to the horizon. The soil which is removed is thrown on a cloth and
carefully mixed. From the whole mass a convenient amount is next removed
care being taken not to include any roots. In a similar manner it is
directed to proceed for the sample of subsoil. At first the subsoil
should be removed to a depth of two to three decimeters. The number and
depth of subsequent samples will depend chiefly upon the character of
the soil. Where samples are taken to the depth of two meters the use of
a post-hole auger is recommended.
The samples taken should not be too small. In general from two to three
kilograms should remain after all preliminary sampling is finished.
=75. Method of König.=—The directions given by König[59] for taking soil
samples are almost identical with those prescribed by Wahnschaffe and do
not require any further illustration.
=76. Special Instruments Employed in Taking Samples.=—In general a sharp
spade or post-hole auger is quite sufficient for all ordinary sampling
but for certain special purposes other apparatus may be used.
The instrument which is used by King[60] consists of a thin metal tube
of a size and length suited to the special object in view, provided with
a point which enables it to cut a core of soil smaller than the internal
bore of the tube and at the same time make a hole in the ground larger
than its outside diameter. Its construction is shown in figure 11, in
which A B represent a soil tube intended to take samples down to a depth
of four feet. A′ is a cross-section of the cutting end of the tube,
which is made by soldering a heavy tin collar, about three inches wide,
to the outside of a large tube allowing its lower end to project about
one-half an inch. Into this collar a second one is soldered with one
edge projecting about one-quarter of an inch and the other abutting
directly against the end of the soil tube. Still inside of this collar
is a third about one-half an inch wide which projects beyond the second
and forms the cutting edge of the instrument.
[Illustration:
FIGURE 11.
]
The construction of the head of the tube is shown at B′. It is formed by
turning a flange on the upper end of the tube and then wrapping it
closely with thick wire for a distance of about three inches, the wire
being securely fixed by soldering. The soil tube should be of as light
weight as possible not to buckle when being forced into the ground, and
the cutting edge thin. The brass tubing used by gas fitters in covering
their pipes has been found very satisfactory for ordinary sampling. With
a one inch soil tube four feet long it is possible to get a clear
continuous sample of soil to that depth by simply forcing the tube into
the ground with the hand and withdrawing it, or the sample may be taken
in sections of any intermediate length. Later in the season when the
soil becomes dryer it is necessary to use a heavy wooden mallet to force
the tube, and this should be done with light blows.
The closeness with which it is possible to duplicate the samples in
weight by this method will be seen below, where from each of four
localities three samples were taken from the surface to a depth of four
feet.
SHOWING VARIATIONS IN THE DRY WEIGHT OF TRIPLICATE SAMPLES OF SOIL.
─────────────────────────┬──────────┬──────────┬──────────
│ A. │ B. │ C.
─────────────────────────┼──────────┼──────────┼──────────
I. Surface to four feet│716.6 gms.│715.5 gms.│710.3 gms.
II. Surface to four feet│715.4 gms.│687.1 gms.│731.2 gms.
III. Surface to four feet│654.0 gms.│688.3 gms.│709.0 gms.
IV. Surface to four feet│714.0 gms.│687.8 gms.│719.3 gms.
These four series of samples were taken at the four corners of a square
twelve feet on a side and serve to show how much samples may vary in
that distance. The large difference shown in III, A is due to the fact
that the soil tube penetrated a hole left by the decay of a rather large
root as shown by the bark in the sample.
=77. Auger for Taking Samples.=—It has already been said that the
ordinary auger used for boring fence post-holes may be used to advantage
in taking soil samples. Large wood augers can also be used to advantage
for the same purpose. For special purposes, however, other forms of
augers may be used.
Norwacki and Borchardt[61] have described a new auger for taking samples
of soil for analytical purposes.
[Illustration:
FIGURE 12.
]
In figure 12, A, B and C show the general exterior and interior form of
the instrument. The handle is hollow and made of iron gas pipe covered
with leather. On the inside of this, in the middle, is fixed a wooden
plug a, which leaves two compartments, one in each end for holding the
brass plug bb,’ and the wicker lubricating wad cc.’ The stem of the
auger a, is heavy and made of eight-sided steel and the under end is
strengthened with a heavy casting fitting into the auger guide g g. The
end of the auger I I′ is triangular and hardened. The auger guide g g,
is made out of a single piece of drawn steel tubing. Above it is
strengthened by a ring-shaped piece of iron or copper and its lower end
is furnished with saw teeth as shown in K and is hardened. The fixing
key e, is bent in the form of a hook and can be passed through the two
holes o o, of the auger stem and through the one hole o′ in the
strengthened part of the auger guide. It permits the auger guide to be
fixed upon the auger stem in two different positions, higher and lower.
On one end it is cut squarely across and on the other provided with a
conical hole drilled into it. It fits on the one hand exactly in the
auger guide and on the other loosely plays in the cavity of the handle
at b, designed to hold it when not in use. The cap d′ is made of heavy
sheet brass and is fastened upon the end of the handle at c c′ after the
manner of a bayonet. The wicker cartridge is made of rolled and sewed
wicker-work. At the upper end it is provided with a metallic button and
before use it is saturated with paraffin oil. It fits on the one side
firmly in the auger guide and on the other in the cavity, of the handle
c where it is kept when not in use. The union h is made of a brass tube
which below is closed with a piece of solid brass upon the inside of
which a hole is bored. In this hole rests the end of the auger stem when
the union is placed firmly upon the auger guide.
The auger is placed together as is shown in A B, the union h is taken
off and it is driven with gentle blows, turning it back and forth, to
the proper depth into the soil. After the key is loosened the auger is
lifted high enough so that the second hole appears and then it is fixed
in position by the key. Then the boring is continued, turning the auger
to the right, by which the auger, eating its way with its saw teeth,
presses deeper into the ground and withdraws the material for analysis.
After the auger guide has been filled through any desired length, say
five to ten centimeters with the sample of soil, the whole auger is
drawn out of the soil, the key removed, the auger stem withdrawn from
the auger guide, the apparatus opened by turning the bayonet fastening
of the stopper on the handle, the brass plug placed in the end and then
with the smooth part forward, from above, it is allowed to fall into the
auger guide until it reaches the soil. The auger stem is then put back,
the point of it fitting into the hole of the plug and the sample of soil
shoved out of the auger guide. The auger guide is again fixed on the
auger stem by the key and then the apparatus is ready for a second
operation. When the borings cease the wicker cartridge is drawn out of
the handle and shoved, the soft end forward, from above, into the auger
guide and the brass plug after it and pushed through with the auger
stem. By this process the wicker cartridge gives up a sufficient amount
of paraffin oil to completely grease the inside of the auger guide and
to protect it from rust. After use the instrument should be cleaned on
the outside by means of a cloth, the plug and wicker wad replaced in
their proper positions, the cap fixed on the handle and the union on the
point of the instrument.
The length of the whole apparatus may reach one meter or more; the
internal diameter sixteen millimeters. The apparatus weighs with a
length of one meter, together with all its belongings, about two
kilograms. For the investigation of peat and muck soils as well as sand,
instead of the steel auger guide one of brass or copper can be used. For
this purpose the length of the apparatus may reach three to four meters.
In comparison with other apparatus which are used for taking samples, it
appears without doubt that with the one just described a better and less
mixed portion of the soil can be obtained at great depths. The apparatus
is said to have many advantages over a similar one known as Fraenkel’s,
and is much more easy to clean. The advantages of the apparatus are said
to be the following: The farmer with this piece of apparatus in a short
time can go over his whole farm taking samples to the depth of ninety
centimeters since a single boring does not take more than one minute.
Geologists and others interested in the soil at greater depths can use
an apparatus three to four meters in length and obtain unmixed samples
from these lower depths. These are also interesting from a bacteriologic
point of view. The entire apparatus is especially valuable for the
investigation of the lower parts of peat and muck soils. The apparatus
has been tried in the collection of samples for the laboratory of the
Department of Agriculture and is too complicated to be recommended for
ordinary use. When however samples are to be taken at great depths as in
peat soils it is highly satisfactory.
=78. Soil Sampling= depends for its success more on the judgment and
knowledge of the collector than on the method employed and the apparatus
used. One skilled in the art and having correct knowledge of the purpose
of the work will be able to get a fair sample with a splinter or a
jack-knife while another with the most elaborate outfit might fail
entirely in collecting anything of representative value.
There are some special kinds of soil sampling, however, which cannot be
left to the method of the individual and it is believed that with the
descriptions given above nearly all purposes for which samples are
desired may be served.
For the study of nitrifying organisms, however, special precautions are
required and these will be noted in a more appropriate place.
In taking samples for moisture determinations the method of Whitney is
recommended as the best. For the general physical and chemical
analytical work the standard methods are all essentially the same. The
principles laid down by Hilgard will be found a sufficient guide in most
cases.
TREATMENT OF SAMPLE IN THE LABORATORY.
=79. The Sample=, or mixed sample, taken by one of the methods above
described, is placed on a hard smooth board, broken up by gentle
pressure into as fine particles as possible and all pieces of stone and
gravel carefully removed and weighed; all roots, particles of vegetable
matter, worms, etc., are also to be weighed and thrown out. This can be
done very well by using a sieve of from one to two millimeter mesh. Care
should be taken that the soil be made to pass through, which can be
accomplished by subjecting the lumps to renewed pressure with a
rubber-tipped pestle. In the above operation the soil should be dry
enough to prevent sticking. The relative weights of the pebbles, roots,
etc., and the soil should be determined.
=80. Order of Preliminary Examination.=—Hilgard[62] commences the
examination of a soil sample by washing about ten grams of it into a
beaker with a water current of definite velocity, stirring meanwhile
actively the part carried into the vessel. The residue not carried by
the current is examined macro- and microscopically to determine the
minerals which may be present, and the condition in which the fragments
exist—whether sharp or rounded edges, etc.
This examination will give some general idea of the parent rocks from
which the sample has been derived and of the distance the particles have
been transported. Next follows the hand test, _viz._, rubbing the soil
between the thumb and fingers first in the dry state and afterwards
kneading it with water and observing its plasticity. Following this
should come a test of the relations of the sample to water, _viz._, its
capacity for absorbing and retaining moisture. Finally the separation of
the soil into particles of definite hydraulic value and a chemical
examination of the different classes of soil concludes the analytical
work.
=81. Air Drying.=—The sifted soil should be thoroughly mixed and about
one kilogram spread on paper and left for several days exposed in a room
with free circulation of air and without artificial heat. The part of
the sample to be used for the determination of nitrates should be dried
more quickly as described in another place. The sample is then placed in
a clean, dry glass bottle, corked, sealed, and labeled. The label or
note book should indicate the locality where the sample was taken, the
kind of soil, the number of places sampled, and other information
necessary to proper description and identification.
=82. Caldwell=[63] directs that having taken the sample to the
laboratory, the stones and larger pebbles should be separated from the
finer parts by the hand, or by sifting with a very coarse sieve, and
examined with reference to their mineralogical character, weight and
size, making note, in this last respect, of the number that are as large
as the fist or larger, the number as large as an egg, a walnut,
hazel-nut, and pea, or give the percentage of each by weight.
Pulverize the air-dried soil in a mortar with a wooden pestle, and
separate the fine earth by a sieve with meshes three millimeters wide;
this sieve should have a tightly fitting cover of sheepskin stretched
over a loop, and it should be covered in the same manner underneath, so
that no dust can escape during the process of sifting.
Wash the pebbles and vegetable fibers remaining on the sieve with water,
dry and weigh the residue; the water with which this gravel was washed
should be evaporated to dryness at a temperature not exceeding 50°
towards the close of the evaporation, and the residue mixed with what
passed through the dry sieve.
The sifted fine earth is reserved for all the processes hereinafter
described, and is kept in well-stoppered bottles, marked air-dried fine
earth. The sieve mentioned above is too coarse for the more modern
methods of analysis.
=83. Wolff=[64] directs that the air-dried earth (in summer dried in
thin layers at room temperature, in winter in ovens at 30° to 50°) be
freed from all stones, the latter washed, dried, and weighed. The soil
is next passed through a three millimeter mesh sieve, the residual
pebbles and fiber washed, dried and weighed. The fine earth passing the
sieve is used for all subsequent examinations. It is air-dried at
moderate temperatures and preserved in stoppered glass vessels.
=84. The French=[65] commission calls especial attention to the method
of subsampling, and prescribes that the sample of earth which has been
taken in the manner indicated, and of which the weight should be greater
as the material is less homogeneous should not be analyzed as a whole.
It should be divided into two parts. The first includes the finer
particles constituting the earth, properly so-called, with the elements
which alone enter into play in vegetable nutrition and on which it is
necessary to carry out the analysis. The second embraces the coarser
particles to which only a superficial examination should be given and
which may have a certain importance from a physical point of view but
which cannot take any part from a chemical point of view, in the
nutrition of plants. It is, however, useful to examine its mineralogical
constitution and to look for the useful elements such as lime, potash,
etc., which it may be able to furnish to the earth, and in proportion as
it is decomposed, finer particles which may be useful in plant
nutrition.
How are we to distinguish between the fine and coarse elements? All
grades of fineness are observed in the soil, from the particles of
hydrated silica so small that with the largest magnifying power of the
microscope it is scarcely possible to distinguish them, up to grains of
sand which are of palpable size and visible to the naked eye, and
extending to pebbles of varying sizes. All intermediate stages are found
between these and if it should be asked what is the precise limit at
which it is necessary to stop in distinguishing the fine from the coarse
elements of the soil, the answer is that this can only be determined by
a common understanding among analysts. In general, it may be said, that
the mark of distinction should be the separation which can be secured
with a sieve having ten meshes per centimeter.
=85. Loose Soils.=—Having agreed upon a sieve of the above size, the
process of separation in loose soils is as follows: The earth is exposed
to the air and when the touch shows that it is sufficiently dry the
conglomerated particles should be simply divided without breaking the
rocky material which exists in a state of undivided fragments. There are
some special precautions to be taken. Rubbing in a mortar must be
forbidden since it reduces the earth to particles which are unnatural in
size, by securing the breaking up of the fragments consisting of the
débris of rocks. When it is possible the earth should be rubbed simply
in the hand and after having separated that which passes the sieve, the
large particles which have not passed should be again rubbed with the
hand, until all the particles which can be loosened by this simple
treatment have passed the sieve. The separation should be as complete as
possible in order that a sample of the particles passing the sieve
should represent as nearly as possible, a correct sample of the fine
particles of the soil.
In regard to the pebbles, they should be washed with water upon the
sieve in order to carry through the last of the particles of earth
adhering to them. They are then dried and their weight taken. The fine
part of the earth is also weighed. On an aliquot part, say 100 grams,
the moisture is determined and then by simple calculation the whole
sample of the air-dry soil can be calculated to the dry state. The
sample is then placed in a glass flask.
The pebbles are examined with a view of determining their mineralogical
constitution; as for instance, on being touched with a little
hydrochloric acid it can be determined whether or not they are carbonate
of lime. The nature of the rock from which they have been derived is
often to be determined by a simple inspection.
=86. Compact Soils.=—If the soils are not sufficiently loose to be
treated as before described, it is necessary to have recourse to other
means of division, which should not, however, be sufficiently energetic
to reduce the rocky elements to fine particles. For this purpose the
earth may be broken by means of a wooden mallet, striking it lightly and
separating the fine elements from time to time by sifting. A wooden
roller may also be used with a little pressure, for breaking up the
particles or a roller made out of a large glass bottle. These methods
will permit of a sufficiently fine division of the soil without breaking
up any of the pebbles. Sometimes, however, a soil can not be broken up
by such treatment. It is then necessary to have recourse to the
following process: The soil is thoroughly moistened and afterwards
rubbed up with water. The paste which is thus formed, is poured upon the
sieve and washed with a stream of water until all the fine particles are
removed. The wash water and the fine particles are left standing until
the silt is thoroughly deposited when the supernatant water is poured
off and the deposited moist earth is transferred into a large dish and
dried on a sand or water-bath. In this way a firm paste is formed which
can be worked up with the hand until rendered homogeneous and afterwards
an aliquot portion be taken to determine moisture.
=87. Method of Peligot.=—The method recommended by Peligot[66] for the
preparatory treatment of the sample is essentially that already
described. The sample is at first dried in the air and then in an oven
at 120°. When dry and friable 100 grams are placed in a mortar and
rubbed with a wooden pestle. It is then passed through a sieve of ten
meshes per centimeter. The largest particles which remain in the sieve
should have about the dimensions of a pin’s head. The stones are
separated by hand. They should be shaken with water in order to detach
any pulverulent particles adhering thereto. The turbid water resulting
from this treatment is added to that which is used in separating the
sand from the impalpable part of the soil.
=88. Wahnschaffe= prescribes[67] in the further preparation of the
sample for analysis that the coarse pieces up to the size of a walnut be
separated in the field where the sample is taken and their relative
weight and mineralogical character determined. The soil sample is then
to be placed in linen or strong paper bags and carefully labelled. In
order to avoid any danger of loss of label the description or number of
the sample should be put on the cloth or paper directly.
The sample when brought to the laboratory should be spread out to dry,
in a room free of dust. In the winter the room should be heated to the
usual temperature. The air drying should continue until there is no
sensible loss of weight. The samples then are to be placed in dry,
glass-stoppered glass bottles where they are kept until ready for
examination. This method of keeping the samples avoids contact with
ammonia or acid fumes with which a laboratory is often contaminated.
=89. The Swedish= chemists[68] direct that samples which are to be used
for chemical examination in the manner described below, are most
conveniently brought to such a condition of looseness and humidity that
the soil feels moist when pressed between the fingers without, however,
sticking to the skin. To prepare the sample in this manner, spread it in
a large porcelain dish or on a glass plate in a place where it is not
reached by the laboratory atmosphere; stir it frequently till it assumes
the mentioned humidity (if the sample when sent is too dry, moisten it
with distilled water till its condition is as indicated); then pulverize
carefully between the fingers and finally sift through a sieve with five
millimeter holes. In this way free the sample from stones, undecayed
roots and similar parts of plants, pieces of wood, and other matter
strange to the soil, which remain on the sieve; mix the sample carefully
and put it into a glass bottle provided with a stopper well ground in;
keep it in a cool place. Samples prepared in this way will usually
contain 20–30 per cent moisture; boggy soils 60–80 per cent and peat
soils 50 per cent.
=90. Petermann=[69] follows the method below in preparing samples of
soil for analysis.
The soil is gently broken up by a soft pestle and all débris if of
organic nature, cut fine with scissors. About 2500 grams of this soil
are passed through a one millimeter mesh sieve. The organic débris is
removed by forceps, washed free of adhering earth dried at 120° and
weighed. The nature of the organic débris should be noted as carefully
as possible.
The pebbles and mineral débris not passing the sieve are worked in a
large quantity of water by decantation. They are also dried at 120° and
weighed. This débris is examined mineralogically and thus some idea of
the origin of the soil obtained.
=91. The= various methods for the preliminary treatment as practiced by
the best authorities have been somewhat fully set forth in the foregoing
résumé. The common object of all these procedures is to get the soil
into a proper shape for further physical and chemical examination and to
determine the comparative weights of foreign bodies contained therein.