234 QUANTITATIVE AGE/CULTURAL
Choosing Samples.—In choosing soil samples it is very important to
secure representative ones. The sampling should be done when th.e ground.
is dry enough to plow. An area should be selected such that the soil is
typical with respect to texture and color. Note should be made of any
available information concerning the geology of the area, original timber
of the land, the present productivity, or any peculiarities in location which
may aid in interpreting its analysis. The surface accumulations of such.
materials as decaying grass should be removed and the borings for samples
then made with a soil auger or other soil tube. Composite samples are
taken from different depths as follows: (a) surface to 6 in., (&) 6 to 2O in., (c)
20 to 40 in. For each depth ten to fifteen borings are taken and well mixed.
About a pint of soil from each depth should finally be preserved. Sample
(c) need not be mixed with as great care as are samples (a) and (b) since it
is not usually taken for analytical purposes but for obtaining some insight
into the physical nature of the subsoil, drought resistance and drainage
depending to some extent upon the nature of substrata. The borings are
placed in. clean cloth sacks in the field and immediately sent to the laboratory.
Here they are dried and later ground for analysis.
Preparation of Samples.—Spread the samples on paper or in shallow pans
in a dry place, in clean air, and allow to remain until apparently dry. Pul-
verize lumps and divide each sample into two fractions by use of a 4-mesh
sieve. The stone remaining in the sieve is weighed and its per cent of
the total is calculated. Grind the finer soil portion in a porcelain pebble
mill or other pulverizer until it will pass a 40-mesh sieve. Mix fhoroughly
and then grind about 100 gm of this sample until it will pass a 100-mesh
sieve. Riffles of different sizes may be used for .sampling, or rolling on
paper or oilcloth may be employed. (See the discussion of sampling,
Part I, pages 17 to 22.) The samples should be placed in stoppered bottles
and carefully labeled.
Moisture.—The proportion of moisture in air-dried soil
depends largely upon the proportion of organic matter. The
water-holding power of soil is of great significance from a, prac-
tical standpoint.
Determination of Moisture.—Weigh accurately 5 gm of finely pulverized
100-mesh soil into a flat porcelain crucible about 4 cm in diaraeter and pro-
Tided witli a glass cover. Remove the cover and dry the sample at 110°
for five hours. Cover and cool the crucible in a desiccator and then ^weigh.
Preserve the dried sample for the determination of volatile matter. Calcu-
late the per cent of moisture in the prepared soil.
Optimum Moisture of Soils.—The water-holding capacity of a
soil depends upon its content of organic matter and its structure.
The amount of water which just permits a soil to crumble is
considered the optimum. This is about one-third of its total
water-holding capacity.