centimeters, are introduced ten cubic centimeters of a solution of
manganese sulfate containing sixteen grams of the anhydrous salt per
liter, and ten cubic centimeters of a ten per cent solution of potassium
permanganate. The solution is heated for a few minutes, the liquor is
decolorized and manganese bronze is precipitated. One hundred cubic
centimeters of water are added, and four cubic centimeters of sulfuric
acid containing 150 cubic centimeters of monohydrated acid per liter.
There is now added an exactly measured volume of the humic liquid
properly prepared, so that in oxidizing completely it destroys at most
only half of the manganese dioxid. The mixture is submitted to gentle
ebullition for eight hours, the water being kept at a constant volume.
The excess of manganese dioxid remaining is dissolved hot by a measured
portion of decinormal oxalic acid in slight excess, and the excess of
oxalic acid is removed by a solution of potassium permanganate
containing one gram per liter. The volume of oxalic acid not destroyed
by manganese dioxid is calculated from the amount of permanganate
consumed. The volume of oxalic acid, which corresponds to the same
quantity of dioxid as the introduced humus, is also calculated by taking
the difference between the volume of oxalic acid necessary to destroy
all the dioxid formed by ten cubic centimeters of the ten per cent
permanganate solution, and the volume of the oxalic acid which has
destroyed the dioxid remaining after the action of the humus. The first
volume of oxalic acid, that is to say, that which destroys the dioxid
formed by ten cubic centimeters of ten per cent permanganate is
determined in a preliminary titration.
In regard to the humic liquor, it is prepared by treating ten grams of
earth with soda solution in the usual manner. It will be easy to
calculate the volume of the oxalic solution equivalent to the total
volume of the humic solution, of which a determined fraction has been
assayed, and consequently the volume of oxalic solution equivalent to
the humus in ten grams of the dry earth. This number of cubic
centimeters of the decinormal oxalic solution multiplied by 0.8 will
express in milligrams the weight of oxygen necessary to burn the humus
from ten grams of dry earth. Humus not being a definite compound, but a
residue of complex organic matters partially oxidized, it will require
as much more oxygen to complete the combustion as the previous oxidation
has been less pronounced. This weight of oxygen necessary to burn the
humus from ten grams of dry earth may serve to detect the total value as
well as the weight of the humus itself. However, if we wish to have
directly the weight of the humus, resource can be had to a table which,
without being rigorous, can be regarded as sufficiently exact when the
variability of the constitution of humus is taken into account.
Volume of decinormal oxalic acid Corresponding humus, directly
for ten grams of earth. determined.
Cubic centimeters. Milligrams.
50 80
100 150
200 280
300 400
400 510
500 610
600 705
700 790
800 885
900 975
1,000 1,060
1,200 1,225
1,400 1,390
1,600 1,560
1,800 1,720
2,000 1,890
2,500 2,315
3,000 2,735
3,500 3,170
4,000 3,605
4,500 4,035
5,000 4,460
5,500 4,890
6,000 5,310
6,500 5,745
=318. Pasturel’s Method.=—According to Pasturel[205] the process of
Raulin does not furnish figures that are rigorously exact only with soil
of which the humus contains forty-five per cent of carbon. When the
richness in organic carbon is less, the results of the estimation are
too high. Pasturel modifies the process as follows:
_Manganese Sulfate._—Dissolve sixteen grams of the pure anhydrous
manganese sulfate in distilled water and make the solution up to one
liter.
_Potassium Permanganate._—Make a solution of ten grams of potassium
permanganate in one liter of water; 100 cubic centimeters of the liquor
just mentioned are diluted to one liter and constitute the potassium
permanganate solution one to ten.
_Oxalic and Sulfuric Acids._—A solution of oxalic acid is prepared
containing 6.3 grams of the acid in one liter of water, and a dilute
solution of sulfuric acid, by dissolving 150 grams of the monohydrated
acid in one liter of water.
_Humus Solution._—The solution of humus is prepared by the following
process: Ten grams of fine earth are freed from all their carbonates by
dilute hydrochloric acid. After washing, the filter is broken and the
dirt is washed into a small flask. Not more than twenty or thirty cubic
centimeters of water should be employed for this purpose. Twenty cubic
centimeters of a liquor containing two grams of caustic soda are added,
and the flask is placed upon a sand-bath and maintained at a boiling
temperature for six hours. It is then diluted with water, filtered and
washed as long as the waters are colored. The liquor is treated with
dilute sulfuric acid until almost the whole of the soda is saturated. It
is indispensable, however, to maintain a slight alkalinity in order that
the organic matter may rest totally dissolved. The precipitation of
silica which is almost always produced is without inconvenience.
Afterward the volume is completed to 500 cubic centimeters and the humus
solution is then ready for use.
_Estimation of the Humus._—Ten cubic centimeters of the manganese
sulfate are placed in a flask and ten cubic centimeters of the
permanganate added, and the whole is then slightly heated, and afterward
100 cubic centimeters of water and four cubic centimeters of sulfuric
acid are added. The humic liquor is now introduced in such proportion
that the humus which it contains dissolves at the greatest, a half of
the precipitated manganese and the rest of the process is continued as
described by Raulin.
=319. Estimation of Carbonates in Arable Soil.=—The principle of the
determination depends on the liberation of the carbon dioxid from its
compounds in the soil by acting on them with strong acid, and the
desiccation, absorption, and weighing of the evolved gas. Any of the
ordinary forms of apparatus for estimating carbon dioxid may be used in
this determination.
The apparatus of Knorr[206] has been used with satisfaction for many
years in the laboratory of the Department of Agriculture.
[Illustration:
FIGURE 65.
KNORR’S APPARATUS FOR THE DETERMINATION OF CARBON DIOXID.
]
The apparatus consists of a flask A, Fig. 65, in which the carbon dioxid
in the soil is liberated. A condenser, D, fits by means of a
ground-glass joint into the neck of the flask in which the liberated
gas, together with any air or aqueous vapor which may be carried
forward, is cooled. This prevents any excess of vapor of water from
entering the absorbing bulbs, which could easily happen at the end of
the experiments when the contents of A are raised to the boiling point.
The bulb B contains the acid, usually hydrochloric, which is employed
for decomposing the carbonates. It is provided with a guard bulb-tube,
C, which serves to absorb any carbon dioxid which might enter the
apparatus with the air during aspiration at the close of the
determination. The carbon dioxid is dried in the bulb-tube, E, in oil of
vitriol, and absorbed in the potash solution in F. It is advisable to
aspirate a slow current of air through the apparatus by means of the
tube G during the whole of the operation. The quantity of the sample to
be taken depends on its richness in carbonates. Many soils are so poor
in carbonates as to render any attempt at exact determination nugatory.
On the other hand, a comparatively small sample of marls will be
sufficient. A preliminary qualitative test will indicate, in a general
way, the quantity of the sample to be taken. The sample of soil, five to
fifty grams, having been transferred to A, which should be perfectly
dry, is made into a batter with freshly boiled distilled water. When all
the parts of the apparatus are properly connected gas-tight, the cock
between B and A is slowly opened and the hydrochloric (nitric) acid in B
allowed to flow into A at such a rate as will secure a moderate
evolution of gas.
When the carbonate is entirely decomposed, a lamp is brought under A and
its contents gradually raised to the boiling point. The aspiration of
air, free from carbon dioxid, is meanwhile continued until all the
liberated gas has been absorbed in F. Usually about fifteen minutes will
be sufficient to accomplish this purpose.
=320. Bernard’s Calcimeter.=—For a rapid and approximately accurate
method of determining the amount of carbonate in the soil, estimated as
calcium carbonate, Bernard makes use of the well-known method of the
volumetric estimation of carbon dioxid. The sample to be examined should
not be powdered in any way. The sample in a natural state, but well
air-dried, is gently broken up by the fingers and passed through a sieve
having ten meshes to the centimeter. Of the fine earth thus obtained,
one gram is taken, for the determination. If the percentage of carbonate
in the soil exceeds fifty then only half a gram is taken.
[Illustration:
FIGURE 66.
BERNARD’S CALCIMETER.
]
The apparatus employed is one well known. The small erlenmeyer C is
fitted with a rubber stopper carrying an exit tube for the gas and a
small thermometer. This flask is connected by means of a rubber tube and
small glass tube to the measuring burette B. This burette is graduated
from 0 to 100 cubic centimeters. Below, by means of a rubber tube, it is
connected with the open bulb A, which, by means of a cord about its
neck, can be suspended by the hook as shown in the figure. The measuring
tube is filled with water through A until the level of the liquid in B
is slightly above the zero mark. Meanwhile the one gram of earth has
been placed in C, together with the tube D three-fourths filled with an
equal mixture of water and strong hydrochloric acid. The greatest care
must be taken that no part of the acid be spilled.
The rubber stopper is now forced into C until the level of the water in
B is just at the zero mark. Grasping C in the right hand and A in the
left, the operator inclines C until the contents of D are emptied.
Meanwhile as the gas is evolved, A is lowered at such a rate as to
always keep the level of the water in B and A on the same plane. In a
few moments the evolution of gas is complete, and the volume given off
is read at once without correction. This volume multiplied by 0.4 gives
the percentage of carbonate in the sample examined. It is understood
that the determination is made at ordinary temperatures; _viz._, 17° to
22°. Example:
One gram of a soil treated as above, gave of carbon dioxid
(uncorrected) 65 cubic centimeters. 65 × 0.4 = 26.00 = per cent
calcium carbonate in sample.
The above method is useful in the classification of soils and in
determining approximately the quantity of calcium carbonate which they
contain. The practical use of this method is of great value in
determining the character of fertilizer to be applied. It is well to
know the percentage of carbonate in selecting mineral fertilizers.
=321. Soils Deficient in Carbonates.=—When a soil contains but a small
quantity of carbonates, Müller[207] has called attention to the fact
that the carbon dioxid absorbed by the water in which the soil is rubbed
up may vitiate the result. Instead of water a titrated solution of
sodium carbonate is employed. The apparatus is composed of a flask
containing the mixture of the sodium carbonate and the soil on which the
hydrochloric acid is to act. The hydrochloric acid is contained in a
small tube, as in Scheibler’s apparatus. The gas is received in a rubber
tube 1.5 meters long and three to four millimeters interior diameter,
and connected with a burette, the open mouth of which dips into the
water of a cylinder of proper length. The volume of gas is read when the
burette is raised or lowered in the cylinder until the liquid within and
without stands at the same level.
During the action of the acid on the carbonates the flask is constantly
shaken.
Several readings of the volume of gas are made, the evolution flask
being vigorously shaken before each one. Finally, in order to allow for
the variations in temperature and pressure of the exterior air which may
take place between the beginning and the end of the reaction, a second
flask containing air is placed by the side of the evolution flask and
communicating with a narrow =ᥩ= tube half filled with water. Any
variations in the volume of the air in the flask will be shown by
variations in the height of the liquid in the two arms of the =ᥩ= tube,
and the volume of the variation can be easily determined by having the
=ᥩ= tube calibrated.
If now _a_ equals the volume per cent of carbon dioxid in the atmosphere
of the evolution flask at the end of the reaction, _v_ the volume of gas
disengaged, and V the volume of the atmosphere in the evolution flask,
the per cent of carbon dioxid contained in a given length of the rubber
tube will be equal to _a_/2. This arises from the fact that the first
gas which passes into the rubber tube is composed solely of air, while
the last contains a per cent of carbon dioxid. By reason of the shaking
of the flask the mean richness of the contents of the tube in carbon
dioxid, will be sensibly _a_/2.
From the above data the following equations are derived:
1. _v__a_/2 + V_a_ = _v_.
2. _a_ = _v_/(_v_/2 + V)
If the weight of the carbon dioxid dissolved in V′ cubic centimeters of
the liquid in the evolution flask be represented by _q_, the coefficient
of the solubility of pure carbon dioxid in this liquid will be,
according to the law of the solubility of a gas, equal to _k_ =
(_q_)/(V′_a_)
The volume of _k_ has been determined for various strengths of the
sodium carbonate solution, using five cubic centimeters of hydrochloric
acid containing 1.6 grams pure hydrochloric acid. For solutions
disengaging from five to fifty milligrams of carbon dioxid, the mean
value of _k_ was found to be 1.8 milligrams in the absence of calcium
chlorid. When calcium chlorid was present in quantities varying from
0.03 to 0.07 gram per cubic centimeter of liquid in the evolution flask,
the value of _k_ was 1.4 milligrams.
By adopting, according to circumstances, the one or the other of the
above numbers and multiplying it by V_a_, as determined by experiment,
results are obtained differing only 0.2 to 0.3 milligram from those
secured by direct weighing of the evolved gas.
Dietrich[208] has called attention to the necessity of adding the volume
of the dissolved gas to the measured volume in such determinations, and
this volume or weight is easily determined by the above formulas.
=322. Belgian Method.=—The method pursued at the Gembloux Station[209]
consists in taking from five to fifty grams of the sample of soil,
according to its content in carbonate, rubbing it up in a porcelain dish
with distilled water in order to make a thin paste. The mass is worked
to drive out all the air, the whole washed into a flask of 300 cubic
centimeters capacity, and the amount of carbon dioxid estimated by
setting free with an acid, and collecting the carbon dioxid evolved in
potash bulbs.
DIGESTION OF SOILS WITH SOLVENTS.
=323. General Considerations.=—There are two points in connection with
the determination of mineral matters in the soil which must always be
kept in view; _viz._, first, the estimation of the total quantities of
material in the soil, and second, the study of those materials which are
more easily brought into solution and thus made available for the food
of plants.
It is well understood that the soil particles do not give up entirely to
the plant the food materials which they contain. The practical value
therefore of an analysis of a soil depends more upon the exact
determination of the plant food available than upon its total quantity.
From a mineral and geological point of view, on the other hand, an idea
of the total composition of the soil is the object to be attained.
For the determination of the available plant food, various solvents have
been proposed, none of which, perhaps, imitates very accurately the
natural solvent action of organic life and moisture on the soil
materials. A description of the standard methods of preparing soil
extracts will be the subject of a few succeeding paragraphs.
=324. Estimation of the Quantity of Materials Soluble in
Water.=[210]—Five hundred grams of the air-dried soil are treated in a
flask with 1,500 cubic centimeters of water, less the quantity of water
already contained in the air-dried soil, which is volatile at 125°. The
mass is frequently shaken and, after seventy-two hours, 750 cubic
centimeters of the liquid filtered. The filtrate is evaporated to
dryness in a platinum dish, dried at 120° and weighed. This is then
incinerated and, after treatment with ammonium carbonate and gentle
ignition, is again weighed. The further examination of the residue for
acids and bases is made by some of the methods hereafter described.
=325. Treatment with Water Saturated with Carbon Dioxid.=—Two thousand
five hundred grams of the air-dried soil are treated with 8000 cubic
centimeters of distilled, and afterwards with 2000 cubic centimeters of
water, which have previously, at room temperature, been saturated with
carbon dioxid. The mixture is left in a closed flask for seven days,
frequently shaken, after which 7,500 cubic centimeters of the liquid are
filtered. The clear filtrate, after treatment with a little hydrochloric
acid and a few drops of nitric acid, is evaporated to dryness. After the
separation of the silica the traces of iron, alumina, lime, sulfuric
acid, magnesia, potash, and soda, are estimated in the liquid in the
manner hereinafter to be described. Phosphoric acid is always present in
such a case, in such small quantities as to make its estimation
unnecessary.
=326. Treatment with Water Containing Ammonium Chlorid.=—In the flask
containing the residue from the last experiment; _viz._, the soil with
2,500 cubic centimeters of liquid, are added 1,500 cubic centimeters of
water saturated with carbon dioxid, and 8,000 cubic centimeters of pure
water in which five grams of ammonium chlorid are dissolved. The mixture
is then left for seven days, with frequent shaking, and 7,500 cubic
centimeters of the liquid are then filtered, and the substances
dissolved, determined in the filtrate. In addition to the usual
quantities of lime and magnesia, from two to four times as much alkali
is dissolved by this treatment as is found in the solution from the
water containing carbon dioxid alone.
=327. Treatment with Water Containing Acetic Acid.=—The acetic acid
should be of such a strength that after it has fully acted on the soil
it should still contain twenty per cent of free acid. 1000 grams of the
soil dried at 100° are taken and the acid added in proper proportions
and treated in the manner to be described for determining the solvent
action of hydrochloric acid.
=328. Treatment with Citric Acid Solution.=—In ascertaining the
quantities of soil materials soluble in a solution of citric acid,
Dyer[211] recommends the use of a carefully prepared citric acid
solution. The digestion is carried on as follows: Place in a flask or
bottle, holding about three liters, 200 grams of air-dried soil and two
liters of distilled water, in which are dissolved twenty grams of pure
citric acid. The soil is left, at room temperature, in contact with the
one per cent acid for seven days, with thorough shaking several times a
day. At the end of the digestion the solution is filtered and 500 cubic
centimeters of the filtrate, corresponding to fifty grams of the soil,
are taken for analysis for each ingredient to be determined.
The digestion in citric acid is especially recommended by Dyer because
of its supposed near resemblance to the methods of solution of plant
food practiced by the rootlets of plants. It is evident, however, that
this process is in no sense an imitation of natural methods. The
solution is to be used exclusively for the estimation of potash and
phosphoric acid. Dyer concludes, from a comparison of the action of a
solution of citric acid on soils of known fertility, that when as little
as 0.01 per cent of phosphoric acid is dissolved from a soil by this
treatment it is justifiable to assume that it stands in immediate need
of phosphatic manure. The methods used by Dyer to determine the
phosphoric acid and potash in the citric acid solution will be given in
their appropriate place.
=329. Treatment with Hydrochloric Acid.=—The solutions of soils usually
subjected to chemical analysis are those obtained by long treatment with
hot mineral acids, among which the most common is hydrochloric.
It has long been assumed by soil analysts, perhaps not with justness,
that such treatment removed from the soil, all those elements of plant
food which could possibly be available for the needs of the growing
crop. In this connection, however, the analyst must not forget that
nature, in a series of years, with her own methods may easily accomplish
what he in five days, even with the help of a hot mineral acid, may not
be able to secure. Since, however, this method of solution has been so
long practiced it is not the place here to throw doubt on its
effectiveness without being able to suggest a better way. Of the mineral
acids available no one possesses solvent powers for soils in a higher
degree than hydrochloric. A somewhat detailed description will therefore
be given of the methods of its use.
=330. Strength of Acid to be Employed.=—The fact that hydrochloric acid
of nearly constant strength; _viz._, specific gravity 1.115, equivalent
to 22.9 per cent hydrochloric acid, may be obtained by distillation, led
Owen to use acid of this density in his classic work on soil analysis.
Hilgard has lately reviewed the conditions of constant strength in the
solvent with results confirming the statements of Owen.[212] He
evaporated on a steam-bath, to one-half its bulk, fifty cubic
centimeters of hydrochloric acid, specific gravity 1.116, obtained by
using the distillate from a stronger acid after rejecting the first
third. The same operation was conducted with similar acid diluted with
ten per cent of water. The acid used contained 22.96 per cent
hydrochloric acid. The residual acid contained 21.49 per cent
hydrochloric acid. These results lead Hilgard to believe that the
changes arising from evaporation in hydrochloric acid during soil
digestion are insignificant, compared with those due to its action on
the soluble matters, and that evaporation during digestion is effective
in maintaining a definite strength in the solvent. For this reason it is
contended that evaporation in a porcelain beaker covered by a
watch-glass is more effective in constancy of conditions than digestion
in a closed flask under pressure.
=331. Influence of Time of Digestion and Strength of Acid.=—Loughridge
has made an interesting study of the influence of the strength of acid
and time of digestion on the extraction of soils.[213] The method of
preparing the soil for the determination of the above points is as
follows:
The soil, having been passed through the appropriate number of sieves to
obtain the fine earth is pulverized with a wooden pestle and thoroughly
mixed. The hygroscopic moisture is determined, after exposing it in a
place saturated with vapor, in a layer not exceeding one millimeter in
thickness for twelve hours, and subsequently drying at 200° in a
paraffin-bath. Of this dried substance, from two to three grams are used
in the general analysis, the methods employed being in general those
adopted by Peter.[214]
The quantities of materials dissolved by acids of different densities
are shown below. The determinations were made by methods hereafter to be
described.