the point of view from which the sample is to be taken. If the object is
a general study, having for its aim the determination of the general
composition of the soils of a definite geologic formation, the sample
should be taken in such a way that the different characteristics of the
soil alone should enter into consideration without paying any attention
to its accidental components, which have been determined by local
causes, such as are produced by continued high cultivation, the
application of abundant fertilizers, or the practice of a particular
line of agriculture. The samples of soil therefore, with such an object
in view, should be taken on parts of the earth which are beyond the
reach of the causes mentioned above and which tend to modify the nature
of the primitive soil. In such a case it is the soil which has not been
modified, or better still, virgin soil, such as is found in the
woodlands and prairies, which should be taken for a sample, choosing
those places in which the geologic formation is most perfectly
characterized. In such a case a soil taken in one spot corresponding to
the conditions before mentioned, would be the best for the purposes in
view. The sample would thus represent a true type to be studied, not one
of a mean composition got by taking samples from different localities
and mixing them into a homogeneous parcel. This last method of
proceeding could introduce into the sample earth modified by culture or
by influences purely accidental. However, it would be wise, in a region
characterized by the same geologic formation, to take a certain number
of samples in different localities, and examine singly each one of them
in order to be assured that there is a uniformity of composition in the
whole of the soils.
If, on the contrary, it is the purpose of the investigation to furnish
information to the cultivator concerning the fields which are worked, it
is necessary to approach the problem from a different point of view. In
this case the earth which is under cultivation should be first of all
considered with all the modifications which nature causes or practical
culture has caused, in it. But it often happens that upon the same farm
the natural soil is variable, caused either by the washings from the
adjacent soils, by the accumulation at certain points of deposits formed
from standing water, or from other reasons. In such a case it would be
necessary to take samples from every part of the field which exhibited
any variation from the general type, in order to get a complete mean
sample of the whole. It is necessary to be on guard against making a
mixture of these different lots which would neither represent the
different soils constituting the farm nor their mean composition. It
would be better to examine each of these samples alone and then from
those parts which appear to have a similar composition, to take a
general sample for the mean analysis.
Most often it is necessary to confine our studies to the really
important part of the farm the composition of which would have a
practical interest. The aspect of the spontaneous vegetation in such a
case, will often serve as a guide to determine the parts of the farms
which are similar in nature. The sample should represent the arable
layer, properly so-called, that is, that part of it which is stirred by
the agricultural implements in use and in which the root system of the
plant takes its greatest development and which is the true reservoir of
the fertilizing materials.
When a trench is dug in the soil it is easy to distinguish the arable
layer from the subsoil. In the first place, its color is different,
generally being modified by vegetable débris which forms the supply of
humus. The depth of the arable layer is variable, but it is most
frequently between 200 and 300 millimeters. In the analysis the depth
and layers should be indicated since the chemical composition of the
earth varies according as the sample is taken to a greater or less
depth. As an example of this it may be said that the quantity of
nitrogen decreases in general in proportion as the depth of the layer is
increased. The sample, therefore, should be limited exactly to the
arable layer of soil.
=61. Caldwell=[44] advises that according to the purpose of the analysis
samples be taken:
_a_, from one or from several spots in the field, in order to subject
each sample to a separate analysis; or
_b_, for an average representation of the soil of the whole field; in
this case, several portions of earth are taken from points distributed
in a regular manner over the field, all of which are most carefully
mixed together, and 4–6 kilograms of the mixture, free from any large
stones, are preserved as the average sample.
An excavation in the soil 30–50 centimeters deep, or through to the
subsoil, and 30–50 centimeters square, with one side as nearly vertical
as possible is made and a slice taken from this side of uniform
thickness throughout, weighing 4–5 kilograms. If the subsoil is to be
examined, a sample of it should be taken out in the same manner as
directed for the upper soil, to the depth of about 60 centimeters.
If the character of the soil varies materially in different parts of the
field, samples from several spots should be analyzed separately.
A small portion of the sample should be put at once in a well-stoppered
bottle; the remainder may be allowed to become air-dried, by exposing it
in a thin layer, in summer, to the common temperature in the shade, or,
in winter, to that of a warm room, or a moderately warm drying-chamber,
heated to 30°–40°; in either case it should be carefully protected from
dust.
At the time of taking the sample of the soil, observations should be
made in regard to the following points:
_a._ The geognostic origin of the soil.
_b._ The nature of the underlying strata, to the depth of 1–2 meters, if
practicable.
_c._ The meteorology of the locality, by consulting meteorological
records, if possible; otherwise, by the general opinion of the
neighborhood; in this connection, the height of the locality above the
level of the sea should be noted also.
_d._ The management and rotation of crops in previous years.
_e._ The character of the customary manuring.
_f._ The amount of the crops removed in the preceding year, and, if
possible, the average amount of each of the more important crops yielded
by the field.
_g._ The practical judgment of neighboring farmers in regard to the
field.
Caldwel’s method is practically identical with that of Wolff[45] which
was one of the earliest of the systematic schemes for taking soil
samples.
=62. Wahnschaffe=[46] insists on rather a fuller preliminary statement
to accompany soil samples but gives essentially the method of Wolff with
some unimportant variations which add little to the value of the
process.
=63. Method Of Peligot.=—According to Peligot[47] the taking of samples
of soil of which the physical and chemical properties are to be
determined is a delicate operation.
These samples should represent as nearly as possible both the good and
bad qualities of a soil.
In the field selected are chosen a certain number of places at least
four or five per hectare. The spots selected should have a homogeneous
appearance—resembling as nearly as possible the general aspect of the
field.
By means of a spade a few kilograms of earth are removed to the depth of
the subsoil being careful to include in the sample no accidental
detritus which the upper part of the soil especially may contain.
The samples should be taken immediately after the crop is harvested and
before any fresh fertilizer is applied. The samples are carefully mixed
and placed in a glass bottle or flask.
The sample of subsoil is obtained in the same manner. If the field
presents notable differences in surface or fertility all the samples
taken should be examined separately.
=64. Method of Whitney.=[48]—An ordinary wood auger, 2½ inches in
diameter is so arranged as to admit of additions to the stem to enable
the operator to take samples at different depths. It may be fitted with
a short piece of gas pipe for a handle and the several pieces of which
it is composed may be taken apart and carried in a knapsack.
In taking a soil sample the boring is continued until a change in color
shows that the subsoil has been reached. The auger cuts a very clean
sample save in excessively sandy soil. After the soil sample is secured
the hole is cleaned out and the sample of subsoil taken by the same
instrument. The soil is conveniently preserved in heavy cloth bags of
which the usual size is 6 by 8½ inches. Where larger samples are
required the size of the bag is correspondingly increased. Each bag is
to be tagged or labeled to correspond with the entry in the note book.
Samples to determine the amount of empty space in a soil are taken as
follows: The sampler is a piece of brass cylinder about nine inches long
and about 1½ inches in diameter. A piece of clock spring is soldered in
one end and sharpened to give a good cutting edge. This arrangement
permits the sample to pass into the cylinder without much friction. The
area enclosed by the clock spring is accurately determined and a mark is
placed in the cylinder six inches from the cutting edge. The cylinder is
driven into the soil to a depth of six inches, a steel cap being used to
prevent the hammer from injuring the cylinder. The earth is next removed
from about the cylinder with a trowel, and the separated cylinder of
earth is cut smoothly off by a sharp knife and removed together with its
brass envelope. The sample is taken to the laboratory in a cloth bag,
dried and weighed.
=65. Taking Samples for Moisture Determination.=—A number of brass tubes
is provided nine inches long and ¾ inch in diameter and with a mark six
inches from the bottom.
The tube is pushed down into the soil to the mark and the sample of soil
removed with the tube. There is but little danger of the sample dropping
out of the tube even in sandy soils. When the tube is withdrawn each end
is capped with a rubber finger tip making a perfectly air tight joint.
The tubes containing the samples can be kept several days with no fear
of losing moisture. This method is especially useful in having samples
taken by observers in different localities who can enclose the tubes in
a cloth sack and send them to the laboratory by mail daily or at stated
intervals. A tube of the size given holds about fifty grams of soil.
=66. Taking Samples to Determine the Permeability of Soil to Water or
Air.=—Whitney[49] determines the permeability of soil or subsoil to
water or air in the following manner:
An excavation two feet square and eighteen inches deep is made in the
soil. On one side of this hole the sample of soil or subsoil is taken by
means of a narrow saw blade and a sharp carving knife. The sample of
soil taken should be two inches square and 3½ to 4 inches long.
It is placed in a brass cylinder three inches long and 3¼ inches in
diameter. The open space in the cylinder is filled with paraffin heated
just to its melting point. As the paraffin cools the upper surface
should be kept stirred to prevent the mass when set from receding from
the square column of soil. Care must be taken to keep the paraffin from
the ends of the soil columns and these should be left, as far as
possible in their natural condition.
The rate of percolation of the water may be determined at the time the
sample is taken. For this purpose an additional section of brass tube
two inches deep is secured to the one holding the sample by a rubber
band. An iron rod is driven into the earth carrying a retort stand ring
supporting a funnel filled with fine gravel. The lower end of the soil
column in the brass cylinder is placed on this gravel. Water is next
carefully poured upon the top of the sample of soil being careful not to
disturb the surface. The surface of the sample may be protected with a
little fine sand. The water should be poured on the paraffin thus
affording an additional protection to the soil surface. When the water
begins to drop from the funnel a graduated glass is set under it and the
time required for a given volume to pass through under an initial
pressure of two inches is noted. The volume required represents one inch
in depth over the four square inches of soil surface, _viz._: four cubic
inches.
=67. Sampling of Soil for Staple Crops.=—Some variations from the usual
methods are recommended by Whitney when the samples are taken from
fields growing staple crops.
The immediate object of the work, for which these samples are desired,
is to make a thorough study of the physical and chemical properties of a
number of typical soils adapted to the different staple crops, such as
grass, wheat, truck, and the different types of tobacco. They should be
taken for a careful study of the texture of the soils, the relative
amount and arrangement of sand and clay, the relation of the soils to
moisture and heat, and the ease with which they can maintain a proper
water supply for the different staple crops under existing climatic and
cultural conditions. The ultimate object of such a study is to see how
these conditions can be changed so as to make the soils more productive,
and make them yield a better quality of crop, or to change the
conditions in other soils, which differ from these, so that the culture
of the different staple crops can be extended over wider areas by
improved methods of cultivation and manuring.
The soil selected for sampling for these investigations should be
typical, should represent fairly well a considerable area of land. It
should represent either the very best type of land for the staple crop
or crops of the locality, or the very poorest lands for these same
crops. Both of these extremes are desired for contrast. For example, if
the staple crop of the locality is wheat or a certain type of tobacco,
select the soil best adapted to this staple crop, and another soil, if
possible, in the same locality, representing considerable area of land
upon which this staple crop cannot be successfully grown on account of
the inferior yield, quality, or the time of ripening of the crop. The
soil sampled should be, or should recently have been, under actual
cultivation in the crop or crops best adapted to it, so that the real
agricultural value of the land can be accurately known.
The samples should be taken inside the field, some distance away from
fences, roads, or trees. If there are plants growing in the field, the
sample should be taken about midway between two plants. The samples
should be taken where they will typify fairly well the average soil of
the field and of the large area of land which they are to represent.
The samples are taken in some one of the ways described herein. Each
sample should be carefully labelled at the time of taking. The following
blank form will be found convenient for this purpose:
│Locality:
LABORATORY No.: │
────────────────────────┼──────────────────────────────────────────────
No. of sack: │Description: (virgin or cultivated).
│ (_a_) Natural herbage:
│ (_b_) Crops best adapted to land (grass,
│ wheat, tobacco, truck, barren).
│ „
------------------------│ „
Date: │ „
│ „
------------------------│ „
Collector: │ „
│ „
------------------------│------------------------
Depth of sample: │
(Soil or Subsoil?) │Geologic formation:
... in. to ... inches. │
=68. Method of the Royal Agricultural Society.=[50]—Have a wooden box
made, six inches long and wide, and from nine to twelve inches deep,
according to the depth of soil and subsoil in the field. At one of the
selected places mark out a space of twelve inches square; dig around it
in a slanting direction a trench, so as to leave undisturbed a block of
soil, with its subsoil, from nine to twelve inches deep; trim this block
to make it fit into the wooden box, invert the open box over it, press
down firmly, then pass a spade under the box and lift it up and gently
turn it over.
In the case of very light, sandy, and porous soils, the wooden box may
be at once inverted over the soil and forced down by pressure, and then
dug out.
Proceed in the same way for collecting the samples from all the selected
places in the field, taking care that the subsoil is not mixed with the
surface soil. The former should be sampled separately.
In preparing the plot for the gathering of the sample, take care to have
it lightly scraped so as to remove any débris which may be accidentally
found there.
The different samples thus procured are emptied on a clean, boarded
surface, and thoroughly mixed, so as to incorporate the different
samples of the same field together. The heap is then divided into four
divisions, and the opposite quarters are put aside, taking care to leave
the two remaining ones undisturbed; these are thoroughly mixed together,
the heap divided into quarters, and the opposite ones taken away as
before. This operation of mixing, dividing into quarters and taking away
the opposite quarter is continued until a sample is left weighing about
ten or twelve pounds.
Thus is obtained the average sample of the soil. Of course where only a
single sample is taken from the field this method of quartering is not
resorted to, but the bottom of the box is nailed directly on and sent to
the laboratory, where the soil is to be analyzed.
=69. Grandeau=[51] suggests that in taking soil samples there are two
cases to be considered; first a homogeneous soil and second, a soil
variable in its appearance and composition. First, if the soil is
homogeneous, being of the same geologic formation it will be sufficient
to take a mean sample in accordance with the following directions:
The field is first divided by diagonals or by transverse lines the
direction of which need not be fixed in advance but as inspection of the
form and configuration of the field may indicate. In the ordinary
conditions, of homogeniety (marly, granite, argillaceous or silicious
soils) it will be sufficient to select about five points per hectare
from which the samples are to be taken. These points having been
determined the surface is cleaned in such a way as to remove from it the
detritus which may accidentally cover it; such as dry leaves, fragments
of wood, foreign bodies, etc. The surface having been prepared, (five to
six square meters) a hole is dug four-tenths of a meter long and as wide
as the spade employed. The sides should be as nearly vertical as
possible. As to depth it varies with the usage of the country in regard
to tillage. The layer of arable earth is what in effect properly
constitutes the soil. It ought not to be mixed with any fragments of the
subsoil. When the hole is properly cleaned the samples are secured with
a spade from the sides of the excavations. About five kilograms are
taken. The soil is placed in a proper receptacle as it is removed from
the hole.
This operation is repeated on as many points as may be necessary to
obtain a mean sample of the soil of the whole field.
All the samples are now collected on a table sufficiently large, and
intimately mixed together. Two samples, each of about five kilograms,
are then taken from the mixed material. One sample is immediately placed
in bottles and carefully stoppered and sealed; the other is dried in the
sun or on the hearth of a furnace. When sufficiently dry the second
sample is also placed in bottles and well stoppered. While mixing the
samples, pebbles, etc., of the size of a nut and larger are removed, the
weight of the rejected matter being determined. The nature of the
pebbles should also be noted; whether silicates, limestone, etc.
The sample of subsoil is taken in exactly the same manner, using the
same holes from which the samples of soil were taken. The nature, the
arrangement and the appearance of the strata will indicate the depth to
which the subsoil should be taken. In general, a depth equal to that of
the sample of soil will be sufficient. The depth to which the roots of
cultivated plants reach is also a good indication in taking a sample of
the subsoil. In forests the sample of subsoil should be taken from four
to five-tenths of a meter below the surface.
If the soil in respect of its geologic formation, its fertility or its
physical aspect presents great differences, special samples should be
taken in each part in accordance with the directions given above.
=70. Method of the Official Agricultural Chemists.=—In the directions
given by the Association of Official Agricultural Chemists[52] it is
stated that the soil selected should be as far as possible in its
natural condition, not modified by recent applications of manure, or
changed by the transporting action of water or wind. Surface
accumulations of decaying leaves, etc., should be removed before taking
the sample.
To eliminate accidental variations in the soil, select specimens from
five or six places in the field which seem to be fair averages of the
soil, remove two or three pounds of the soil, taking it down to the
depth of nine or ten inches[53] so as to include the whole depth. Mix
these soils intimately, remove any stones, shake out all roots and
foreign matter, and dry the soil until it-becomes friable.[54] Break
down any lumps in a mortar with a wooden pestle, but avoid pulverizing
any mineral fragments; pass eight to ten pounds of the soil through a
sieve, having circular perforations one twenty-fifth of an inch in
diameter, rejecting all pebbles and materials too coarse to pass through
the sieve. Once more mix intimately the sifted soil. Expose in thin
layers in a warm room till thoroughly air dry (or dry it in an air-bath
at a temperature of 40°), place six to eight pounds in a clean bottle,
with label of locality and date, and cork the bottle containing the
soil, for analysis.
The soil is rapidly dried to arrest nitrification; it is not heated
above 40° lest there should be dissipation of ammonia compounds, or a
change in solubility. The normal limit to which the soil may be heated
in place by the sun’s rays should not be exceeded in preparing a sample
for an agricultural chemical analysis.
The relative amount of fragments too coarse to pass through the sieve
should be made a matter of record. They are soil material, but not yet
soil, so far as agricultural purposes are concerned.
=71. Method of Lawes.=—In a late method of sampling proposed by Sir J.
B. Lawes[55] a steel frame ten by twelve inches, and nine inches deep
open at top and bottom is driven into the earth until its upper edge is
level with the surface of the soil. All above-ground vegetation is then
cut off as closely as possible with scissors. The soil within the frame
is then removed exactly to the depth of the frame, and immediately
weighed. It is then partially dried, and mechanically separated by a
series of sieves, all visible vegetable matter being at the same time
picked out. The stones and roots and the remaining soil are thus
separated, and the determinations of dry matter, nitrogen, etc., are
made in the separated soil after being finely powdered. The loss of
water at each stage of preparation and on drying the samples as analyzed
is also carefully determined. This method, which requires the soil to be
taken to an arbitrary depth of nine inches, could not be used when
samples of strictly arable soil are to be taken.
=72. In taking= a sample by the French commission[56] method it is
necessary to remove from the surface, the living and dead vegetation
which covers the soil. With a spade a square hole is then dug to the
depth of about 500 millimeters; in other words, to a depth considerably
exceeding that of the arable layer. Afterwards on each of the four sides
of the hole there is removed by the spade, a prismatic layer of the
arable portion of a thickness equal to its depth. The samples thus
obtained are united together and carefully mixed for the purpose of
forming a sample for analysis. If there are large stones they are
removed by hand and their proportion by weight determined.
In all cases it would prove useful to take a sample of the subsoil which
is far from playing a secondary rôle. The rootlets bury themselves
deeply in it and seek there a part of their nourishment. The subsoil,
therefore, furnishes an important addition to the alimentation of the
plants. For taking a sample of the subsoil a ditch is dug of sufficient
depth, say one meter, and the arable soil carefully removed from the top
portion. Afterwards pieces are taken from the four sides of the hole at
variable depths, which should always be indicated, and which should
extend in general, from six to eight-tenths of a meter below the arable
soil since it is demonstrated that the roots of nearly all plants go at
least to this depth. The analysis of the subsoil, however, is less
important than that of the soil, properly so-called, because the
agronomist does not act directly upon it and takes no thought of
modifying it and enriching it as he does the layer of arable soil. But
the composition of the subsoil is a source of information capable of
explaining certain cultural results and capable sometimes, of leading to
the correct way of improving the soil, as in cases where the subsoil can
be advantageously mixed with the superficial layer.
=73. Wolff=[57] suggests that a hole thirty centimeters square be dug
perpendicularly and a section from one of the sides taken for the
sample. To the depth of thirty centimeters the sample shall be taken as
soil and to the additional depth of thirty centimeters as subsoil. The
thickness of the section taken may vary according to the quantity of the
sample desired. For analytical purposes, five kilograms will usually be
sufficient. When culture experiments are also contemplated a larger
quantity will be required.
=74. Method of Wahnschaffe.=—The method of sampling advised by
Wahnschaffe[58] is but little different from that of Wolff already
mentioned.