matter at
fifty-eight
Between 100° Between 120° Between 150° per cent
and ignition. and ignition. and ignition. carbon.
Kind of soil. Per cent. Per cent. Per cent. Per cent.
Old pasture 9.27 9.06 8.50 6.12
New pasture 7.07 6.88 6.55 4.16
Arable soil 5.95 5.70 5.61 2.44
Clay subsoil 5.82 5.39 4.76 0.65
The loss on ignition is seen to be in all cases very considerably in
excess of the organic matter calculated from the carbon, even when the
soil has been dried at as high a temperature as 150°. The error of the
ignition method is least in soils rich in organic matter, as, for
instance, the old pasture soil in the above table. The error reaches its
maximum in the case of the clay subsoil, which contains very little
carbonaceous matter, but is naturally rich in hydrated silicates, which
part with their water only at a very high temperature.
The above methods of Warington and Peake have been given in detail, and
in almost the verbiage of the authors for the reason that the working
directions are clearly set forth, and may serve, therefore, as guides to
the previous methods where only general indications of manipulation have
been given.
=309. Estimation of Organic Hydrogen.=—The estimation of the total
hydrogen is made without difficulty either by burning the sample in a
current of oxygen or with lead chromate, and weighing the water
produced. This water comes from two sources, the pre-existing water and
organic hydrogen. There is no direct method of distinguishing one from
the other. They may, however, be estimated indirectly. The method of
calculating the organic hydrogen has already been given (paragraph
=299=). Experience shows that the hydrogen thus calculated is a little
greater than is necessary to form water with the whole of the oxygen
found in the organic matters.
=310. Estimation of Organic Oxygen.=—The determination of this oxygen
cannot be made directly. It is obtained by calculation, according to
Berthelot and André,[198] from the oxygen in the proteid and ligneous
matters.
Let p represent the weight of the proteid bodies in a sample of soil.
Then O = (p × 33.5)/(100)
Let p′ = weight of ligneous bodies.
Then O′ = (p′ × 49.4)/(100)
The total oxygen = O + O′.
An approximate result is thus obtained, very useful to have when account
is taken of the oxidizing processes which go on in the soil during
agricultural operations.
=311. Estimation of Humus (Matière Noire).=—The original method of
determining this substance is due to Grandeau.[199] It is carried on as
follows: Ten grams of the fine earth are mixed with coarse sand
previously washed with acids and ignited. The mixture is placed in a
small funnel, the bottom of which is filled with fragments of glass or
porcelain. The mass is moistened with ammonia diluted with an equal
volume of distilled water, and allowed to digest for three or four
hours. The ammonia dissolves the dark matter without attacking the
silica. The ammoniacal solution is displaced by treating the mass with
pure water, or water to which some ammonia has been added, and the whole
of the dark matter is thus obtained in a volume of twenty to fifty cubic
centimeters of filtrate. The filtrate is evaporated to dryness in a
weighed platinum dish, and the weight of residue is determined and the
percentage of _matière noire_ calculated therefrom. The residue is
incinerated, and when in sufficient quantity the phosphoric acid is
determined in the ash. In soils poor in humus a larger quantity than ten
grams may be taken. If the soil be previously treated with hydrochloric
acid, Grandeau recommends that the phosphoric acid be determined always
in the ash of the dark matter.
The method has undergone various modifications and, as given by Hilgard,
is now practiced as follows:
About ten grams of soil are weighed into a prepared filter. The soil
should be covered with a piece of paper (a filter) so as to prevent it
from packing when solvents are poured on it. It is now treated with
hydrochloric acid from five-tenths per cent to one per cent strong
(twenty-five and one-third cubic centimeters of strong acid and 808
cubic centimeters of water) to dissolve the lime and magnesia which
prevent the humus from dissolving in the ammonia. Treat with the acid
until there is no reaction for lime; then wash out the acid with water
to neutral reaction. Dissolve the humus with weak ammonia water,
prepared by diluting common saturated ammonia water (178 cubic
centimeters of ammonia to 422 cubic centimeters of water). Evaporate the
humus solution to dryness in a weighed platinum dish at 100°; weigh,
then ignite; the loss of weight gives the weight of humus.
The residue from ignition is carbonated with carbon dioxid, heated and
weighed, thus giving the ash. It is then moistened with nitric acid and
evaporated to dryness. The residue is treated with nitric acid and
water, allowed to stand a few hours, and the solution filtered from the
insoluble residue, which is ignited and weighed, giving the silica.
The soluble phosphoric acid is determined in the solution by the usual
method, as magnesium pyrophosphate. It usually amounts to a fraction,
varying from one-half to as little as one-tenth of the total in the
soil. While the phosphoric acid so determined is manifestly more soluble
and more available to vegetation than the rest of that found by
extraction with stronger acid, it is clearly not as available as that
which, when introduced in the form of superphosphates, exerts such
striking effects even though forming a much smaller percentage of the
whole soil. Nevertheless, very striking agreement with actual practice
is often found in making this determination.
The estimation of humus by combustion, in any form, of the total organic
matter in the soil, gives results varying according to the season, and
having no direct relation to the active humus of the soil. The same
objection lies against extraction with strong caustic lye.
=312. Modification of Grandeau’s Method for Determining Humus in
Soils.=—According to Huston and McBride[200] the function of the
vegetable matter in the soil has long been a matter of contention among
those interested in the science of agriculture. Two factors have
contributed to the uncertainty existing in this matter: First, the very
complex and varying nature of the compounds resulting from the
decomposition of vegetable matter in the soils; and second, the lack of
uniformity in the methods of determining either the total amount of
organic matter present in a soil, or the amount that has been so far
decomposed as to be of any immediate agricultural value. Prominent among
these methods are the methods in which a combustion is resorted to, the
substance being either burned in air or in a combustion tube with some
agent supplying oxygen. The loss on ignition is no measure of the amount
of organic matter present since it is practically impossible to remove
all the water from the soil previous to ignition, and neither of the
methods gives information regarding the extent of the decomposition of
the organic matter. Pure cellulose and the black matter of a fertile
soil are of very different agricultural value.
Determinations of carbon in soils by oxidation with chromic and sulfuric
acid, and with alkaline permanganate have been used. The method with
alkaline permanganate agrees fairly well with combustion with copper
oxid or lead chromate, but the chromic sulfuric acid method gives only
about eighty per cent of the carbon found by combustion processes.
However valuable these processes may be for determining the total carbon
in the soil, they furnish no information regarding the condition of the
carbonaceous soil constituents, and as the determination is really one
of carbon, the organic matter must be calculated by using an arbitrary
factor. Generally the organic matter of the soil is considered to have
fifty-eight per cent carbon; yet different values are given from forty
to seventy-two per cent.
There is a general opinion that the black or dark brown material of the
soil, resulting from the decay of vegetable matter, has a much higher
agricultural value than the undecomposed vegetable matter. No very sharp
dividing line can be drawn, for changes in the soil are continually
going on, and material may be found in almost every stage between pure
cellulose and carbon dioxid. The character of the intermediate products
will vary according to the conditions of tillage and the supply of air
and water.
For agricultural purposes some means of determining the amount of
decomposed matter is very desirable. Several solvents have been tried
for this purpose. The earlier attempts were made by treating the soil
with successive quantities of boiling half-saturated solution of sodium
carbonate until the soil appeared to yield no more coloring matter to
the solvent. The solutions were then united, rendered acid with HCl,
which precipitated the humic acid, which was then washed, dried, and
weighed. This was considered the more soluble portion of the humic acid.
The soil was afterward treated with caustic potash solution in the same
manner, and the humus thus extracted was called insoluble humus. This
last process was really more in the nature of manufacturing humus, for
sawdust treated with caustic potash yields humic acid, and the inert
organic matter in the soil was decomposed to some extent by the caustic
alkali. Neither of the processes provided for the separation of the
humic acid from the lime, magnesia, alumina, and iron with which it is
usually combined in the soil.
In case results of different workers are to be compared, it is of the
greatest importance that methods should be used that are of such a
nature that errors resulting from difference of manipulation, and from
difficulty of reproducing duplicate work can be reduced to a minimum.
Hence, a simple modification of the Grandeau method has been tried which
has the advantage of keeping a definite amount of the soil in contact
with a definite volume of ammonia for a fixed time, the strength of the
ammonia remaining constant.
The process is as follows: The soil is washed with acid and water as
usual. It is then washed into a 500 cubic centimeter cylinder with
ammonia, the cylinder closed and well shaken and allowed to remain for a
definite time, usually thirty-six hours. The material is shaken at
regular intervals. The cylinder is left inclined as much as possible
without having the fluid touch the glass stopper, thus allowing the soil
to settle on the side of the cylinder and exposing a very large surface
to the action of the ammonia. During the last twelve hours the cylinder
is placed in a vertical position to allow the soil to settle well before
taking out the aliquot part of the solution.
The process of washing the soil with hydrochloric acid, water and
ammonia, is very tedious when performed in the usual way with the
wash-bottle. A simple automatic washing apparatus was devised by which a
fixed volume of the washing fluid can be delivered at regular intervals,
giving ample time for the thorough draining between each addition of the
fluid, and requiring no attention. By this apparatus work can be
continued day and night. Instead of washing on the usual form of filter
paper in funnels, it is preferable with this apparatus to hold the soils
on a disk of filter paper resting on a perforated porcelain disk in the
bottom of the funnel. This removes the necessity of washing out the
filter papers, does not permit of the accumulation of humus on the edge
of the filter paper when the Grandeau process is used, and insures that
all the washing fluids pass through the soil and not around it. This
form of apparatus reduces the labor to a minimum and permits many
determinations to be carried on at once.
This form of apparatus was only lately devised and has only been used
long enough to test it and to show its advantages. The reported results
were obtained by the ordinary methods of washing.
In all the work reported, five grams were used, as the soils contained
so much humus that this amount gave enough humus for good work in the
final weighings. The results obtained so far appear in the following
tables:
TABLE I.
COMPARISON OF METHOD OF GRANDEAU WITH HUSTON’S MODIFICATION AND OF
INFLUENCE OF STRENGTH OF AMMONIA SOLUTION. TIME OF DIGESTION IN MODIFIED
METHOD THIRTY-SIX HOURS.
Two per cent Four per cent
NH₃. NH₃.
Grandeau. Huston. Grandeau. Huston.
1. Peat soil, 16.40 20.06
Bogus „ 13.98 20.80
„ „ 17.43
————— —————
Mean 15.94 20.43
2. Peat subsoil, 13.98 19.38
Bogus „ 13.85 20.30
————— —————
Mean 13.92 19.84
3. Peat soil, 9.05 15.60 14.71 21.24
Good „ 10.27 15.88 15.34 20.20
————— ————— ————— —————
Mean 9.61 15.74 15.03 20.72
4. Peat subsoil, 16.75 24.34
Good „ 18.60 23.52
————— —————
Mean 17.68 23.93
5. Black soil, A 3.90 6.90 (1.86) 7.42
„ „ „ (1.67) 6.98
„ „ B 3.88 7.00 4.42
„ „ „ 4.20
————— ————— ————— —————
Mean 3.99 6.95 (3.05) 7.20
„ 4.31
6. Clay loam, 1.86 4.20 2.40 4.26
West side, A 4.28
„ „ B 1.76 4.36 2.48 (3.40)
„ „ „ (3.10)
————— ————— ————— —————
Mean 1.81 4.28 2.44 (3.76)
„ 4.27
7. Clay loam, A 1.90 4.12 (1.60) (4.59)
Lysimeter soil, B 1.61 4.22 (1.41) (4.58)
„ „ C 1.80 4.12
„ „ D 1.95 4.04
„ „ E 1.92 3.85
„ „ F 1.95 4.08
„ „ G 1.90 3.93
„ „ H 1.90 3.80
————— ————— ————— —————
Mean 1.76 4.17 (1.80) (4.12)
„ 1.90 3.97
Seven per cent Eight per cent
NH₃. NH₃.
Grandeau. Huston. Grandeau. Huston.
1. Peat soil,
Bogus „
„ „
Mean
2. Peat subsoil,
Bogus „
Mean
3. Peat soil, 19.77 21.70 16.05 21.42
Good „ 19.85 21.90 15.40 21.80
————— ————— ————— —————
Mean 19.81 21.80 15.73 21.61
4. Peat subsoil,
Good „
Mean
5. Black soil, A
„ „ „
„ „ B
„ „ „
Mean
„
6. Clay loam, 2.14 4.02 1.85 4.12
West side, A
„ „ B 2.13 4.48 1.90 4.40
„ „ „
————— ————— ————— —————
Mean 2.14 4.25 1.88 4.26
„
7. Clay loam, A
Lysimeter soil, B
„ „ C
„ „ D
„ „ E
„ „ F
„ „ G
„ „ H
Mean
„
NOTE.—Numbers in parentheses indicate results, generally the earliest
ones, which the authors do not consider strictly comparable with the
rest of the work. They are given solely for the purpose of exhibiting
all the work that has been done to date. When a mean is included in
parentheses it indicates that it is calculated from all the results
obtained, including those not considered strictly comparable. Bogus is
a name given to a peaty soil which is very sterile.
TABLE II.
INFLUENCE OF TIME OF DIGESTION. FOUR PER CENT OF AMMONIA USED
THROUGHOUT. HUSTON’S METHOD.
Thirty-six Forty-eight Sixty-eight Ninety-eight
hours. hours. hours. hours.
Peat Soil, 21.24 22.28 24.04
Good „ 20.20 21.70 23.94
————— ————— —————
Mean 20.72 21.99 23.99
Clay loam, 4.28 4.00 4.40
„ 4.26 4.01 4.85
West side (3.40)
„ „ (3.05)
—————— ————— ————
Mean 4.27 4.01 4.63
TABLE III.
INFLUENCE OF TIME OF EXTRACTION. TIME, TEN DAYS. GRANDEAU’S METHOD,
FOUR PER CENT AMMONIA. PEAT SOIL.
A. B. Mean. Remarks.
Per Per Per
cent. cent. cent.
1st extraction, 750 cc 16.90 18.96 17.93
2nd „ 250 „ 2.80 2.38 2.59
3rd „ 250 „ 1.77 1.10 1.44
4th „ 250 „ 1.34 1.30 1.32 Stood over night.
5th „ 250 „ 0.89 0.85 0.87
6th „ 250 „ 1.41 1.65 1.53 Stood overnight.
7th „ 250 „ 2.10 1.80 1.95 Washed again with HCl for Ca.
Trace found. HCl washed out,
but trace of chlorids found in
ash. Probably HCl absorbed
from air as humus showed small
quantity of a white volatile
solid on evaporation.
8th „ 250 „ 0.67 0.65 0.66
9th „ 250 „ 0.57 0.50 0.53
———— ————— ————— —————
Total 2750 „ 28.45 29.19 28.82
=313. Summary of Results.=—1. The modified method gives much higher
results than the original method of Grandeau.
2. In the Grandeau method marked irregularities follow a change in the
strength of the ammonia solution. These differences in results bear no
relation to the strength of the solution used. They seem to be errors
due to the difficulty of securing uniform and complete washing of the
soil by the ammonia solution.
In the modified method the change in the strength of the ammonia
solution makes practically no difference in the amount of humus
extracted, except in the case of the peat soil where two per cent
ammonia failed to extract all the humus. But the results show no
considerable increase when the strength is increased to over four per
cent.
3. The factor of time has not been fully investigated, but the results
so far obtained indicate that the time exerts less influence in the
modified than in the Grandeau method.
4. Table III shows that considerable quantities of the peat soil are
still passing into solution in the Grandeau method at the end of ten
days. With ordinary soils this is not true; but in the case of soil No.
5, a black soil, the solutions were colored at the end of a week. On the
peat soil the modified method extracted from ten to fifty per cent more
than the Grandeau, and on the ordinary soil from two to three times as
much humus.
5. In comparing duplicate results by both methods it is found that with
soil No. 3, peat soil, the following differences appear calculated to
percentage of the total amount involved in the determination:
Per cent. Per cent. Per cent. Per cent.
Strength of ammonia 2. 4. 7. 8.
Modified 1.7 5. 1.0 1.8
Grandeau 13.0 4.3 0.5 3.4
Special attention was paid to this point in case of soil No. 7, an
ordinary soil; taking all results into consideration the greatest
difference in percentage of total amount involved was, by the modified
method, nineteen per cent, and by the Grandeau, thirty per cent. In the
set of six special determinations made by both methods to test this
point and which are strictly comparable with each other, the maximum
range was by the modified method 7.8 per cent and by the Grandeau 8.3
per cent of the total amount involved in the determination. From which
it appears that the modified method is on the whole capable of yielding
rather more concordant results than the Grandeau.
=314. Estimation of Free Humic Acids.=—This process, due to Müntz[201]
is essentially that of Huston and McBride. Twenty grams of the soil are
reduced to a fine powder and saturated with fifty cubic centimeters of
concentrated ammonia and allowed to digest two or three days in a warm
place. The volume is then made up to one liter with water, well shaken,
and set aside for one day in order to permit the subsidence of the solid
matter. At the end of this time 500 cubic centimeters of the supernatant
liquor are taken and acidified with hydrochloric acid in order to
precipitate the humic bodies. The humus is collected on a filter, dried
and weighed. It is then ignited and the weight of ash deducted from the
first weight thus giving the actual weight of the humus obtained, free
from mineral matter. This process gives the free humic acids. By
previous treatment of the sample with hydrochloric acid as in the
process of Huston and McBride, the total humus is obtained. The
estimation of the free humic acids is of importance in determining the
quantity of lime or marl which should be added to acid lands.
=315. Humus Method of Von Bemmelén.=[202]—Von Bemmelén obtains the
content of humus by the multiplication of the content of carbon in the
soil by the factor of Wolff; _viz._, 1.724. The estimation of carbon,
water, and of the loss on ignition is conducted in combustion tubes in a
current of oxygen. The nitrogen estimation is carried on according to
the method of Dumas.
In soils containing calcium carbonate the carbon content is derived from
the carbon dioxid taken up by the potash bulbs during combustion (a);
from other carbonates not decomposed on ignition and which are
subsequently determined in the residue by treatment with hydrochloric
acid in a carbon dioxid apparatus, (b) and the total carbon dioxid
derived from the carbonates in the soil (c).
For each estimation from three to five grams of the soil are taken,
because with smaller quantities the errors of analysis too strongly
influence the results. The carbon is then calculated according to the
formula:
Carbon = ³⁄₁₁ (a + b − c).
_The Carbon Dioxid of Carbonates._—It is necessary to expel the carbon
dioxid at ordinary temperatures, because on heating to boiling, carbon
dioxid would be formed from the humus. In a flask, as small as possible,
the soil is treated at ordinary temperature, with dilute sulfuric or
citric acid, the escaping gas dried over sulfuric acid and taken up with
soda-lime. Behind the soda-lime is a small tube filled with pieces of
glass and moistened with sulfuric acid, which retains any moisture taken
out of the soda-lime. A stream of about one liter of air, free from
carbon dioxid, is sufficient to drive out all of the carbon dioxid when
the estimation is made at ordinary temperatures.
A volcanic earth from Deli, which contained five per cent of humus,
gave, at a temperature plus or minus 15°, 0.01 per cent CO₂. At boiling
temperature two analyses gave 0.54 and 0.56 CO₂. This soil contained no
carbonate, and the carbon dioxid found at the boiling temperature, must
have come from the humus substances under the influence of the dilute
acids.
A heavy clay containing 6.9 per cent of humus gave, at plus or minus
15°, 3.60 per cent CO₂; at 100° without boiling, it gave an additional
0.53 per cent, and with boiling an additional 0.11 per cent, or a total
of 4.24 per cent CO₂. A light clay containing 3.2 per cent of humus,
gave, at 15°, 5.09 per cent CO₂; at a boiling temperature an additional
0.43 per cent, and by continued boiling an additional 0.27 per cent.
=316. Estimation of Humus by the German Method.=—The German experiment
stations follow the method of Loges,[203] depending on the oxidation of
the humic bodies with copper oxid after evaporation of the sample with
phosphoric acid. The object of the preliminary evaporation is to set the
humic acids free in order that they may be better and more easily
oxidized than when burned in the combined state.
The sample of soil is placed in a Hoffmeister dish (Schälchen),
moistened with dilute phosphoric acid and evaporated to complete
dryness. The dish and its contents are rubbed up with pulverized copper
oxid and placed in a combustion tube of sixty centimeters in length,
open at both ends. There is then placed in the tube, and held in place
by asbestos plugs, granular copper oxid to a length of twenty
centimeters.
The combustion tube is placed in a proper furnace and one end connected
with two washing-flasks, the first containing potash lye, and the other
a solution of barium hydroxid. These flasks are to free the aspirated
air from carbon dioxid. The other end of the combustion tube is
connected with an appropriate apparatus for absorbing the carbon dioxid.
Loges recommends the Pettenkofer absorption tube and a Fresenius drying
cylinder.
Between the absorption apparatus and the aspirator, is also placed a
washing-flask containing barium hydroxid solution, serving to detect any
unabsorbed carbon dioxid. The layer of granular copper oxid is first
heated, the air being slowly aspirated through the apparatus meanwhile,
but not through the absorption bulbs. All the carbon dioxid is thus
removed from the apparatus.
The absorption system being connected, the tube is heated slowly from
the front, backwards, and after the tube is well heated a slow current
of air is drawn through and continued until the combustion is complete,
which is usually in about three-quarters of an hour.
After the tube is cool the powdered copper oxid and residue of
combustion are removed, and for this reason the tube is stopped with a
cork at both ends instead of being drawn out and sealed at one end. The
tube can thus be refilled without disturbing the granular layer of
copper oxid.
The drying cylinder used between the combustion tube and the absorption
system has its upper part filled with cotton to avoid the deleterious
effects of the nitric oxid produced in the combustion. With this
arrangement the use of metallic copper in the combustion tube to reduce
the nitric oxid can be dispensed with, the moist cotton holding back the
acid fumes. The per cent of humus is obtained by multiplying the per
cent of carbon found by 1.724.
=317. Method of Raulin for the Estimation of Humus.=[204]—The volumetric
estimation of humus in soil by a solution of potassium permanganate
would be convenient and practical if the combustion of the organic
matter were complete, and if the browning of the liquor did not render
the end of the reaction uncertain. The process of Schmidt, modified as
below, has given satisfactory results.