minus 15°, in a dry room, still remains in the soil, is chiefly combined
according to Von Bemmelén with the colloidal bodies therein. Its
estimation, presents, naturally, difficulties and is not capable of any
great exactness. The quantity of strongly combined water, on the one
hand is determined from the difference between the loss on ignition and
the quantity of humus present, calculated from the content of carbon; on
the other hand, from the difference between the water obtained by
elementary analysis and the water which corresponds to the calculated
quantity of humus. If the hydrogen content of humus is correctly taken
and no appreciable error is introduced through the factor 1.724, both of
these differences must agree. On the other hand the hydrogen content of
the humus can be computed from the difference between the water found
and the calculated content of the firmly combined water.
The hydrogen content of humus bodies, dried at 100°, varies between four
and five per cent. Eggertz has found the content from 4.3 to 6.6 per
cent of hydrogen in thirteen soils which he first treated with dilute
hydrochloric acid then extracted with ammonia or potash lye and
precipitated this alkaline extract with acid. The method of applying
these principles to soil analysis is indicated in the following scheme:
A volcanic earth from Deli gave, on elementary analysis:
Per cent.
Carbon 2.94
Water 14.78
Nitrogen 0.28
Loss on ignition 17.54
FIRST CALCULATION.
Per cent.
Loss on ignition 17.54
Humus = carbon, 2.94 × 1.724 = 5.07
Difference = firmly combined water 12.47
Assuming that a humus dried over sulfuric acid contains five per cent of
hydrogen, the second calculation is made as follows.
SECOND CALCULATION.
5.00 humus × 5 per cent = 0.25 per cent of
hydrogen in humus corresponding to 2.28 per
cent of water.
Per cent.
Water found 14.79
Difference = firmly combined water 12.51
THIRD CALCULATION.
Per cent.
Firmly combined water 12.47
Water from the hydrogen in humus 2.28
Total water 14.75
Found 14.79
In this way, in three other volcanic earths and in an ordinary alluvial
clay from Rembang, there were found by analysis and by calculation the
following percentages of water:
1. 2. 3. 4. 5.
Percentage of water calculated 14.75 7.74 8.06 4.90 6.01
„ „ „ found 14.97 7.63 8.05 4.70 6.00
On the contrary, when the calculation is made from sea-slime taken from
under the water a higher content of hydrogen must be assumed; _viz._,
about six per cent. In two samples of sea-slime calculated in this way
the following numbers were obtained:
Percentage of water calculated 8.61 3.71
„ „ „ found 8.53 3.57
It is, therefore, quite evident that the organic compounds of soil taken
from under the sea-water are richer in hydrogen than those exposed to
the air or in cultivation.
=303. General Conclusions.=—In the foregoing paragraphs have been
collected the most widely practiced methods of determining moisture in
soil in both a free and combined state. The following conclusions may
serve to guide the analyst who endeavors to determine the water in any
or all of its conditions:
(1) In determining water in fresh samples the method of Whitney is
satisfactory. Although the samples taken by this method are small they
may be easily secured in great numbers over widely scattered areas, and
can be easily transported without change. These samples should be dried
at 100° to 110° for rapid work, or where time can be spared may be
air-dried.
(2) For a simple determination of the water left in the soil after
air-drying (hygroscopic water) the method of the Association of Official
Agricultural Chemists may be followed. There is much difference of
opinion in respect of the proper temperature at which this moisture is
to be determined. Much here depends on the nature of the soil. An almost
purely mineral soil may safely be dried at 140° or 150°. A peaty soil,
on the contrary, should not be exposed to a temperature above 100°. For
general purposes the temperature chosen by the official chemists is to
be recommended.
(3) Water of composition can only be determined by ignition. As has been
fully shown, this process not only eliminates the water, but also
destroys organic matter, decomposes carbonates and sulfids, and, to some
extent, chlorids. Subsequent repeated treatment with ammonium carbonate
may restore the loss due to carbon dioxid, but in many cases not
entirely. The water which comes from organic matter may be approximately
calculated from the humus content of the sample, but as will be seen
further on the methods of estimating humus itself are only approximate.
Nevertheless, in distributing the losses on ignition properly to the
several compounds of the soil there is no better method now known than
that of taking into consideration the humus content and carbonates
present. The principles of procedure established by Berthelot and André,
and Von Bemmelén, are to be applied in all such cases, modified as
circumstances may arise according to the judgment of the analyst.
=304. Estimation of the Organic Matter of the Soil.=—The organic matter
in the soil may be divided into two classes. First, the undecayed roots
and other remains of plant and animal life, and the living organisms
existing in the soil. The study of the organisms which are active in the
condition of plant growth will be the subject of a special chapter.
Second, the decayed or partially decayed remnants of organic matter in
the soil known as humus. Such matter may be present in only minute
traces, as in barren sand soils, or it may form the great mass of the
soil under examination, as in the case of peat, muck, and vegetable
mold. It is with the investigation of the second class of matter that
the analyst has chiefly to do at present. The problems which are to be
elucidated by the analytical study of such bodies are the following: (1)
The total quantity of such matter in the soil. (2) The determination of
the organic carbon and hydrogen therein. (3) The determination of total
nitrogen. (4) The determination of the availability of the nitrogen for
plant growth. (5) The estimation of the humic bodies (humus, humic acid,
ulmic acid, etc.).
The importance of humus in the promotion of plant growth is sufficient
excuse for the somewhat extended study of the principles which underlie
the analytical methods, and the methods themselves, which follow.
=305. Total Quantity of Organic Matter.=—The total approximate quantity
of organic matter in the soil can be determined by simple ignition, in
the manner noted in paragraphs =294= and =295=. The proper correction
for free and combined water being applied by the further copper oxid or
lead chromate combustion of the sample, and for carbonates and volatile
chlorids, the approximate total of the organic matter of all kinds is
obtained.
=306. Estimation of the Organic Carbon.=—To estimate the organic carbon
in an earth the sample may be burned in a current of oxygen, or after
mixing with lead chromate.
_In a Current of Oxygen._—When burned in a current of oxygen the sample
is held in a boat and the gases arising from the combustion directed
over copper oxid at a red heat. The carbon thus disappears as carbon
dioxid and is absorbed and weighed in the usual way.
_With Lead Chromate._—The lead chromate employed should be previously
tested since it often contains other compounds, especially lead acetate
and nitrate, furnishing in the one case both carbon dioxid and water,
and in the other hyponitric acid.
From two to ten grams of earth are employed, according to its richness
in organic matter. The total carbon dioxid is obtained in this process
both from carbonates and organic bodies. The water and carbon dioxid are
secured and weighed in the usual manner.
The oxygen method should be used in all cases possible. Although it does
not always give the whole of the carbon dioxid present as carbonates,
the rest can be easily estimated by treating the residue in the boat
with hydrochloric acid, in an apparatus for estimating that gas.
_Calculation of Results._—The whole of the carbon dioxid is determined
either by direct combustion with lead chromate, or by taking the sum of
the amounts by burning in a stream of oxygen and treating the residue in
a carbon dioxid apparatus.
The carbon dioxid contained in the original carbonates should be
determined by direct treatment of the sample in the usual way.
The carbon in organic compounds is determined by subtracting the carbon
present as carbonates from the total.
From the organic carbon contained in the soil the humus is calculated by
Wolff on the supposition that it contains fifty-eight per cent of
carbon. It is, therefore, only necessary to multiply the percentage of
carbon found by 1.724, or the carbon dioxid found by 0.471, to determine
the quantity of humus in the dried soil.
=307. Details of the Direct Estimation of Carbon in Soils by Various
Methods.=—(1) _Oxidation by Chromic Acid._—The method of Wolff by
oxidation with chromic acid has been worked out in detail by Warington
and Peake.[196] It consists in treating the soil with sulfuric acid and
potassium bichromate, or by preference with a mixture of sulfuric and
chromic acids, the carbon dioxid evolved being estimated in the usual
way. This method is recommended by Fresenius as an alternative to a
combustion of the soil with copper oxid or lead chromate. It is
apparently the method which has been most generally employed in
agricultural investigations.
Ten grams of the finely powdered soil are placed in a flask of about 250
cubic centimeters capacity, provided with a caoutchouc stopper, through
which pass two tubes, one for the supply of liquids, the other for the
delivery of gas. The soil is treated with twenty cubic centimeters of
water and thirty cubic centimeters of oil of vitriol; and the whole,
after being thoroughly mixed, is heated for a short time in a
water-bath, the object in view being the decomposition of any carbonates
existing in the soil. Air is next drawn through the flask to remove any
carbon dioxid which has been evolved. The stopper is next removed, and
coarsely powdered potassium bichromate introduced. In the case of a soil
containing three per cent of carbon, six grams of bichromate will be
found sufficient, a portion remaining undissolved at the end of the
experiment. The stopper is then replaced, its supply-tube closed by a
clamp, and the delivery-tube connected with a series of absorbents
contained in =ᥩ= tubes. The first of these tubes contains solid calcium
chlorid; the second, fragments of glass moistened with oil of vitriol;
the third and fourth are nearly filled with soda-lime, a little calcium
chlorid being placed on the top of the soda-lime at each extremity. The
last named tubes are for the absorption of carbon dioxid, and have been
previously weighed. The series is closed by a guard-tube containing
soda-lime, with calcium chlorid at the two ends.
The flask containing the soil and bichromate is now gradually heated in
a water-bath, the contents of the flask being from time to time mixed by
agitation. A brisk reaction occurs, carbon dioxid being evolved in
proportion as the soil is rich in organic matter. The temperature of the
water-bath is slowly raised to boiling as the action becomes weaker, and
is maintained at that point till all action ceases. As bubbles of gas
are slowly evolved for some time, it has been usual in these experiments
to prolong the digestion for four or five hours. When the operation is
concluded the source of heat is removed, an aspirator is attached to the
guard-tube at the end of the absorbent vessels, and air freed from
carbon dioxid is drawn through the flask and through the whole series of
=ᥩ= tubes. The =ᥩ= tubes filled with soda-lime are finally weighed, the
increase in weight showing the amount of carbon dioxid produced. The
object of the calcium chlorid placed on the surface of the soda-lime is
to retain the water which is freely given up when the soda-lime absorbs
carbon dioxid. The second =ᥩ= tube filled with soda-lime does not gain
in weight till the first is nearly saturated; it thus serves to indicate
when the first tube requires refilling. The same tubes may be used
several times in succession.
No increase in the carbon dioxid evolved is obtained by substituting
chromic acid for potassium bichromate.
The organic matter of the soil appears to the eye to be completely
destroyed by the digestion with sulfuric acid and potassium bichromate;
the residue of soil remaining in the flask when washed with water is
perfectly white, or the dark particles, if any, are found to be
unaltered by ignition, and therefore to be inorganic in their nature.
Under these circumstances considerable confidence has naturally been
felt in this method. The complete destruction of the humic matter of the
soil does not, however, necessarily imply that the carbon has been
entirely converted into carbon dioxid as has been pointed out by
Wanklyn. According to his demonstration of the action of chromic acid on
organic matter the oxidation frequently stops short of the production of
carbon dioxid. While oxidation with chromic acid apparently leads to a
complete reaction when the carbon is in the form of graphite, it would
probably yield other products than carbon dioxid when the carbon exists
as a carbohydrate. The doubt thus raised as to the correctness of the
results yielded by the chromate method makes it desirable to check the
work by the use of other methods for the determination of carbon. For
this purpose Warington and Peake recommend:
(2) _Oxidation with Potassium Permanganate._—In the trials with this
method ten grams of soil are digested in a closed flask with a measured
quantity of solution of caustic potash containing five grams of potash
for each twenty cubic centimeters, and crystals of potassium
permanganate. Seven grams of the permanganate are found to be sufficient
for a soil containing 3.3 per cent of carbon. The flask is heated for
half an hour in boiling water, and then for one hour in a salt-bath. The
flask during this digestion is connected with a small receiver
containing a little potash solution, to preserve an atmosphere free from
carbon dioxid; distillation to a limited extent is allowed during the
digestion in the salt-bath.
The first part of the operation being completed a rubber stopper,
carrying a delivery and supply-tube, is fitted to the flask, which is
then connected with the system of =ᥩ= tubes already described. Dilute
sulfuric acid is then poured down the supply-tube, a water-bath
surrounding the flask is brought to boiling, and maintained thus for one
hour, after which air, free from carbon dioxid, is drawn through the
apparatus, the =ᥩ= tubes containing soda-lime being finally disconnected
and weighed.
In the first stage of this method the carbon of the organic matter is
converted into carbonate, and probably also into potassium oxalate.[197]
In the second stage the oxalate is decomposed by the sulfuric acid and
permanganate, and the carbon existing, both as oxalate and carbonate, is
evolved as carbon dioxid, and absorbed by the weighed soda-lime tubes.
Both F. Schulze and Wanklyn have employed potassium permanganate for the
determination of organic carbon, but they have preferred to calculate
the amount of carbon from the quantity of permanganate consumed, as,
however, by so doing everything oxidizable by permanganate is reckoned
as carbon, it seems better to make a direct determination of the carbon
dioxid formed.
From the amount of carbon dioxid found, is to be subtracted that
existing as carbonates in the soil, and in the solution of potash used.
For this purpose an experiment is made with the same quantities of soil
and potash previously employed, but without permanganate, and the carbon
dioxid obtained is deducted from that yielded in the experiment with
permanganate. If the potash used contains organic matter two blank
experiments will be necessary, one with potash and permanganate, and one
with soil alone.
A further difficulty arises from the presence of chlorids in the
materials, which occasions an evolution of free chlorin when the
permanganate solution is heated with sulfuric acid. This error occurs
also with the chromic acid method, but in that case the quantity of
chlorid is merely that contained in the soil, which is usually very
small; in the permanganate method we have also the chlorid present in
the caustic potash, and this is often considerable. Corrections for
chlorin by blank experiments are unsatisfactory, the amount of chlorin
which reaches the soda-lime tubes depending in part on the degree to
which the calcium chlorid tube has become saturated with chlorin. It is
better therefore to remove the chlorin in every experiment by the plan
which Perkin has suggested, by inserting a tube containing silver foil,
maintained at a low red heat, between the flask and the absorbent =ᥩ=
tubes.
The amount of carbon dioxid yielded by oxidation with potassium
permanganate is found to be considerably in excess of that obtained by
oxidation with chromic acid; to ascertain whether these higher results
really represented the whole of the carbon present in the soil, trials
were next made by actual combustion of the soil in oxygen.
(3) _Combustion in Oxygen._—The most convenient mode of carrying out the
combustion of soil is to place the soil in a platinum boat, and ignite
it in a current of oxygen in a combustion tube partly filled with cupric
oxid. A wide combustion tube is employed, about twenty inches long, and
drawn out at one end; the front of the tube is filled for eight inches
with coarse cupric oxid, the hind part is left empty to receive the
platinum boat. The drawn out end of the combustion tube is connected
with a series of absorbent =ᥩ= tubes, quite similar to those employed
for the estimation of carbon dioxid in the chromic acid method. Between
these absorbent vessels and the combustion tube is placed a three-bulbed
Geissler tube filled with oil of vitriol. The oil of vitriol is quite
effective in retaining nitrous fumes. The wide end of the combustion
tube is connected with a gas-holder of oxygen; the oxygen gas is made to
pass through a =ᥩ= tube of soda-lime before entering the combustion
tube, to remove any possible contamination of carbon dioxid.
In starting a combustion the part of the combustion tube containing the
cupric oxid is brought to a red heat, and oxygen is passed for some time
through the apparatus. Ten grams of soil, previously dried, are placed
in a large platinum boat, which is next introduced at the wide end of
the combustion tube. The combustion is conducted in the usual manner, a
current of oxygen being maintained throughout the whole operation. It is
very useful to terminate the whole series of absorbent vessels with a
glass tube dipping into water; the rate at which the gas is seen to
bubble, serves as a guide to the supply of oxygen from the gas-holder,
the consumption of oxygen varying, of course, with different soils, and
at different stages of the combustion. At the close of the combustion,
oxygen, or air freed from carbon dioxid, is passed for some time through
the apparatus to drive all carbon dioxid into the absorbent vessels. One
experiment can be followed by another as soon as the hind part of the
combustion tube has cooled sufficiently to admit a second platinum boat.
The same combustion tube can be employed for several days, if packed in
the usual manner in asbestos.
The presence of carbonates in the soil occasions some difficulty in
working the combustion method, as a part of this carbon dioxid will, of
course, be given up on ignition, and be reckoned as carbon. The simplest
mode of meeting this difficulty is to expel the carbon dioxid belonging
to the carbonates before the combustion commences. The method of
Manning; namely, treatment with a strong solution of sulfurous acid, may
be employed for this purpose. The ten grams of soil taken for combustion
are placed in a flat-bottomed basin, covered with a thin layer of
sulfurous acid, and frequently stirred. After a time the action is
assisted by a gentle heat. When the carbonates have been completely
decomposed the contents of the basin are evaporated to dryness on a
water-bath; the dry mass is then pulverized, and removed to the platinum
boat for combustion in oxygen. For the action of the sulfurous acid to
be complete it is essential that the carbonates should be in very fine
powder, since even chalk is but imperfectly attacked when present in
coarse particles.
=308. Comparison of Methods.=—A considerable number of soils analyzed by
the chromic acid method and by the combustion, method, by Warington and
Peake, with the assistance of Cathcart, shows the following comparisons:
PERCENTAGE OF CARBON FOUND BY TWO METHODS IN SOILS DRIED AT 100°.
Chromic acid method. Combustion method.
No. Kind of Exp. 1. Exp. 2. Mean. Exp. 1. Exp. 2. Mean. Per
soil. cent.
yielded
by
chromic
acid.
1. Old pasture 2.85 2.79 2.82 3.58 3.55 3.57 79.0
2. „ „ 2.83 2.79 2.81 3.57 3.53 3.55 79.1
3. „ „ 2.76 2.76 2.76 3.46 3.46 3.46 79.7
4. „ „ 2.74 2.76 2.75 3.37 3.38 3.38 81.4
5. „ „ 2.64 2.54 2.59 3.31 3.36 3.34 77.5
6. „ „ 2.51 2.43 2.47 3.15 3.15 3.15 78.4
7. „ „ 2.40 2.44 2.42 3.09 3.13 3.11 77.8
8. New pasture 1.92 1.93 1.93 2.41 2.40 2.41 80.1
9. „ „ 1.66 1.81 1.74 2.39 2.43 2.41 72.2
10. Arable soil 1.78 1.78 1.78 2.14 2.13 2.14 83.2
11. „ „ 1.21 1.14 1.18 1.40 1.43 1.42 83.1
12. Subsoil 0.28 0.27 0.28 0.37 0.38 0.38 73.7
Of the above soils the arable soils, Nos. 10 and 11, were the only ones
containing carbonates in any quantity exceeding a minute trace. The two
soils in question were treated with sulfurous acid before combustion,
the others not.
All the determinations by the chromic acid method were made by Mr. P. H.
Cathcart, with the exception of Nos. 9 and 12, which were executed by
another experimenter, and are seen to give distinctly lower results.
Excluding these two analyses the relation of the carbon found by the two
methods is tolerably constant, the average being 79.9 of carbon found by
oxidation with chromic acid for 100 yielded by combustion in oxygen. The
results obtained by the chromic acid method thus appear to be very
considerably below the truth.
Four typical soils were analyzed by the permanganate, as well as by the
chromic acid and combustion methods. The results obtained were as
follows:
PERCENTAGE OF CARBON FOUND BY THREE METHODS IN SOILS DRIED AT 100°.
Permanganate method.
Kind of Chromic Yielded by
soil. acid permanganate
Combustion method. if carbon by
method. Mean. Exp. 1. Exp. 2. Mean. combustion =
Mean. Per Per Per Per Per 100. Per
cent. cent. cent. cent. cent. cent.
Old pasture 3.55 2.81 3.26 3.30 3.28 92.4
New pasture 2.41 1.93 2.29 2.30 2.30 95.4
Arable soil 1.42 1.18 1.28 1.33 1.31 92.3
Subsoil 0.38 0.28 0.34 0.34 0.34 89.5
Oxidation by permanganate thus gives a much higher result than oxidation
with chromic acid; but even the permanganate fails to convert the whole
of the carbon into carbon dioxid, the product with permanganate being on
an average of the four soils 92.4 per cent of that yielded by combustion
in oxygen.
Wanklyn states that a temperature of 160°–180° is necessary in some
cases to effect complete oxidation with permanganate and caustic potash.
Such a temperature is found impracticable when dealing with soil, from
the action of the potash on the silicates present; hence possibly the
low results obtained.
Combustion in oxygen appears from these experiments to be the most
satisfactory method for determining carbon in soil, nor is this method,
on the whole, longer or more troublesome than the other methods
investigated.
Warington and Peake have further determined the loss on ignition of the
four soils mentioned above, with the view of comparing this loss with
the amount of organic matter calculated from the carbon actually
present. In making this calculation they have taken as the amount of
carbon in the soil, that found by combustion in oxygen, and have assumed
with Schulze, Wolff, and Fresenius, that fifty-eight per cent of carbon
will be present in the organic matter of soils. The four soils were
heated successively at 100°, 120°, and 150°, till they ceased to lose
weight; the loss on ignition in each of these stages of dryness is shown
in the following table:
PERCENTAGE LOSS ON IGNITION COMPARED WITH ORGANIC MATTER CALCULATED
FROM CARBON.