space of forty days 17.4 per cent of the total nitrogen contained in the
soil had been converted into ammonia by dilute potash. According to the
above observations the amid principles transformed into ammonia under
the influence of dilute potash, exist in groups which are acted on with
very unequal rapidity.
2. Reaction with hot dilute solution of potash. Take 200 grams of the
soil sample, mix with one and one-half liters of dilute potash solution
containing fifty grams of potash. Place in a flask and heat on boiling
water-bath for six hours. The flask is furnished with a stopper and
tubes, and a current of pure hydrogen is made to pass through the
liquid, having the double object of preventing any oxidizing effect from
the air and of carrying away the ammonia which may be formed. The
escaping hydrogen and ammonia are passed into a bulb apparatus
containing titrated sulfuric acid.
The sample of soil employed contained in 200 grams, 0.3256 gram of
nitrogen. There was obtained at the end of six hours’ heating, 0.0366
gram of nitrogen. In other words, 11.24 per cent of the total nitrogen
in the sample appeared as ammonia.
_Examination of Residue._—After the separation of the ammonia as above
described, pour the residue in the flask on a filter, wash with hot
water, and determine nitrogen in filtrate and in solid matter on the
filter by combustion with soda-lime. The filtrate is, of course, first
evaporated to dryness after being neutralized with sulfuric acid.
The insoluble part contained 0.041 gram of nitrogen, _i. e._, 12.84 per
cent of the entire amount.
The soluble part contained 0.2411 gram of nitrogen, _i. e._, 74.05 per
cent of the whole.
_Summary of Data._—In the sample analyzed the following data were
obtained:
Of the whole.
Nitrogen as ammonia 11.24 per cent.
„ in insoluble part 12.84 „ „
„ „ soluble part 74.05 „ „
„ not determined 1.87 „ „
——————
Sum 100.00 „ „
The same experiment in which the heating on the water-bath was continued
for thirteen hours gave the following data:
Of the whole.
Nitrogen as ammonia 16.03 per cent.
„ in insoluble part 9.98 „ „
„ „ soluble part 74.01 „ „
——————
Sum 100.00 „ „
_Further Treatment of Matter Insoluble in Hot Dilute Potash._—A portion
of the insoluble portion from the last experiment was treated for
thirteen hours longer under the same conditions with dilute hot potash.
The soluble and insoluble portions were determined as already described.
Of the nitrogen insoluble after thirteen hours, 64.21 per cent remained
insoluble after the second thirteen hours. This fact shows that slow and
progressive decomposition of the alkalamids in the soil occurs under the
influence of hot dilute potash.
_Treatment of Matter Insoluble in Hot Dilute Potash with Hydrochloric
Acid._—A part of the material insoluble in hot potash after thirteen
hours is mixed with dilute hydrochloric acid, in such proportion as to
have one-fifth the weight of pure hydrochloric acid to the dry solid
matter. Heat in flask on a boiling water-bath for thirteen hours.
Determine the nitrogen in the insoluble residue.
_Example_: In the case given it was found that 54.91 per cent of the
nitrogen insoluble in dilute hot potash were dissolved by the hot
hydrochloric acid.
This fact shows that insoluble nitrogen compounds contained in the soil
are dissolved by dilute acids even more readily than by dilute alkalies
at the temperature of boiling water.
Several reactions appear to take place simultaneously when potash is
brought into contact with the nitrogenous principles of arable earth.
Some of these principles, during the first period of the action become
soluble and even form compounds which are not precipitable by acids.
When, however, the action of the potash is prolonged, the dissolved
bodies lose little by little a part of their nitrogen as ammonia or as
soluble alkalamids. They become thus changed either to compounds no
longer soluble in the potash, or to those insoluble in the solution when
acidified. These compounds, it is true, contain nitrogen, but are poorer
in this element and have a higher molecular weight, or, in other words,
are condensation products. These last principles are not absolutely
stable in the presence of potash, but are decomposed much more slowly
than the original principles from which they were derived.
In general, it may be said that under the influence of alkalies on the
nitrogenous principles of the soil there is a tendency to form two
classes of bodies, the one more soluble with a lower molecular weight,
the other less soluble with a higher molecular weight. The inverse
relation between solubility and condensation is in agreement with what
is observed in similar reactions with organic bodies in general. It
certainly plays an important rôle in the transformations which an arable
soil undergoes, either through the mild influences of the air and
natural waters, or the more energetic action of vegetables themselves.
The methods of estimating nitric nitrogen will be made the theme of a
special study in connection with the chapter on nitrification. There
will be considered first, therefore, the methods of determining organic
and ammoniacal nitrogen with only such incidental treatment of the
methods for nitric nitrogen as the processes applicable to the other
forms may contain.
=396. Provisional Methods of the Association of Official Agricultural
Chemists.=[261]—The nitrogen compounds in the soil are usually placed in
three classes.
1. The nitrogen combined with oxygen as nitrates, or nitrites, existing
as soluble salts in the soil.
2. The nitrogen combined with hydrogen as ammonia, or organic nitrogen
easily convertible into ammonia. The ammonia may exist as salts, or may
be occluded by hydrated ferric or aluminum oxids and organic matter in
the soil.
3. The inert nitrogen of the soil or the humus nitrogen.
_Active Soil Nitrogen._—The material proposed for reducing the nitrates
to ammonia, and at the same time to bring ammonia salts and organic
nitrogen into a condition for separation by distillation, is sodium
amalgam. Liquid sodium amalgam may be readily prepared by placing 100
cubic centimeters of mercury in a flask of half a liter capacity,
covering the warmed mercury with melted paraffin, and dropping into the
flask at short intervals pieces of metallic sodium, the size of a large
pea (taking care that the violence of the reaction does not project the
contents from the flask), till 6.75 grams of sodium have combined with
the mercury. The amalgam contains one-half per cent of sodium and may be
preserved indefinitely under the covering of paraffin. To estimate the
active soil nitrogen, weigh fifty grams of air-dried soil and place it
in a clean mortar. Take 200 cubic centimeters of ammonia-free distilled
water, rub up the soil with a part of the water to a smooth paste,
transfer this to a flask of one liter capacity, washing the last traces
of the soil into the flask with the rest of the water. Add twenty-five
cubic centimeters of the liquid sodium amalgam and shake the flask so as
to break the sodium amalgam into small globules distributed through the
soil. Insert a stopper with a valve and set aside in a cool place for
twenty-four hours. Pour into the flask fifty cubic centimeters of milk
of lime, and distill, on a sand-bath, 100 cubic centimeters into a flask
containing twenty cubic centimeters of decinormal sulfuric acid, and
titrate with decinormal soda solution, using dimethyl-orange as
indicator. Estimate the nitrogen of the ammonia found as active soil
nitrogen.
If the ammonia produced is too small in amount to be readily estimated
volumetrically, determine the ammonia by nesslerizing the distillate.
_Estimation of Nitrates in the Soil._—When it is desired to estimate
separately the nitrates in the soil the following method may be used:
Evaporate 100 cubic centimeters of the soil extract to dryness on the
water-bath, dissolve the soluble portion, of the residue in 100 cubic
centimeters of ammonia-free distilled water, filtering out any insoluble
residue, place the solution in a flask and add ten cubic centimeters of
liquid sodium amalgam, insert stopper with valve, set it aside to digest
in a cool place for twenty-four hours, add fifty cubic centimeters of
milk of lime, distill and titrate as above, and estimate the nitrogen as
N₂O₅.
Nesslerizing may be substituted for titration when the amount of
nitrates is small.
An approximate estimation of the amount of nitrates will be of value in
determining which method of estimation to use. This may be done by
evaporating a measured quantity of the soil extract, say five cubic
centimeters, on a porcelain crucible cover on a steam-bath or radiator,
having first dissolved a minute fragment of pure brucin sulfate in the
soil extract. When dry pour over the residue concentrated sulfuric acid,
free from nitrates, and observe the color reactions produced.
If the nitrate (reckoned as KNO₃) left upon evaporating the quantity of
water taken does not exceed the two-thousandth part of a milligram, only
a pink color will be developed by adding the sulfuric acid; with the
three-thousandth part of a milligram, a pink with faint reddish lines;
with the four-thousandth part, a reddish color; with the five-thousandth
part, a red color.
By increasing or diminishing the amount of soil extract evaporated to
secure a color reaction of a certain intensity, an approximate estimate
may be made of the amount of nitrates present.
Blank experiments to test the acid and the brucin sulfate will be
required before confidence can be placed in such estimations.
_Total Nitrogen of Soils._—The total nitrogen of soils may be determined
by the usual combustion with soda-lime, but this process is often
unsatisfactory because of the large amount of material required when the
organic matter or humus is small in amount.
A modification of the kjeldahl method is more easy to carry out and
gives results equally satisfactory. Place twenty grams of soil in a
kjeldahl flask, and add twenty cubic centimeters of sulfuric acid (free
from ammonia) holding in solution one gram of salicylic acid. If the
soil contain much lime or magnesia in the form of carbonate, enough more
sulfuric acid must be added to secure a strongly acid condition of the
contents of the flask. Add gradually two grams of zinc dust, shaking the
contents of the flask to secure intimate mixture. Place the flask in a
sand-bath and heat till the acid boils, and maintain the boiling for ten
minutes. Add one gram of mercury and continue the boiling for one hour,
adding ten cubic centimeters of sulfuric acid if the contents of the
flask are likely to become solid. Cool the flask and wash out the
soluble materials with 200 cubic centimeters of pure water, leaving the
heavy earthy materials. Rinse the residue with 100 cubic centimeters of
water, and add this to the first washings. Place this soluble acid
extract in a liter digestion flask, add thirty-five cubic centimeters of
a solution of potassium sulfid, and shake the flask to secure intimate
mixture of the contents. Introduce a few fragments of granulated zinc,
pour in seventy-five cubic centimeters of a saturated solution of
caustic soda, connect the flask with a condenser and distill 150 cubic
centimeters into a flask containing twenty cubic centimeters of acid,
using the same acid and alkali for titration used in the kjeldahl method
under fertilizers.
Enter the nitrogen found in this operation as total soil nitrogen.
The difference between the total soil nitrogen and the active soil
nitrogen will express the inert nitrogen of the soil.
=397. Hilgard’s Method.=[262]—The humus determination will, in the case
of virgin soils, usually indicate approximately the store of nitrogen in
the soil, which must be gradually made available by nitrification.
Ordinarily (outside of the arid regions) the determination of ammonia
and nitrates present in the soil is of little interest for general
purposes, since these factors will vary with the season and from day to
day. Kedzie proposes to estimate the active soil nitrogen (ammonia plus
nitrates and nitrites) by treatment of the whole soil with sodium
amalgam and distillation with lime. The objection to this process is
that the formation of ammonia by the reaction of the alkali and lime
upon the humus amids would greatly exaggerate the active nitrogen and
lead to a serious overestimate of the soil’s immediate resources.
The usual content of nitrogen in black soil-humus is from six to eight
per cent in the regions of summer rains. From late determinations it
would seem that in the arid regions the usually small amount of humus
(often less than two-tenths per cent) is materially compensated by a
higher nitrogen percentage. It thus becomes necessary to determine the
humus nitrogen directly; and this is easily done by substituting in the
grandeau process of humus extraction potash or soda-lye for ammonia
water, and determining the nitrogen by the kjeldahl method in the
filtrate.
The lye used should have the strength of four per cent in the case of
potassium hydroxid, three per cent in that of sodium hydroxid. The black
humus filtrate is carefully neutralized with sulfuric acid, evaporated
to a small bulk in a beaker or evaporating basin, and the reduced liquid
finally evaporated to dryness in the kjeldahl flask itself by means of a
current of air. The beaker or basin is washed either with some of the
alkaline lye, or, after evaporation, with warm concentrated sulfuric
acid, which is then used in the nitrogen determination in the usual way.
For the determination of nitrates in the soil it is, of course, usually
necessary to use large amounts of material, say not less than 100 grams,
and, according to circumstances, five or more times that amount. In the
evaporated solution the nitric acid is best determined by the reduction
method, as ammonia.
Usually the soil filtrate is clear and contains no appreciable amount of
organic matter that would interfere with the determination; yet in the
case of alkaline soils (impregnated with sodium carbonate) a very dark
colored solution may be obtained. In that case the soil may
advantageously be mixed with a few per cent of powdered gypsum before
leaching; or the gypsum may be used in the filtrate to discolor it by
the decomposition of sodium carbonate and the precipitation of calcium
humate. The evaporated filtrate can then be used for the nitrate
determination by either the kjeldahl, griess, or the nessler process,
which will, of course, include such portions of the ammoniacal salts as
may have been leached out.
For the separate determination of these and of the occluded ammonia,
when desired, it is probably best to mix the wetted soil intimately with
about ten per cent of magnesium oxid and distill into titrated
hydrochloric acid. For general purposes, however, this determination is
usually of little interest.
=398. Müller’s Modified Kjeldahl Method.=—Numerous difficulties, as
stated by Müller,[263] have attended the attempts to apply the kjeldahl
method for the estimation of nitrogen to samples of soil, and he has
modified the method to some extent and made comparisons of the quantity
of nitrogen by this modified method and by the soda-lime method.
The principal difficulty encountered by him has been in the regular
heating of the mixture of fuming sulfuric acid and soil. The particles
of soil are deposited at the bottom of the flask and the result is that
the bottom layers become overheated, and, being poor conductors of heat,
fail to transmit a sufficient quantity of heat to penetrate to the upper
layers of the liquid to complete the reaction. In order to avoid this
difficulty Müller heats his flask in a small stove formed with a
straight vertical cylinder of iron or copper, the upper end of which is
covered with a sheet of iron pierced with a hole which allows the neck
of the flask to pass through, while the lower end is closed with a piece
of sheet iron furnished on its upper surface with a layer of asbestos.
This cylinder of metal is surrounded with a second one, concentric with
the first through which passes a current of heated gases furnished by an
ordinary bunsen. By heating the flask in this stove or furnace an even
distribution of the heat is secured to all parts of the mixture, but the
little drops of sulfuric acid, which are condensed on the cold part of
the neck, sometimes lead to the fracture of the glass as they run down
the sides of the flask to the hot portions. To prevent the reflux of
this condensed acid, which only needs to be done near the end of the
reaction, when it is necessary to heat to a very high temperature, the
neck of the flask is bent at the point immediately above its emergence
at the upper surface of the furnace, and carried into a flask of about
seventy-five cubic centimeters capacity, which will receive the drops of
sulfuric acid condensed during the operation. The furnace has the
following dimensions; height, twelve centimeters; diameter of interior
cylinder, five and one-half centimeters; diameter of exterior cylinder,
seven and one-half centimeters.
It is supported on a triangle of large iron wire and is heated by an
ordinary bunsen, or by a concentric bunsen, according to the temperature
which it is necessary to obtain. The proportions which should be
observed between the amount of earth employed and the sulfuric acid are
about as follows:
Of the dry earth, fifteen grams; of the fuming sulfuric acid, thirty
cubic centimeters. There should also be added to the mixture about
three-tenths of a gram of pure stearic acid, or better, benzoic acid.
When the soil to be analyzed does not contain carbonate, the sulfuric
acid should be added in two portions. At first add about twenty cubic
centimeters of the acid, and after shaking it, the other ten cubic
centimeters, running it in from a burette or a pipette in such a manner
as to wash thoroughly the neck and sides of the flask. If the earth
contain carbonate, however, it is necessary to add the fuming acid in
small portions of about five cubic centimeters at a time, waiting each
time until the disengagement of the gas caused by the previous addition
has ceased. A soil which contains from thirty to forty per cent of
calcium carbonate should be carefully treated in a porcelain capsule
with a slight excess of sulfuric acid, pure and dilute. The mixture is
afterward to be evaporated to dryness upon a sand-bath and the residue
heated in a drying oven to 110°. The mass is then pulverized, introduced
into the flask, treated with three-tenths of a gram of benzoic acid and
thirty cubic centimeters of fuming sulfuric acid, and treated as
indicated above. In all cases it is necessary to continue the heating
until the contents of the flask are colorless.
With soils containing considerable quantities of iron, however, a slight
red color will probably be observed which will not interfere with the
accuracy of the tests.
The heating should at first be gentle and the temperature afterward
elevated little by little, and finally the heat should be sufficiently
great to distill about one and one-half cubic centimeters of sulfuric
acid. The operation lasts from twelve to thirteen hours. As the reaction
is terminated the cooled mass is taken up with water absolutely free
from ammonia. It is filtered into a flask, and washed upon the filter
until the volume of the filtered liquid is about 350 cubic centimeters.
Afterward an excess of soda-lye, at 50° baumé is added, then a few
pieces of quartz to facilitate boiling. The flask is then connected with
a condenser, the liquid distilled and received in a conical flask closed
by a cork having two holes, of which one permits the entrance of the end
of the condenser, and the other a glass tube which is connected with a
small flask containing water, the neck of the receiving flask being
inclined toward the condenser to avoid the entrainment of any of the
alkaline liquid which may be distilled. The receiving flask rests upon
two or three pieces of sheet iron and is heated with an ordinary burner,
and ebullition is perfectly regular. From 170 to 180 cubic centimeters
of the liquid are distilled in from three and one-half to four hours.
The distilled liquid, treated with a few drops of litmus, is titrated by
a solution of sulfuric or hydrochloric acid, of which one cubic
centimeter corresponds to 0.001 cubic centimeter of nitrogen.
=399. Modification of the Kjeldahl Method by Arnold and
Wedemeyer.=[264]—For the oxidizing liquid a mixture of three grams of
benzoic acid with forty cubic centimeters of H₂SO₄ is employed. After
placing in the digestion flask with the nitrogenous body the whole is
gently shaken for a few minutes to prevent clotting. The temperature is
then raised until acid vapors begin to come off, when one gram of copper
sulfate and one gram of mercuric oxid are added; and after ten to
fifteen minutes, to avoid foaming, ten to twenty grams of potassium
sulfate. The sublimate noticed on the walls of the flask is benzoic acid
and does not interfere with the accuracy of the determination.
This method has given good results with the alkaline nitrates, the
nitrates of barium, mercury, silver, lead, and with strychnia, ammonia,
pyridin, azobenzol, dinitrobenzol, and picric acid.
=400. Prevention of Bumping During Distillation.=—Daffert has employed
the modified kjeldahl method, but found considerable difficulty in using
the same owing to the violent bumping of the liquid in the distillation.
This was especially the case where the sample contained a large
proportion of sand. To overcome this annoyance and danger he devised the
following process:[265]
Fit into the mouth of a large-mouthed distillation flask a stopper
having two perforations. Through one of the perforations pass the usual
distillation tube, through the other a similar tube connected with a
supply of steam. Bring the contents to a brisk boil, after which a small
current of steam is turned on, allowing the same to pass in a small
stream throughout the distillation. By this means, not only is all
danger from bumping avoided, but the time required for the distillation
shortened. By the old method it usually requires from fifteen to twenty
minutes, whereas the former requires from six to ten minutes.
It is advisable to filter all samples of soils having a large proportion
of sand.
=401. Determination of Organic Nitrogen by the Soda-Lime Method.=—In the
description of the method following, the directions of the French
Commission of Agricultural Chemists have been taken as the basis of the
analytical process.[266] This method is, in this country, almost
superseded by the moist combustion process with sulfuric acid. By reason
of its long use, however, and because it is still regarded as the best
method by the agricultural chemists of France, Italy, and England, it
merits a full description. It is recommended also by Berthelot and
André,[267] by the International Congress of Chemists, held in Paris in
1889, by the Italian chemists, and by the official Belgian method,[268]
in all cases where nitrates are not present in notable quantities.
The nitrogen which is found in soils in the organic state is transformed
into ammonia when it is heated with soda-lime. This reaction is the base
of the process of analysis which has so long been used for this class of
bodies. The analytical process is conducted as follows: