268

QUANTITATIVE AGRICULTURAL ANALYSIS

year is of interest and importance. Various weak acids, which
imitate the action of the plant roots, have been used for extract-
ing available plant food. Dyer1 has shown that root acidity
(expressed as citric acid) varies from 0.34 to 3.4 per cent of the
weight of the plant. He found the average acidity of one
hundred plants (root and top) to be about equal to that of 1-
per cent citric acid and so used this acid for soil extraction.
Fifth-normal nitric and oxalic acids are other solutions that have
been used for this purpose. Fifth-normal nitric acid has given
best results2 in field tests and this has been quite widely adopted.
The amount of acid capable of being neutralized by materials
already present in the soil also is a factor of importance in
fertility work. This is estimated by titrating the solution after
the extraction has been completed. The amount of acid con-
sumed depends considerably upon whether the soil is calcareous,
it being much greater in this case.
Flocculation and Deflocculation of Clay.—When "silt" soil
is suspended in water it may be easily flocculated by a calcium
salt, such as calcium nitrate. However, if calcium hydroxide is
added so that the solution becomes basic, flocculation is more
difficult. A clay responds in just the opposite manner, being
easily precipitated from suspension by a basic solution.
Determination of Comparative Degree of Flocculation and Defloccula-
tion.—Place about 3 gm (not accurately weighed) of a clay soil in a mortar
and add sufficient water to make a thin paste when rubbed, then dilute to
one liter and mix. Repeat this process, using a "silt" soil. Pipette 25 cc
of each turbid liquid into each of nine test tubes and add each of the following
solutions in order to test its power to flocculate or deflocculate clay and silt
soils. The solutions should have approximately the concentrations indi-
cated but they need not be accurately standardized.
(1) Use as a control—water and soil suspension only.
(2) 5 cc of tenth-normal sodium chloride.
(3) 5 cc of tenth-normal monosodium phosphate.
(4) 5 cc of tenth-normal sodium hydroxide.
(5) 5 cc of tenth-normal hydrochloric acid.
(6) 5 cc of tenth-normal ammonium sulphate.
(7) 5 cc of tenth-normal monocalcium phosphate.
(8) 10 cc of twentieth-normal calcium hydroxide.
(9) 5 cc of tenth-normal calcium nitrate.
1 J. Chem. Soc., 65, J15 (1894).
2 Ohio Exp. Sta. Bull, 261 (1913).