FERTILIZERS

291

in 400 cc of water. Mix the solutions, dilute to 1000 cc and mix. To 100 cc
of this solution add 65 cc of distilled water and 5 cc of glacial acetic acid,
mix and allow to stand over night. This diluted solution is unstable and it
should not be kept for use more than five days.

Measure 17 cc of solution (6) into each tube, the temperature being not
lower than 22°. Be sure that the graduated stems are filled, with entire

absence of air bubbles. To one of the tubes add 5 cc of .___________,

solution (a) and to each of the others 5 cc of the diluted
sample. Whirl immediately for one minute at the rated
speed for the centrifuge that is being used. Remove the
tubes and tap those in which the upper surface of the
column of precipitate is not practically plane. Whirl
again for 15 seconds. The reading of the precipitate in
the tubes containing the sample solutions should be
within 5 divisions (either way) of that of the standard.
If this is not the case, repeat the experiment, using more
concentrated or more dilute solutions, as indicated.

From the relative volumes of the precipitates and the
known potassium content of the standard solution, calcu-
late the per cent of potassium (or of potassium oxide) in
the sample.

Methods of Pot and Field Culture.—From
analyses alone it is difficult to foretell just what
will be the response of a plant to any given ap-
plication of fertilizer to a soil. The great variety
of soil components, including toxic substances
often contained in them, is responsible for this,
and it is very desirable that pot and field tests
be conducted for the purpose of gaining more
information as to the needs of the soil for the
growth of any particular crop. This is analogous
to conducting feeding experiments with animals
for testing the degree of utilization and the physiological effects
of the feeds.

Much valuable information has been gained through sand and
water culture experiments, in which solutions of certain com-
pounds are added. Reference may be made to the experiments
of Knop,1 Shives2 and Tottingham.3

lLandw. Vers. Sta., 7, 93 (1865).

2 N. J. Exp. Sta. Bull, 319 (1917).

3 J. Am. Soc. Agron., 2, 1 (1919).

FIG. 60.—
Graduated tube
for determina-
tion of potas-
sium by the
Sherrill centrif-
ugal method.