hydrochloric acid, and the hydrogen sulfid produced is made to pass
through an acidulated solution of copper sulfate in such a way as to
transform the sulfur in the hydrogen sulfid into a sulfid, which is
afterwards collected and weighed in the usual way. The use of a titrated
solution of iodin is not advisable on account of the organic matter
which may be present.
_Estimation of Sulfur in Ethereal Compounds._—These compounds can be
decomposed by boiling with a solution of potash or concentrated
hydrochloric acid. The resulting sulfuric acid is precipitated with
barium chlorid. Subtract from the sulfates thus obtained those
pre-existing as sulfates; the difference represents the sulfur present
in ethers.
_Estimation of Sulfur in Other Organic Compounds._—This is estimated
indirectly by subtracting from the total sulfur that present as
sulfates, sulfids, and ethers.
=386. Method of Von Bemmelén.=—As Von Bemmelén[251] observes, the
estimation of sulfuric acid in soils presents a number of difficulties.
A small part of it can be present as sulfate insoluble in water. In
addition to this, there is always some sulfur in the organic bodies
present. If the soil is extracted with water then the sulfuric acid can
be estimated therein when only a trace of humus substance has gone into
solution. On the contrary, if there is much humus substance in solution,
and also iron oxid, as is the case when the extraction is made with
hydrochloric acid, then both of these must be removed, otherwise the
estimation is very inexact. By fusing the residue of the solution with
sodium carbonate and a little potassium nitrate the organic substance is
destroyed, and after treatment with water the iron oxid is separated. If
any sulfur has been dissolved in the organic substance present, this is
then oxidized to sulfuric acid. The estimation of the sulfuric acid and
of the sulfur, therefore, remains unsatisfactory.
In a sample of clay from Java, which was rich in calcium carbonate, but
which contained no basic iron sulfate, there was found the following
percentages of sulfuric acid:
Exhausted in the cold with very weak hydrochloric acid, 0.04 per cent;
the residue treated in the cold with concentrated hydrochloric acid, the
solution evaporated and fused with sodium carbonate and potassium
nitrate, 0.07 per cent; again, the residue treated with aqua regia to
oxidize the sulfur, the solution evaporated to dryness, fused with
sodium carbonate and potassium nitrate, 0.14 per cent; in all 0.25
percent. A sample of the same soil treated directly with aqua regia, and
then evaporated and fused as above, gave two-tenths per cent sulfuric
acid. A sample of the same soil ignited in a crucible with sodium
carbonate and potassium nitrate gave 0.16 per cent of sulfuric acid. The
difference between 0.04 and 0.07 per cent can be attributed to the
sulfur in the organic substance which was dissolved by the concentrated
hydrochloric acid; the quantity, however, is too small to draw any safe
conclusion. Possibly it might have been that the very dilute
hydrochloric acid did not dissolve all of the sulfate. The quantity of
sulfur combined in the organic substance in the above soil may be
derived from the following equation; _viz._, (0.2–0.07)/(80) × 32 = 0.05
per cent of sulfur.
The estimation of the sulfur in a sample of soil from Deli was carried
on with still greater exactness by three different methods.
The quantities of hydrochloric acid, nitric acid, and sodium carbonate
employed were measured or weighed, and the minute content of sulfuric
acid therein estimated and subtracted from the final results. The
methods employed were as follows:
(A) Extraction with water and afterwards with very dilute hydrochloric
acid.
(B) Extraction with cold hydrochloric acid, one part to three of water.
(C) Extraction with aqua regia.
(D) Ignition with sodium carbonate and potassium nitrate.
(F) Ignition in a combustion tube with sodium carbonate in a stream of
oxygen.
The percentages of sulfuric acid obtained by the different methods were
as follows:
(A) 0.058 per cent.
(B) 0.070 „ „
(C) 0.140 „ „
(D) 0.125 „ „
(F) 0.106 „ „
=387. Method of Wolff.=—In regard to the sulfuric acid Wolff calls
attention to the fact that in soils which have been ignited, a larger
quantity of this acid is found than in soils containing humus.[252]
This, doubtless, arises from the oxidation of the organic sulfur. The
following special method for determining the sulfuric acid is therefore
proposed:
Fifty grams of fresh air-dried soil are placed in a platinum dish with a
concentrated solution of pure sodium nitrate. After drying, the heat is
raised gradually to redness. In this way the complete ignition of the
humus present takes place. After cooling, the mass is diluted with
hydrochloric acid, with the addition of a little nitric acid, and
boiled. In the solution, the silicic acid is first separated and the
sulfuric acid estimated in the usual way with barium sulfate.
=388. Method of the Italian Chemists.=—The determination of the sulfuric
acid is conducted as follows by the Italian chemists:[253] The soil is
completely extracted by diluted hydrochloric acid and the sulfuric acid
precipitated in the solution with barium chlorid. If a soil is very rich
in calcium sulfate it should first be treated with a warm solution of
sodium carbonate to decompose the calcium sulfate, and the sulfuric acid
determined in the solution after having added hydrochloric acid.
=389. Estimation of the Chlorin.=—The estimation of the chlorin is of
great importance in certain cases. When this element is lacking in the
soil, which, however, is rare, certain plants appear to suffer from its
absence. The quality of the forage plants in particular is influenced by
it; but when the chlorids are too abundant, which is a frequent case,
they prevent or arrest completely the progress of vegetation. Salty
soils are, in general, completely sterile. In the proportion of one
pound in a thousand in the earth, sodium chlorid is to be regarded as
injurious. It is necessary, therefore, in analysis to take account of
two cases; _viz._, those of soils poor in chlorids and those of soils
rich in chlorids.
For soils poor in chlorids the French method directs that[254] 200 grams
of the earth are to be washed on a filter with boiling water. The liquor
is evaporated to dryness and gently heated to a temperature inferior to
redness in order to destroy the organic matter. The residue is taken up
by small quantities of water and to the filtered liquor the volume of
which should not exceed forty to fifty cubic centimeters are added ten
cubic centimeters of pure nitric acid and a sufficient quantity of
silver nitrate to produce a complete precipitation. The precipitate is
vigorously shaken and allowed to stand for a few hours in a darkened
locality. The precipitate is collected upon a double filter and the
silver chlorid, after proper desiccation, is weighed.
When the soil is rich in chlorids it is washed as has just been
described upon a filter. The wash-waters are made up to one liter and
fifty cubic centimeters, equivalent to ten grams of the soil, are taken
for analysis. This quantity is treated exactly as described above.
=390. Wolff’s Method of Estimating Chlorin in Soils.=[255]—Three hundred
grams of the soil are treated with 900 cubic centimeters of pure water
containing a little nitric acid for forty-eight hours with frequent
shaking. Four hundred and fifty cubic centimeters are then filtered and
the clear liquid evaporated to 200 cubic centimeters. The chlorin is
then precipitated with silver nitrate. The quantity obtained,
corresponds to that found in 150 grams of the air-dried soil.
A second method, Mohr’s, is as follows: Fifty grams of the soil are
placed in a platinum dish and moistened with a concentrated solution of
potassium nitrate, free from chlorin. The mass is evaporated to dryness
and gradually heated to a red heat. After cooling it is moistened with
water and washed into a beaker and the solid mass quickly separated. The
clear liquid is poured off and the residue again washed with water. The
clear liquid obtained is saturated with acetic acid, carefully
evaporated to dryness and after solution in water, filtration and the
addition of a little nitric acid, the chlorin therein is precipitated by
a silver nitrate solution, and the precipitate collected and weighed as
usual.
=391. Method of Petermann.=[256]—Chlorin in the soil is estimated at the
Gembloux station by digesting 1,000 grams of the sample with two liters
of distilled water with frequent shaking for thirty-six hours. After
allowing to stand for twelve hours with the addition of one gram of
powdered magnesium sulfate to facilitate the deposition of suspended
matter one liter of liquid is siphoned and evaporated in a platinum dish
with the addition of a few drops of a solution of potassium carbonate
free from chlorin and nitric acid. The concentrated solution is
filtered, washed, and made up to 250 cubic centimeters. Take 100 cubic
centimeters of the solution add some nitric acid and precipitate the
chlorin with silver nitrate. The rest of the solution is reserved for
the estimation of nitrate.
=392. Estimation of Silicic Acid.=—_Direct Estimation._—The sample of
soil in the method of Berthelot and André[257] is mixed with two or
three times its weight of pure sodium carbonate and fused in a silver
crucible until complete decomposition has taken place. The residue is
dissolved in water and dilute hydrochloric acid. The silicates are
decomposed by this treatment and the solution is evaporated to dryness
on the water-bath, and when dry slightly heated. The silicic acid
(silica) is by this treatment rendered insoluble. It is collected on a
filter, washed, ignited, and weighed. The resulting compound should be
mixed with ammonium fluorid and sulfuric acid, and after the
disappearance of the silica the residue should be dried and weighed. The
loss in weight represents the true silica. The loss in weight should be
corrected by calculating the sulfates of the alkalies back to oxids.
This correction can be neglected when the work has been carefully done,
and the washing of the original silica has been well performed.
_Indirect Estimation._—The total silica may be estimated indirectly by
subtracting from the total weight of the sample the sum of the weights
of the other constituents resulting from the separate estimation of each
of them after decomposing the sample with hydrofluoric acid.
=393. Simultaneous Estimation of Different Elements.=—The operations and
processes for the estimation of each of the elements have been
described, but it is often best to carry on an operation in such a way
as to gain time by making a single decomposition upon a quantity of soil
of some considerable magnitude, and using the results of the solution
for the determination of the different substances. From the operations
already described it will be easy to make a combination of methods by
which all or nearly all the important constituents in a soil may be
determined in a single sample. Of the various methods proposed, that of
the commission of the French agricultural chemists may be taken as a
type.[258] In the case of the estimation of lime, potash, magnesia, and
sulfuric acid, in which the operation is carried on in a soil which is
not incinerated, time may be saved by digesting a considerable quantity
of the soil with concentrated nitric acid for a period of five hours. It
is best to take 100 grams of the soil and increase proportionally the
nitric acid. The filtrate, after washing, is made up to one liter and
thoroughly shaken. From this amount of liquid, portions are taken
corresponding to the weights of soil upon which the operation for the
determination of each of the constituents would be conducted. For
example, for the estimation of lime in the case of a very calcareous
earth, ten cubic centimeters representing one gram of the original
sample, in the case of a soil poor in carbonates 100 cubic centimeters
representing ten grams, and for the estimation of potash, magnesia, and
sulfuric acid 200 cubic centimeters, representing twenty grams of the
soil, should be taken. This method avoids frequent weighings of the
earth and separate treatments thereof by the acid.
On the other hand, in the same portion of the solution, the different
elements can be estimated. For example, for the estimation of the potash
as has been indicated, in the place of precipitating as a whole the
sulfuric acid, lime, etc., and of afterwards separating the magnesia in
the sole aim of eliminating these bodies, they can be collected
separately and weighed, thus securing at a single operation several
determinations.
At the end, some barium chlorid is added and if the barium sulfate is
then collected and weighed, the estimation of the sulfuric acid is
effected. To the filtrate there are afterwards added some ammonia and
ammonium carbonate to precipitate, at once, the excess of barium and the
iron and aluminum oxids, the lime and the phosphoric acid. This
separation being effected the filtrate contains still the magnesia and
the alkalies. The first can be separated by carbonation by means of
oxalic acid, collected, and weighed. Finally the potash itself can be
estimated in the state of perchlorate. It has thus been possible in the
same suite of operations to estimate in a given quantity of the liquid,
the sulfuric acid, the magnesia, the lime, and the potash.
=394. Estimation of Kaolin in Soils.=—True kaolin is a hydrated aluminum
silicate, having the formula H₄Al₂Si₂O₉. This substance is, even in
concentrated hydrochloric acid, almost completely insoluble. It
contains, theoretically, 13.94 per cent of water of combination. The
following methods, due to Sachsse and Becker,[259] can be used for its
determination.
_Estimation of the Water of Combination._—Heat from one to two grams of
kaolin, dried at 100°, for half an hour in a covered platinum crucible
to a temperature which shows an incipient red heat when the crucible is
partly protected from the daylight with the hand. This treatment does
not quite give the whole of the water of combination but nearly all of
it. A kaolin is changed by this treatment into a substance which is
easily soluble in dilute hydrochloric acid.
_Estimation of the Kaolin in Impure Kaolins._—Mineral kaolin, or the
kaolin obtained by silt analysis, is dried at 100° to constant weight.
It is then heated with strong hydrochloric acid until all the matters
which will pass into solution have been dissolved. The residual kaolin
is then washed thoroughly with water and ignited for half an hour at a
low red heat. The residual mass is a second time extracted with
hydrochloric acid until silica no longer passes into solution. The
soluble silica is then estimated in the usual way and calculated to
kaolin. The result will give the pure kaolin in the sample examined.
The estimation may also be made as follows: Two samples of the impure
kaolin are taken and dried to constant weight at 100°. One is extracted
with hydrochloric acid in the manner described above and the amount of
silica determined. The second is treated directly by ignition to low
redness for half an hour, dissolved in hydrochloric acid and the amount
of silica determined. The difference in the two percentages of silica
corresponds to the silica equivalent to the pure kaolin.
_Statement of Results._—It is convenient to incorporate the data
obtained by the above methods with the complete mass analysis of the
silicate examined. In the sample given below the analysis was made on a
clay silt obtained with a velocity of two-tenths millimeter per second.
The mass analysis gave the following data:
Loss on ignition 10.04
SiO₂ 51.52
Al₂O₃ 17.93
Fe₂O₃ 7.42
CaO 1.57
MgO 6.27
K₂O 4.1
Na₂O 1.61
The loss on ignition was made up of the combined water and a trace of
humus. On gentle ignition only 7.52 per cent of water came off.
The examination of the non-ignited and the gently ignited silica by
means of dilute hydrochloric acid, gave the following data:
Non-ignited. Gently ignited. Difference.
Water 10.04 10.04
Insoluble residue 40. 34.54 –5.46
Al₂O₃ 9.04 10. +0.96
Fe₂O₃ 5.96 7.27 +1.31
SiO₂ 25.27 28. +2.73
Alkalies and alkaline earths 9.69 10.15 +0.46
By a comparison of these data with those obtained by the mass analysis,
the following representation of the distribution of the various
components in the clay is obtained:
23.52 per cent SiO₂ in the form of quartz and undecomposed silicates.
2.73 per cent SiO₂ in the form of kaolin.
25.27 per cent in the form of easily decomposable silicates and of the
hydrates of SiO₂.
7.93 per cent Al₂O₃ in the form of undecomposed silicates.
0.96 per cent Al₂O₃ in the form of kaolin.
9.04 per cent Al₂O₃ in the form of easily decomposed silicates and of
hydrates.
0.15 per cent Fe₂O₃ in the form of undecomposed silicates.
1.31 per cent Fe₂O₃ in the form of kaolin.
5.96 per cent Fe₂O₃ in the form of easily decomposable silicates and
hydrates.
3.40 per cent of alkalies and alkaline earths in the form of
undecomposed silicates.
9.69 per cent of alkalies and alkaline earths in the form of easily
decomposable silicates.
10.04 per cent of water, including a trace of humus.
Collecting these results the following statement is obtained.
The clay analyzed contained:
10.04 per cent of water, a trace of humus.
35.15 per cent of undecomposed silicates and quartz.
5.00 per cent of kaolin.
50.27 per cent of easily decomposable silicates, hydrates of SiO₂ and
hydroxids.
ESTIMATION OF NITROGEN IN SOILS.
=395. Introductory Considerations.=—The great economic and biologic
value of nitrogen as a plant food renders its estimation in soils of
especial importance. It is necessary, first of all, to remember that the
nitrogen present in soils may be found in three forms; _viz._, first, in
organic compounds, second, as ammonia, and third, as nitric or nitrous
acid. Further than this each of these classes of nitrogen may be
subdivided. The organic nitrogen may be in a form easily nitrified and
rendered available for plant food, or it may be inert and resistant to
nitrification, as in humus, or exist in an amid state. The ammoniacal
nitrogen may exist in small quantities as gaseous ammonia, or be
combined with mineral or organic acids. As nitric or nitrous acid the
nitrogen will be found combined with bases, or perhaps in minute
quantities as free acid, in passing under the influence of the
nitrifying ferment from the organic to the inorganic state. To the
latter state it must finally come before it is suited to absorption by
plants.
In general, far the largest part of soil nitrogen, excluding the
atmosphere diffused in the pores of the soil, is found in the organic
state and is derived from the débris of animal and vegetable life and
from added fertilizers. As ammonia, the nitrogen can only be regarded as
in a transition state, arising from the processes of decay, or
incomplete nitrification. As nitric acid, it is found as a completed
product of nitrification, or as the result of electrical action. The
processes of nitrification and the isolation and determination of the
nitrifying organisms will be considered in a special chapter of this
manual. By reason of the great solubility of the nitrates, and the
inability of the soil to retain them, there can never be a great
accumulation of nitric acid in the soil save in localities deficient in
rain-fall or in specially protected spots, such as caves. The nitric
acid, therefore, produced in the soil passes at once into growing
vegetation, or is found eventually in the drainage waters.
The formation of ammonia in soil containing much vegetable matter is
thought by Berthelot and André[260] to be due to the progressive
decomposition of amid principles under the influence of dilute acids or
alkalies, either in the cold or at an elevated temperature. Soils of the
above description, of themselves, contain neither free ammonia nor
ammoniacal salts, and the ammonia which is found in the analysis of
these soils comes from the reaction above indicated. The ammonia which
comes from these soils, in place of what is given off to the surrounding
atmosphere, comes from the same class of decompositions, and these
decompositions, in this case, are effected by the water itself, and by
the alkaline carbonates of the soil. The amid principles which are thus
decomposed belong either to the class of amids proper, derived by the
displacement of hydrogen in ammonia by acids, or to the class of
alkalamids derived from nitrogenous bases, both volatile and fixed.
Among these alkalamids some are soluble in water and some insoluble, and
the decomposition of these last by acids or by alkalies may furnish
bodies which themselves are either soluble or insoluble in water.
To determine the nature of the nitrogenous principles in a soil rather
rich in humus, Berthelot and André applied the following treatment:
A soil containing 19.1 grams of carbon and 1.67 grams of nitrogen per
kilogram was first subjected to treatment, at room temperature, with a
concentrated solution of potash. By this treatment 17.4 per cent of the
nitrogen content was set free under the form of ammonia. One-quarter of
this was obtained during the first three days; one-eighth during the
next three days. Afterward the action became much more feeble and was
continued during forty days longer, and the evolution of the gas was
diminished almost proportionately to the time. It appears from the above
observations that the amid principles of the soil, decomposable by
potash, belong to two distinct groups, which are broken up with very
unequal velocities. The soil, treated on the water-bath for three hours
at 100° with strong potash, showed the following behavior in respect of
its nitrogenous constituents: Nitrogen eliminated in the form of
ammonia, sixteen per cent; nitrogen remaining in the part soluble in
potash, ten per cent; nitrogen remaining in the part insoluble in
potash, seventy-four per cent.
_Treatment with Acid._—The nitrogenous compounds of the soil are also
decomposed by dilute acids, and often more rapidly than by the alkalies.
The method of treatment is substantially the same as that set forth
above. The decompositions effected either by alkalies or by acids tend
in general to lower the molecular weights of the resulting products. The
prolonged action of alkalies at the temperature of boiling water
rendered soluble, after twenty-four hours of treatment, 93.6 per cent of
the organic nitrogen found in the vegetable mould. By treating the earth
successively with alkalies and acids 95.5 per cent of the total nitrogen
were decomposed. These experiments show how the insoluble nitrogen in
humic compounds can be gradually rendered assimilable. The action of
vegetables is not assuredly identical with those which acids and
alkalies exercise. However, both present certain degrees of comparison
from the point of view of the mechanisms set in play by the earthy
carbonates and carbon dioxid, as well as by the acids formed by
vegetation. The reactions which take place naturally, while they are not
so violent as those produced in the laboratory, make up by their
duration what they lack in intensity.
For a more detailed study of the nature of the nitrogenous elements in
soil the following method of treatment, due to Berthelot and André, is
recommended:
_Treatment of the Soil with Alkalies._—1. Reaction with cold, dilute
solution of potash. Take fifty grams of the sample, dried at 110°, and
mix with a large excess of ten per cent potash solution and place under
a bell-jar containing standard sulfuric acid. The mixture is left for a
long time in order to secure as fully as possible the ammonia set free.
Example: Fifty grams of a soil contained 0.0814 gram of nitrogen.
Treated as above it gave the following quantities of nitrogen as
ammonia:
Nitrogen as Ammonia.
After 3 days 0.0034 gram.
„ 6 „ 0.0054 „
„ 11 „ 0.0065 „
„ 17 „ 0.0078 „
„ 25 „ 0.0093 „
„ 41 „ 0.0107 „
„ 46 „ 0.0141 „