if
any, the liquid has turned pink. Repeat, using a narrower series whose
limits are indicated by the results on the first series. The least volume of
calcium hydroxide solution required to cause a pink tint is equivalent to the
lime requirement of 10 gm of soil. Calculate the pounds of calcium carbon-
ate needed on the basis of 2,000,000 Ib. of soil per acre.
The Tniog Method.2—If barium chloride and zinc sulphide are
added to an acid soil, evolution of hydrogen sulphide takes place:
2RCOOH + BaCl2 -» (RCOO)2Ba + 2HC1, (1)
ZnS + 2HC1 -» H2S + £nC!2. (2)
This gas coming in contact with lead acetate paper produces a
degree of blackening somewhat in proportion to the amount of
acid present.
H2S + Pb(C2H302)2 -> PbS + 2HC2H302. (3)
1 J. Am. Chem. Soc., 26, 261 (1904).
2 Wis. Exp. Sta. Bull, 249 (1915).