chemical analysis should consist of the fine earth which has passed at
least a one-millimeter mesh sieve and subsequently been completely
air-dried. According to Petermann the air-drying of a soil should
continue for about four days for an ordinary arable soil, and about six
days for one very rich in organic matter. With peat and muck soils I
have found that ten or twelve days with frequent stirring, even when in
thin layers, are necessary to attain approximately a constant weight.
The soil is conveniently spread on a zinc or other metal sheet of
sufficient area so that the layer will be only one or two centimeters in
thickness. The weight before and after desiccation will give the
percentage of moisture lost on air-drying, which, of course, will depend
chiefly on the degree of saturation of the sample when taken and the
atmospheric conditions prevailing during drying.
If samples of soil are taken in very dry times it is often necessary to
moisten them with distilled water in order to prepare them properly for
air-drying.
The quantity of hygroscopic water which the sample loses at 100°–105°
should be determined, and all subsequent calculations of the percentages
of the various constituents be based on the water-free material. When a
soil which has been dried at 100°–105° to a constant weight is heated to
140°–150° it loses additional weight not due to loss of water of
constitution. A part of this loss may be due to hygroscopic moisture
which is not given off at 100°–105°, and a part may be hydrocarbons, or
other easily volatile organic or inorganic bodies. Before estimating the
total loss on ignition it is recommended by most chemists to dry at
140°–150°. The samples of soil, however, intended for chemical
examination should never be dried beyond the point which is reached by
exposure in thin layers at ordinary room temperatures. The state of
aggregation, degree of solubility, and general properties of a soil, may
be so changed by absolute desiccation as to render the subsequent
results of chemical investigation misleading. In the methods which
follow the actual processes employed have been given, which in some
instances transgress the general principle stated above, but in all
cases standard and approved methods are given in detail, even if some of
their provisions seem unnecessary or imperfect.
=291. Order of Examination.=—First of all in a chemical study of the
soil should be determined, its reaction (with litmus), its water-holding
power in the air-dried state (hygroscopicity), its content of combined
water (giving hydrous silicates of alumina), its organic matter (humus
and organic nitrogen), its content of carbon dioxid (carbonates of the
alkaline earths), and the part of it soluble in acids. A determination
of these values gives the analyst a general view of the type of soil
with which he is engaged, and leads him to adopt such a method of more
extended analysis as the circumstances of the case may demand.
For this reason those operations which relate to the above
determinations are placed first in the processes to be performed, while
the estimation of the more particular ingredients of the soil is left
for subsequent elaboration.
Next follows a description of the standard methods of estimating the
more important elements passing into solution on treatment of a soil
sample with an acid. The method of treating the insoluble residue, and
the detection and estimation of rare or unimportant soil constituents,
closes the analytical study of the soils.
With respect to the determination of nitrogen as nitric or nitrous acid
in the soil and drainage waters, it has been thought proper to collect
all standard methods relating particularly thereto into one group, and
they will appear separate from the methods under nitrogen analysis in
fertilizers.
The question of the utility of chemical soil analyses is one which has
been the subject of vigorous discussion, a discussion which finds no
proper place in a work of this character. Unless, however, intelligent
soil analysis be productive of some good it would be a thankless task to
collect and arrange the details of the processes employed. An accurate
determination of the constituents of a soil may not enable the chemist
to recommend a proper course of treatment, but it will help in many ways
to develop a rational soil diagnosis which will permit the physician in
charge of the case, who last of all is the farmer, to follow a rational
treatment which in the end will be productive of good.
The analyst will find in the methods given all that are approved by
bodies of official or affiliated chemists, or by individual experience,
and among them some method may be found which, it is hoped, will be
suited, in the light of our present knowledge, to each case which may
arise.
=292. Reaction of the Soil.=—In soils rich in decaying vegetable matter
the excess of acid is often great enough to produce a distinct acid
reaction.
On the contrary, in arid regions the accumulation of salts near the
surface may produce the opposite effect.
The reaction of the soil may be determined with a large number of
indicators among which, for convenience, sensitive litmus paper, both
red and blue, stand in the front rank. A sample of the soil, from
fifteen to thirty grams, is mixed with water to a paste and allowed to
settle. The litmus paper is then dipped into the supernatant liquid.
=293. Determination of Water in Soil.=—The following problems are
presented:
(a) _The Determination of Water in Fresh Samples taken in situ._—The
content of water in this case varies with the date and amount of
rain-fall, the capacity of the soil for holding water, the temperature
and degree of saturation of the atmosphere, and many other conditions,
all of which should be noted at the time the samples are taken.
(b) _The Determination of Water in Air-Dried Samples._—In this case the
soil is allowed to remain in thin layers, and exposed to the air until
it ceases to lose weight. The quantity of water left is dependent on the
capacity of the soil to hold hygroscopic water and to the temperature
and degree of saturation of the air.
(c) _The Determination of the Total Water by Ignition._—This process not
only gives the free and hygroscopic moisture, but also combined water
present in the hydrous silicates and otherwise. The estimation is
complicated by the presence of carbonates and organic matter.
=294. Determination of Water in Fresh Samples.=—This determination
requires that the sample, when taken in the field, should be so secured
as to be weighed before any loss of moisture can take place. For this
purpose it can be sealed up in tubes or bottles and preserved for
examination in the laboratory.
According to Whitney, the relations of soils to moisture and heat are
such prominent factors in the distribution and development of
agricultural crops, that the determination of the actual moisture
content of soils in the fields should be considered a necessary part of
the meteorological observations, and of far more importance, indeed, or
having far more meaning to the agriculturist than the simple record of
the rain-fall.
In order to determine the relation of the soil to moisture, uninfluenced
by the varying conditions of cultivation and of the different size of
crop, he recommends that a small plot of ground be reserved at each
station, adjacent to the soil thermometers, where the samples may be
taken for the moisture determinations. No crops should be allowed to
grow on this area and the soil is not to be disturbed, except that weeds
and grass are carefully removed by hand when necessary. Samples of the
soil should be taken every morning at 8 o’clock, by correspondents in
the principal soil formations from the different parts of the area under
observation, and sent by mail to the laboratory.
The samples should be taken as described in paragraph =65=. The locality
and date are written on a label attached to the tube. The tube contains
about sixty or seventy grams of soil, and the moisture determination is
made on this in the laboratory in the usual way.
It would be desirable to have this sample represent a depth of from six
to nine inches, thus rejecting the surface three inches which are more
liable to sudden and accidental changes. These tubes are very
inexpensive, and a sufficient number should be purchased to keep each
station supplied. The sample represents a definite depth, and it does
not have to be subsampled or even transferred in the field. This record
of the moisture of the soil will show the amount of moisture which the
different soils can maintain at the disposal of the plants, which,
together with the temperature of the soil, is believed to be a most
important factor in crop distribution and development.
=295. Method of Berthelot and André.=—The estimation of the water
according to Berthelot and André[189] should be made under three forms;
_viz._,
1. Water eliminated spontaneously at ordinary temperatures.
2. Water eliminated by drying to constant weight at 110°.
3. Water eliminated at a red heat.
The water may be determined directly on a sample weighed at the time of
taking and afterwards dried in the open air, and finally, if necessary,
in a desiccator. For a general idea the desiccation should be made on a
sample of 100 grams, for exact work on ten grams. The dish in which the
drying takes place should be shallow, and during the time the sample
should be frequently stirred and thoroughly pulverized with a spatula
which is weighed with the dish. The drying in the air should continue
several days. The data obtained are not fixed since they depend on the
temperature and the degree of saturation of the air with aqueous vapor.
The variations due to these causes, however, are not very wide. The
process may be regarded as practically finished when successive weights
sensibly constant are obtained. In this state the soils contain very
little water eliminable at 110°.
=296. Estimation of Water Remaining after Air-Drying.=—The sifted sample
is placed in quantities of five or ten grams in a flat-bottomed dish and
dried at 110° to constant weight. This treatment not only removes the
moisture, but all matters volatile at that temperature.
Petermann,[190] in the Agricultural Station, at Gembloux, practices
drying the sample to constant weight at 150°.
It is further recommended by Petermann to determine total volatile and
combustible matters by igniting to incipient redness, allowing to cool,
moistening with distilled water, and drying at 150°.
The German experiment stations[3] estimate hygroscopic moisture for
analytical calculations by drying to constant weight at 100°. In
determining loss on ignition, however, the preliminary drying is made at
140°, with the exception of peaty samples where so high a temperature is
not admissible.
The Official Agricultural Chemists[191] place five grams of air-dried
soil in a flat-bottomed and tared platinum dish; heat in an air-bath to
110° for eight hours; cool in a desiccator, and weigh; repeat the
heating, cooling, and weighing, at intervals of an hour till constant
weight is found, and estimate the hygroscopic moisture by the loss of
weight. Weigh rapidly to avoid absorption of moisture from the air.
In the German laboratories, according to König,[192] from ten to twenty
grams of the fine earth, properly prepared by air-drying and sifting,
for analysis, are heated at 100° to constant weight. For control, five
grams are placed in a desiccator over sulfuric acid for two or three
days.
Wolff directs that a small portion of the well-mixed earth, for example,
twenty grams, be spread out on a flat zinc plate, and its changes in
weight observed through several days. These observations are continued
until the variations are so slight that the means can be determined with
sufficient exactness from the last weighings. The soil is then dried at
125° in a hot air-chamber. The loss in weight will give the mean
hygroscopic moisture in the soil under the conditions in which the
experiment is made.
=297. Drying in a Desiccator.=—The sample dried as indicated previously
by the method of Berthelot and André is placed in a desiccator over
sulfuric acid. It is better to have the sample traversed by a current of
perfectly dry air, and in this case it should be placed in a tube, which
is closed while weighing, to prevent absorption of moisture. Much time
is also required for this operation, and it does not possess the
practical value of the method of drying in the free air.
=298. Water Set Free at 110°.=—This is determined by Berthelot and André
on a weight of five to ten grams of soil. The sample which has been
employed for the preceding determination may be used. While this is
going on in an air-bath heated to 110°, about ten times as much soil
should be dried for the same time at the same temperature, and this
should be preserved in a well-stoppered flask. All subsequent
determinations are to be made with the soil dried at 110°.
The loss of weight in a soil increases with the temperature to which it
is exposed. The apparent quantity of water, therefore, determined at
140° or 180° is always greater than that obtained at 110°. But when the
temperature exceeds 110° there is danger of decomposing organic bodies
with the loss of a part of their constituent elements. Carbon dioxid and
ammonia may also be lost, as well as acetic acid and other volatile
bodies.
=299. Loss on Ignition.=—The loss on ignition represents any hygroscopic
moisture not removed by previous drying, all water in combination with
mineral matters as water of constitution, all organic acids and
ammoniacal compounds, all organic matter when the ignition is continued
until the carbon is burned away, all or nearly all of the carbon dioxid
present in carbonates, and, finally, some of the chlorids of the
alkalies, if the temperature have been carried too high or been
continued too long.
The loss of carbon dioxid in carbonates may be mostly restored by
moistening the ignited mass two or three times with ammonium carbonate,
followed by gentle ignition for a few minutes to incipient redness, to
remove excess of the reagent. The apportionment of the rest of the loss
justly among the remaining volatile constituents of the original sample
is a matter of some difficulty but may be approximately effected by the
methods to be submitted.
=300. Determination of Loss on Ignition.=—_Method of the Official
Agricultural Chemists._ The platinum crucible and five grams of soil
used to determine the hygroscopic moisture may be employed to determine
the volatile matter. Heat the crucible and dry soil to low redness. The
heating should be prolonged till all organic material is burned away,
but below the temperature at which alkaline chlorids volatilize. Moisten
the cold mass with a few drops of a saturated solution of ammonium
carbonate, dry, and heat to 150° to expel excess of ammonia. The loss in
weight of the sample represents organic matter, water of combination,
salts of ammonia, etc.
According to Knop[193] the total loss on ignition is determined as
follows: About two grams of the fine earth are carefully ignited until
all organic matter is consumed. The sample is then mixed with an equal
volume of finely powdered, pure oxalic acid, and again heated until all
the oxalic acid is decomposed. After cooling, the sample is weighed,
again mixed with oxalic acid, ignited, cooled, weighed, and the process
continued until the weight is constant.
The method recommended by König consists in igniting about ten grams of
the fine earth at the lowest possible temperature until all the humus is
destroyed. Thereafter the sample is repeatedly moistened with a solution
of ammonium carbonate and ignited after drying at 100°, until constant
weight is obtained. In soils rich in carbonates some carbon dioxid may
be lost by the above process. For this a proper correction can be made
by estimating the carbon dioxid in the sample, both before and after the
execution of the above described process.
The method described by Frühling as much used in the German
laboratories, consists in igniting ten grams of the fine earth,
previously dried at 140° in a crucible placed obliquely on its support
and with the cover so adjusted over its mouth as to give a draft within
the body of the crucible. The ignition, at a gentle heat is continued
until on stirring with a platinum wire no evidence of unconsumed carbon
is found. The moistening with solution of ammonium carbonate, should not
take place until the contents of the crucible are cool. Subsequent
ignition, at a low heat for a short time, will remove the excess of
ammonium salt.
=301. Method of Berthelot and André.=[194]—The earth dried at 110°
contains still a greater or less quantity of combined water. This is the
water united with alumina, silica and certain salts, but not the water
of constitution belonging to organic bodies. The exact estimation of
this water offers many difficulties. The determination of loss obtained
at a red heat embraces:
(1) The water combined with zeolitic silicates, with alumina and with
organic compounds.
(2) The water produced by the combustion of the organic compounds.
(3) The carbon dioxid resulting from the partial decomposition of the
calcium and magnesium carbonates.
(4) The carbon burned and the nitrogen lost during ignition. The measure
of the loss of weight in an earth heated to redness in contact with the
air is not therefore, an exact process of estimating water or even
volatile matters.
A better defined result is obtained in carefully burning a known weight
of earth either in a current of free oxygen, or with lead chromate. The
water produced in such a combustion is secured in a =ᥩ= tube filled with
pumice stone saturated with sulfuric acid, the carbon dioxid being
absorbed afterwards in potash bulbs and by solid potash. The weight of
earth burned is chosen so as to furnish a convenient weight of both
water and carbon dioxid. In general about five grams are sufficient.
When the combustion is made with oxygen, the soil is contained in a boat
and the products of the combustion are carried over a long column of
copper oxid heated to redness. The residue left in the boat is weighed
at the end of the operation, and in this residue it is advisable to
determine any undecomposed carbonate. Should the sample burn badly and
be mixed with carbonaceous matter at the end of the operation, it will
be necessary to substitute the lead chromate method. In this case, of
course, the residue left after combustion is not weighed. Whichever
method is employed gives a quantity of water originally combined with
the soil, plus the quantity arising from the combustion of the hydrogen
of the organic matter. The details of the processes for organic
combustion, will be given in a subsequent part of this manual. It is not
possible to divide the water between these two sources directly, but
this can be done by calculation, which gives results lying within the
limits of probability. The method follows:
The organic nitrogen, determined separately, by soda-lime, the method of
Kjeldahl, or volumetrically, is derived from proteid principles
resembling albuminoids containing about one-sixteenth of their weight of
nitrogen. The nitrates contained in the earth are in such feeble
proportion, as to be negligible in this calculation. The total weight of
these nitrogenous principles in the soil is therefore easily calculated.
The carbon contained in the proteids is then calculated on a basis of 53
per cent of their total weight, and the hydrogen on a basis of 7.2 per
cent. From the weight of the total organic carbon (determined as
described further on) is subtracted the carbon present in the proteids.
The remainder corresponds to the organic carbon present as
carbohydrates, (ligneous principles) containing 44.4 per cent carbon and
6.2 per cent hydrogen. By adding together the weight of the hydrogen
contained in the ligneous principles, and the hydrogen contained in the
proteids, and multiplying the sum by 9, the weight of water formed by
the combustion of all the organic matter in the sample is obtained. This
is subtracted from the weight of the total water obtained by direct
determination as described above. The difference represents the weight
of water combined with the silicates, etc., as well as with organic
matters.
=302. Method of Von Bemmelén.=[195]—According to the view of Von
Bemmelén, the soil contains colloidal humus and colloidal silicate,
which complicate the determination of water. The colloids retain water
in varying quantities, depending upon the following conditions:
(1) Upon their composition and state of molecular equilibrium.
(2) Upon the pressure of the aqueous vapor of the room.
(3) Upon the temperature.
At each degree of temperature, the quantity of absorbed water which a
colloid can retain in a room saturated with aqueous vapor, is different.
The quantity of water which air-dried earth gives off at 100°, has
therefore, no special significance unless all conditions are known.
In addition to the estimation of the quantity of water which soils, in
their natural condition, are capable of taking up and holding, at
ordinary temperatures, the estimation of the quantity of water which
they can take up in different temperatures in rooms saturated with
aqueous vapor should be of interest. It follows, therefore, that there
is no special value in data obtained by drying earth at 100° or 110°.
For the purpose of comparison, he prefers to select that point at which
the soil is dried over sulfuric acid, the point at which the tension of
the water vapor in the earth, at a temperature of plus or minus 15°,
approaches zero. The water which still remains in the earth under these
conditions is characterized as firmly combined water.
Von Bemmelén truly observes that only in soils which contain no
carbonates and no chlorids and sulfids, can the loss on ignition be
regarded as the sum of the humus and water content. By moistening with
ammonium carbonate, the correction for lime or carbon dioxid cannot be
correctly made as has been the custom up to the present time. In the
first place, ignited magnesia, when it has lost its carbon dioxid, does
not take this up completely on moistening with ammonium carbonate; in
the second place, reactions with the chlorids may take place; and in the
third place, the lime which is in the humus will be converted into
calcium carbonate. Chlorids on ignition may be volatilized or oxidized.
The sulfuric acid formed from the sulfids, on ignition, can expel carbon
dioxid; further than this the iron of pyrites takes up oxygen on
ignition. All these influences make the numbers obtained from loss on
ignition extremely variable.
With sea soils, Von Bemmelén has weighed the soil after the elementary
analysis and estimated, in addition to the carbon dioxid, both chlorin
and sulfuric acid therein. The comparison of these estimations with
those of CO₂, Cl, SO₃ and S, made in the original soil, gave the
necessary corrections; _viz._, for the increase in the weight through
oxidation of sulfur and iron, and for the decrease in weight through the
volatilization of sodium chlorid, sulfur, and carbon dioxid. A trace of
chlorin was evolved as ferric chlorid, nevertheless, the molecular
weight of sodium chlorid, 58.5, is scarcely different from the
equivalent quantity of ferric chlorid 54.1. For this reason the
estimation of loss of water, on ignition, of sea soils is less exact
than that of soils which are free from carbonates and sulfids and which,
as is usually the case with tillable soils, contain only small
quantities of chlorids and sulfates.