Examination of soil samples

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1911

iBsued May 9, 1011.

United States Department of Agriculture,

BUREAU OF SOILS— CIECULAR No. 26.

Milton Whitney, Chief.

EXAMINATION OF SOIL SAMPLES.

Several hundred letters are received annually by the Bureau of
Soils asking for more or less specific information regarding samples
of soils sent in for examination. A review of this correspondence
shows that, while all of the States and Territories are represented,
about half of the requests come from New York, Pennsylvania,
Texas, Virginia, California, Illinois, Maryland, New Jersey, Wash-
ington, and Florida, in about the order named.

It appears that in about one-third of the cases submitted for an
opinion the requests are for advice as to crop adaptation, about one-
third for information regarding fertilizer requirements, and one-
third for a definite chemical examination to determine the amount of
some one or more of the mineral elements — including potash, phos-
phoric acid, calcium, and magnesium — of nitrates, of alkali, and of
the lime requirement. About 1 per cent of the requests are for spe-
cific information requiring a special laboratory examination for the
purpose of determining harmful constituents, usually suspected to
be of organic nature.

In the event that the examinations show obscure conditions, and
especially if they indicate the presence of deleterious substances of
organic nature, and with all samples submitted for the specific pur-
pose of determining harmful organic constituents suspected of caus-
ing abnormally low yields a long and often tedious investigation is
required, based upon the methods described in part in Bureau of
Soils Bulletin 74.

In very few cases is it considered necessary or advisable to make a
chemical analysis by any one of the well-known acid digestion
methods, as the result of such an examination appears to throw little
light upon the adaptation of crops to soils; and as far as yield is con-

80517°— Cir. 26— 11

I

cerned there are many contributing factors not shown by the analysis
which affect this as much or more than the relative proportions of the
mineral constituents. This matter is treated more fully in Bureau of
Soils Bulletins 55 and 57 and Farmers' Bulletin 257.

When a sample is sent to the bureau for advice as to crop adapta-
tion a determination is made, so far as can be done from a small sam-
ple, of the nature of the material, to identify the soil series to which
it belongs. If the exact locality from which the sample is taken is
given and information is furnished regarding the deeper subsoil,
the drainage, elevation, slope, location along streams or in the up-
lands, the occurrence of hardpan, subsurface gravel or rock layers, or
other information bearing upon the origin and mode of formation of
the material, an examination of the soil and subsoil samples by the
aid of the keys given in Bureau of Soils Bulletin 78 will usually de-
termine the soil series to which it belongs. This gives at once the
main characteristics of the soil and its crop adaptation under certain
physical and topographical conditions which will be referred to later.

If the soil is found to belong to the Hagerstown, Sassafras, Hous-
ton, Norfolk, Knox, Marshall, Dunkirk, Clyde, or other known
series, adaptations are advised on the basis of the known normal
peculiarities of the series, as determined by the soil surveys. The
soil sample or an aqueous extract of the same is then tested by the
wire-basket method, described in Bureau of Soils Circular 18, or by
the bottle-culture method described in Bureau of Soils Bulletins 36
and 70, using carbon black in the soil extract to be sure that there is
no obscure substance present that will unfit the soil for normal crop
development.

A sample submitted for fertilizer requirements is subjected to ex-
amination by the wire-basket method, using a normally productive
soil from the same series for comparison, adding to the soil which is
to be tested manure, lime, and commercial fertilizers in approximately
the same amounts as used in field culture.

The requests for information regarding the mineral composition of
soils are complied with through the ordinary analytical methods
when it appears that the results may throw any light upon peculiar-
ities of constitution or character of the soil that may influence its
crop production. Many of these cases, it is believed, can more prop-
erly and more advantageously be considered questions of fertilizer
requirement. It is oft^n found that the more expeditious method of
mineralogical examination or analysis gives fuller information re-
garding the mineral constitution and composition of the soil than
does the chemical analysis.

All soils sent in for examination from arid regions, or generally
where the rainfall is less than 20 inches per annum, are examined by

IClr. 26]

the electrolytic bridge method, described in Bureau of Soils Bulletin
61, for accumulations of alkali salts, and this is carried further, if
for any reason it seems desirable, by an analysis of the individual
saline constituents in the water extract.

The foregoing is substantially the procedure of the bureau in ex-
aminations of soil samples submitted by correspondence.

It is necessary to bear in mind that the most thorough examina-
tion of a soil may reveal nothing of a deleterious nature or nothing
indicating that success could not be had with the crops adapted to
that kind of material, and yet these crops might fail if certain field
indications are ignored. Success is not possible if the drainage con-
ditions are not satisfactory, if the elevation and topography of the
farm are not suitable, if the methods of cultivation and cropping
are not efficient, and if the market demands and transportation facil-
ities are inadequate. Some of these factors, which leave nothing in
the sample that can possibly be revealed and others which may or
may not leave eifects that could only be determined by the most re-
fined methods and after exhaustive research, are so important and so
much more easily determined through a field examination that con-
siderable space will be given their consideration.

Local limitations as to yields of crops arise partly through mis-
control and partly through misadaptation of soils to crops. The
most common fault in the control of soils comes through the method
and means of cultivation.

The equipment of a farm for the proper and efficient cultivation
of any particular soil type should be regulated by the grade of soil.
Soils ranging in grade from loams to clays require for their efficient
control a farm equipment of large teams, of heavy draft animals, and
correspondingly heavy tools and implements, with substantial and
commodious buildings. A silt loam is usually heavier to handle than
a loam, and a clay is usually heavier to handle than a silt loam, and
provision should be made accordingly. On the other hand, soils
ranging from sand to fine sandy loams require for their most efficient
control smaller teams, lighter animals, and generally lighter tools and
implements, and less commodious buildings. The heavier grades of
soil not only offer more resistance in handling, but they require deeper
and more thorough cultivation. Where a sand may be efficiently
cultivated to a depth of 4 or 5 inches, a silt loam or clay may require
8 or 9 inches in depth of cultivation.

The kind of implements most efficiently used on the different grades
of soil and particularly on some of the different soil types should be
adapted to the physical character of the soil. A loose sand or sandy
loam should be worked with implements that tend so far as possible
to compact the subsoil, while a silt loam that tends naturally to com-

[Clr. 26]

h

pactness unfavorable to crops should be handled with implements
that tend to keep it in the loosest possible condition.

A second condition of importance in soil control is the content and
character of the organic matter. As a general rule soils respond to
applications of organic matter whether in the form of stable manure
or of plant remains; or, to state it in another yvay, soils to do their
best require that a constant supply of organic matter be fed to them,
and that this organic matter be of such character as to be 6asily
digested by the soil. No standard can be fixed as to the quantity of
organic matter a soil should contain, but it is essential for its most
effective use that it should at all times contain readily digestible
organic matter.

A soil is most efficient when crops are grown upon it in certain
order of rotation. It is not to be expected that a soil will produce
as large a crop of corn immediately following potatoes as it would
if the corn followed sod; nor is it to be expected that the soil will
produce as large a crop of wheat following wheat as it would if a
crop of clover had intervened between the two wheat crops. The
general principle in a rotation should be a sod followed by com, then
by a small grain, and then by sod again. This is a safe general guide
where such crops are well adapted to a soil type, but the soil types
differ so markedly in their adaptations and in their peculiarities that
no general rotation can be expected to be successful on all types of
soil; and yet, as stated above, the soil will maintain its maximum
capacity only if crops are rotated in certain orders, depending upon
the soil material, the soil class, and the crop adaptations.

Drainage is another factor upon which the efficiency of the soil
depends. There is an optimum water content for each type and
grade of soil with which the highest efficiency of the soil may be
expected. If the soil is so situated as to have excessive or deficient
drainage, the highest efficiency of the soil can not be expected until
these conditions are alleviated.

The presence of certain weeds, and in general an abundance of
weeds, lowers the efficiency of the soil for all cultivated crops, and
weeds are often indicators of soil conditions which make the soil
poorly adapted to certain crops.

These factors of miscontrol do not leave as a rule sufficient evi-
dence in the soil to be determined by a laboratoi*y examination of a
sample, and where failure to maintain the soil in its highest state of
efficiency results through such mismanagement, it is the part of the
expert farmer or soil scientist, skilled in handling soil material in
the field, rather than that of the soil chemist, confined to laboratory
examination, to determine the cause of failure and the remedial
measures.

(Clr. 26]

Failure to maintain a high degree of efficiency of the soil for any
particular crop is very often seen in the misadaptation of soil mate-
rial or the use of a soil series which does not function properly for
the crop grown. As an illustration, it is not to be expected that
soils of the Portsmouth, Volusia, and Susquehanna series will pro-
duce under normal conditions of cultivation satisfactory crops of
wheat, and with other series there are degrees of adaptation from
which it is possible to eliminate many soils from consideration for
commercial wheat production.

In the matter of class of material, also, the sands and up to and
including the fine sandy loams are generally unsuited to wheat pro-
duction, while the heavier members of the series may be admirably
adapted to this crop. This illustration holds for most soils and for
most crops, and it is not to be expected that equal success will be
attained by the indiscriminate use of soils for any or all crops.

The structure of a soil and subsoil and often of the deep subsoil
affect the adaptation to crops without imparting to the soil sample
evidences of the unfavorable conditions which exist. Such condi-
tions can only be determined by a field examination.

In many instances the depth to the subsoil determines the charac-
ter, grade, or yield of crops, particularly in highly specialized indus-
tries. For example, in the production of the Sumatra leaf tobacco
on the Norfolk fine sandy loam in Florida, a soil of medium depth
of 6 or 8 inches gives by far the better quality, and where the sub-
soil is found closer to the surface or considerably deeper equally
good results are not to be expected.

Topography and elevation play an important part in the adapta-
tion of certain crops, sometimes limiting sharply the kinds and at
other times the quality or the yield.

The color of the soil is often a safe guide in the adaptation of soils,
indicating as it does not only material differences in constitution,
but also the general nature of the processes taking place in the soil.

The presence of concretionary formations, either as graved or as
more or less continuous layers of hardpan, influence the adaptation
and frequently the yield, acting in such a way as to leave no sensible
effect upon the sample submitted for examination, and requiring a
field investigation rather than a laboratory analysis.

All of these subjects are treated in the soil survey reports and
adequate information is supplied for such areas as have been sur-
veyed, but where misadaptation of material owing to the presence of
any of th^e factors gives an insufficient crop, the results of labora-
tory investigations upon the sample, removed from its place in the
field and changed in its physical conditions through packing and

[Cir. 26]

transportation, do not apply. It is only in the absence of these
limiting conditions that such results are of value.

It, thus, is essential to the proper investigation of a sample of soil
by the bureau that the purpose for which the examination is required
be stated, and that information be given which will enable the bureau
to identify the material, the agencies which have led to its formation,
and those that have influenced the present condition of the soil. The
general purposes for which such examinations may be requested may
be briefly stated :

1. Adaptation of the soil for crop&

2. Fertilizer and manurial requirements.

3. Alkali determination.

4. Special chemical and mineralogical examination.

5. Investigations to reveal the presence of deleterious substances suspected to
be of organic nature.

The purpose for which the soil is to be used should be stated — that
is, whether for greenhouse, garden, or field crops, and the kind of
crops it is intended to grow.

The general topography and elevation of the field should be stated
and its relative position with respect to surrounding fields. If the
soil occurs as prairie or not, this fact should be stated, together with
the nature and species of the native plants, whether weeds, grass, or
trees.

The occurrence of concretionary formations such as ferruginous
or calcareous hardpan or nodules will often aid in the identification
of material and in the consideration of the problem of adaptation
and yield. If the soil is of recent alluvium, it should be stated
whether of first bottom or of second bottom, and a note should be
made of the drainage conditions. Any unusual phenomena or ab-
normal growth or death of crop should be stated, and general infor-
mation as to the crops previously grown and the character and
efficiency of cultivation.

It is advisable always to have a sufficiently large sample to serve
for the different kinds of examinations that will be required. For
this purpose there should be submitted at least 10 pounds of the top
soil taken down to a change of color or of texture marking the sub-
soil; or if tnere is no particular difference between the soil and sub-
soil, the sample should be taken to a depth of at least 8 or 9 inches.
There should also be submitted a sample of about 5 pounds of the
subsoil immediately underlying; and if there is a gravel substratum
below this, the fact should be stated. Care should be exercised in
securing the sample to select a spot representing as accurately as
possible the conditions that are to be investigated. If the examina-

[Cir. 26]

tion is to be for the general characteristics of a field, it is better to
make a composite sample from 8 or 10 different parts of the field.
In all cases the exact location of the farm should be stated, giving
distances in miles and direction from some post-office, town, or vil-
lage or indicating position by land lines.

Approved :

James Welson,

Secretary of Agriculture.

Washington, D. C, April 7, 1911.

tClr. 26]

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