=278. Classification.=—Nitrogen, phosphoric acid, and potash are
the most important of the plant foods both from a commercial and
physiological point of view. They are the chief constituents of the
most important fertilizers and manures, but are by no means the sole
essential elements of plant nutrition. Lime, magnesia, soda, sulfur,
chlorin, and many other elements are found constantly in plants and
must be regarded as normal constituents thereof. It is the purpose
here, however, to speak only of those substances which are used as
fertilizers and which constantly or occasionally are subjected to
chemical examination for determining their commercial or agronomic
value. These bodies may be conveniently divided into two classes;
_viz._, mineral and organic. Among those of mineral nature may be
mentioned lime, gypsum, marls, wood-ashes, common salt, and ferrous
sulfate; among those of organic nature may be included guano, hen
manure, stall manure, composts, and muck.
=279. Forms of Lime.=—By the term lime is meant the product obtained
by subjecting limestone or other lime carbonates to the action of heat
until the carbon dioxid contained therein is expelled. The resulting
lime, CaO, when exposed for some time to the air, or at once on the
addition of water, is converted into the hydrate CaO₂H₂, known as
slaked lime. On longer exposure to the air, the hydrate gradually
absorbs carbon dioxid and becomes converted into carbonate. In whatever
form, therefore, lime is applied to the soil, it is found in the end,
as carbonate. A distinction should also be made between lime obtained
from mineral substances and that got from organic products such as
shells. Strictly speaking this is not a definite matter, inasmuch as
limestones are sometimes but little more than aggregations of fossil
shells. Practically, however, the distinction is made, and some farmers
prefer shell lime to that of any other kind. Gas lime, that is lime
which has been used for the purification of illuminating gas made from
coal, is hardly to be considered in this connection, since it may
contain very little even of the hydrate. In this case the lime has been
converted largely into carbonate and sulfid.
=280. Application of Lime.=—For many reasons it is important that
the lime be transported to the field before it has had time to be
converted into hydrate. The transportation costs less in this state
and it can be handled with far less inconvenience than when slaked.
The lime should be placed in small piles and left thus, best covered
with a little earth, until thoroughly slaked. It is then spread evenly
over the surface. The quantity used per acre depends largely on the
nature of the soil. Stiff clays and sour marsh lands require a larger
dressing than loams or well aerated soils. From three to six thousand
pounds per acre are the quantities usually employed. When the lime
is once thoroughly incorporated in the soil it is rapidly converted
into carbonate, but while in the caustic state it may act vigorously
in promoting the decay of organic matter and may prove injurious in
promoting the decomposition of ammonium salts with attendant loss of
nitrogen.
=281. Action of Lime.=—The benefits arising from the application of
lime to agricultural lands, although in many cases great, do not
arise from any distinct fertilizing action of its own. Plants need
lime for growth and need plenty of it, but as a rule any soil which
is good enough to grow crops will contain enough lime to furnish that
constituent of the crops for many years. Its action is both mechanical
and chemical. By virtue of the latter property it renders available
for plant food bodies already existing in the soil but existing in
such shape as to be unavailable for plants. The supply of plant food
available for the crops of one year is increased but this increase is
at the expense of the following years. Lime is a stimulant. There is an
European proverb that “lime enriches the father but beggars the son.”
Nevertheless, a limestone country is usually a fertile one and soils
containing plenty of lime naturally, are nearly always rich soils. It
is said that the trees and plants which farmers pick out as indicative
of rich land are nearly always those which prefer lime soils.
The mechanical action of lime on soils tends to lighten heavy clays
and loams and to render firmer and more consistent the light and
shifting sandy soils. When a lump of clay is stirred up in a bucket
of rain water the water becomes muddy and remains that way for many
days. If, however, to the bucket of muddy water a little lime water
be added the suspended particles of clay begin to flocculate and soon
the water is clear and the clay falls to the bottom, nor does it again
make the water muddy for a long time when stirred up with it. The
flocky character of the precipitate is tenaciously retained and it is
necessary to knead the clay for some time to induce it to reassume its
original heavy character. An action like this takes place when lime is
added to heavy soils so that the soil becomes more porous and assumes
a better tilth on plowing. With sandy soils an altogether different
action takes place. In making mortar, as is well known, sand is stirred
in with milk of lime and after being exposed to air for a while the
mixture becomes hard and firm, the firmness increasing with age. This
is due to the fact that when the mortar dries the lime begins to absorb
carbon dioxid from the air and is converted into grains of carbonate
which adhere strongly to neighboring sand grains and to each other
so that the whole soon gets to be a solid mass. Something like this
takes place in the soil and the sand grains are to some extent bound
together. The increased firmness of the soil thus gained is often of
considerable advantage.
Besides these actions, which are more or less mechanical, lime exerts
a chemical action on many soil constituents. Feldspar and other
common rocks contain potash, and this potash is in such a form as to
be inaccessible to plants. These rocks exist in the shape of small
particles in many soils and on them lime exerts a decomposing action,
setting the potash free. Lime also hastens the decomposition of the
nitrogenous organic matter and at the same time renders the soil more
retentive of the products formed. The conversion of ammonia, resulting
from the decomposition of such organic matter into nitrites and
nitrates, is not easily accomplished without a proper amount of calcium
carbonate. The microorganisms producing this change, which is known
as nitrification, apparently require its presence for neutralizing
the acid formed. In general, it may be said that the presence of lime
hastens the putrefactive process. This is the reason it is so largely
used in making composts.
It is difficult to say just what soils will be improved by liming and
what will not, and it is a matter which must be settled by experiment
in each case. As a rule heavy clays and loams are benefited, yet of two
such soils, apparently identical, one may not be affected in any marked
degree while the other may readily respond to treatment. Sandy soils
are often improved but sometimes not. Sour, boggy lands, are usually
improved by the addition of enough lime to neutralize their undue
acidity. Marsh grasses and plants are more tolerant of acid in the soil
than tame grasses are, so that in unlimed soil the former run out the
latter. The application of lime alone to a very poor soil does not pay.
The particles of lime resting in the soil are partially dissolved by
the next rainfall after application, or by the soil moisture, forming
lime water, and the lime is distributed in this form through the soil
to some extent. It all probably soon becomes converted into carbonate
as ground air is usually quite rich in carbon dioxid. Indeed, for
many soils, it is immaterial whether lime be applied as lime or as
carbonate, granting, of course, that the latter be ground to a fine
powder. Economy is in favor of the lime, however, not only because it
needs no grinding, but because it is lighter than the corresponding
amount of carbonate, making a saving in transportation. The difference
is quite considerable, fifty-six pounds of lime being equivalent in
effect to 100 pounds of carbonate. For these reasons as well as because
it possesses some valuable properties not shared by the carbonate, it
is possible that for most localities lime is to be preferred to any
form of ground oyster shells, ground limestone, marble dust or the like.
One of these valuable properties not possessed by limestone, is said
to be that of acting as a fungicide and insecticide. As a rule, fungi
prefer acid reaction in the substances in which they grow, so that the
strongly alkaline properties of lime may make a limed soil unsuitable
for their growth.[239]
=282. Analysis of Lime.=—Lime, which is prepared for use as a
fertilizer, is rarely submitted to a chemical examination. It is easy
to see, however, that such an examination is of some importance. If
the real value of a sample be dependent on the content of lime, the
actual quantity present as determined by analysis, must fix the value
for agricultural purposes. The more important things to be determined
are the quantities of lime, and of slaked lime, of undecomposed calcium
carbonate, and of insoluble matter. It will be also of interest to
determine the respective quantities of lime present as oxid, hydrate,
and carbonate. If any question be raised in the case of slaked lime in
respect of its origin, it can usually be answered by an examination of
the unburned or unslaked residues. In perfectly slaked lime containing
no débris, the analyst will be unable to discover whether it has been
made from limestone, marble or shells. The lime used for agricultural
purposes should be reasonably free of magnesia, and should not be
air-slaked before transportation to the field. In dry air-slaking, a
considerable quantity of carbonate may be formed.
=283. The Process.=—(1) _Insoluble and Soluble Constituents._—A
representative sample of the lime having been secured, it is reduced to
a powder and passed through a half millimeter mesh sieve or ground to
a fine powder in an agate mortar. Digest two grams of this sample with
an excess of hydrochloric acid, for two hours with frequent stirring;
filter, wash the residue with hot distilled water until chlorin is
all removed, and dry to constant weight. The lime, magnesia, silica,
and other constituents of the filtrate, are determined by the usual
processes of mineral analysis.[240]
(2) _State of Combination of the Lime._—In a lime containing only
small quantities of magnesia the lime carbonate may be determined by
estimating the carbon dioxid by any one of the reliable processes in
use.[241] In every case sufficient acid must be employed to combine
with all the bases present. Tartaric or hydrochloric acid may be used.
From the volume or weight of the carbon dioxid obtained the quantity of
calcium carbonate may be calculated. Since magnesium carbonate is more
easily decomposed by heat than the corresponding calcium compound, any
residual carbonate in a well-burned sample is probably lime. The total
percentage of lime in the sample is to be determined in the usual way
by precipitation as oxalate and weighing as carbonate or oxid. The lime
existing as oxid can be determined by exposing a weighed sample in an
atmosphere of aqueous vapor until all the lime is slaked. After drying
at 100° the increase in weight is determined and the calcium oxid
calculated from the formula, CaO + H₂O = CaO₂H₂.
If now the total lime be represented by _a_; the lime combined as
carbonate by _b_; and that present as oxid by _c_; the quantity _x_
existing as hydrate may be calculated from the equation
_x_ = _a_ - (_b_ + _c_).
Example: Let the total lime be 88 per cent.
CaO as carbonate, 2 “ “
CaO as oxid, 78 “ “
Then the CaO as hydrate = 88 - (2 + 78) = 8 per cent. The total lime as
oxid and hydroxid may also be separated from that present as carbonate
by solution in sugar.[242] One gram of calcium oxid is completely
soluble in 150 cubic centimeters of a ten per cent sucrose solution.
Magnesia, iron and alumina do not interfere with the determinations.
=284. Gypsum or Land Plaster.=—This substance is highly prized as a top
dressing for grass and for admixture with stall manure for the purpose
of fixing ammonia. Its value in both cases depends upon its percentage
of hydrated calcium sulfate. The quantity of gypsum mined in the
United States in 1893 was a little over 250,000 tons. Of this amount
only about 50,000 tons were used as fertilizer.[243] In the same time
there were imported into the United States, in round numbers, 170,000
tons. If the same proportionate part of this were used for fertilizing
purposes, it may be said that the annual consumption of land plaster in
the United States at the present time for agricultural uses is about
75,000 tons.
Gypsum, being a very soft mineral, is easily ground and should be in
the state of a fine powder when used for fertilizing purposes. It is
soluble in about 500 parts of rain water, so that when applied as a top
dressing it is carried into the soil by rain. Its favorable action is
both as a plant food and mechanically in modifying, in an advantageous
way, the physical constituents of the soil. It is also valuable for
composting and for use in stables by reason of its power of fixing
ammonia by the formation of lime carbonate and ammonium sulfate:
(H₄N)₂CO₃ + CaSO₄ = (H₄N)₂SO₄ + CaCO₃.
=285. Analysis of Gypsum.=—For agricultural purposes it will be
sufficient to determine the quantity of sulfuric acid, and to calculate
therefrom the amount of calcium sulfate in the sample: Or the lime may
be determined and the quantity of sulfate calculated therefrom.
(1) _Insoluble Matter._—In the conduct of the work the sample of
gypsum is rubbed to an impalpable powder in an agate mortar. The
washed sample, about one gram, is dissolved in a large excess of
dilute hydrochloric acid, the digestion being continued at near the
boiling-point, with frequent stirring, for at least two hours. The
solution is made alkaline, filtered, and the residue washed and dried
to constant weight.
(2) _Sulfuric Acid._—The washings and filtrate from the above
determination are made up to a definite volume with water and divided
into two equal parts. The sulfuric acid is estimated in one part by
adding to it sodium carbonate until the acidity is nearly neutralized.
The sulfuric acid is then thrown down at near boiling temperature by
the gradual addition of barium chlorid solution. The barium sulfate
formed is separated, washed, dried, and weighed in the usual manner.
(3) _Iron and Alumina._—To the other half of the solution a little
nitric acid is added and boiled to convert any ferrous into ferric
iron. On the addition of ammonia the iron and alumina are separated as
hydroxids, collected on a gooch, washed, dried, ignited, and weighed as
oxids.
(4) In the filtrate the lime is thrown out as oxalate, and separated
and weighed in the usual way as oxid. One part of CaO is equal to
2.4286 parts of CaSO₄.
(5) _Moisture._—Dry about two grams of the sample to constant weight at
80°.
(6) _Water of Crystallization._—Heat the residue from the above to
150°, until a constant weight is obtained. The loss represents water of
crystallization.
(7) _Carbonates._—Determine the quantity of carbon dioxid evolved by
the usual process, and calculate the calcium carbonate.
=286. Solution in Sodium Carbonate.=—Gypsum is also easily decomposed
by boiling with a solution of about ten times its weight of sodium
carbonate. The calcium, by this operation, is converted into carbonate
and can be collected on a gooch, washed, and estimated as usual, but
in this case it will contain all the insoluble matters, from which the
lime can be separated by solution in hydrochloric acid.
In the filtrate from the above separation the excess of sodium
carbonate is removed by the addition of hydrochloric to slight acidity,
and the sulfuric acid estimated as described in the preceding paragraph.
Pure gypsum has a composition represented by the following formula:
CaSO₄·H₂O.
It contains:
Sulfur dioxid 46.51 per cent.
Lime 32.56 “ “
Water 20.93 “ “
A commercial sample of ordinary gypsum should have about the following
composition:[244]
CaSO₄·H₂O 88.15 per cent.
CaCO₃ 3.50 “ “
Fe₂O₃ and Al₂O₃ 1.50 “ “
Insoluble 2.80 “ “
Organic matter 0.50 “ “
Water and undetermined 3.55 “ “
=287. Common Salt.=—Common salt is highly esteemed in many quarters as
a top dressing for lawns and meadows, and also for cultivated crops.
Its action is chiefly of a mechanical and katalytic nature, since it
does not form a very large percentage of the mineral food of plants.
On account of its affinity for moisture it is also said to have
some value as a condenser and carrier of water in times of drouth.
On account of its great cheapness, selling often for less than ten
dollars a ton, its use in moderate quantity entails no great expense.
Its ability, however, to pay for its own use in the increased harvest
is of a doubtful character when it is applied at a cost of more than
a few dollars per acre. In the chemical examination of a sample of
common salt which is to be used as a fertilizer, a complete analysis
is rarely necessary. When desired it can be conducted according to the
usual methods of mineral analysis. For practical purposes the moisture,
insoluble matter, magnesia and chlorin should be determined and the
quantity of sodium chlorid calculated from the latter number. Traces of
iodin or bromin which may be present are of no consequence.
The moisture is determined by drying two grams of the well-mixed and
finely-powdered sample to constant weight at 100°. The chlorin is
obtained by precipitation of an aliquot part of a solution of the salt
by set silver nitrate, using potassium chromate as indicator.
In the determination of insoluble matter it should not be forgotten
that a little gypsum may be present, and this should be dissolved by
rubbing to a finer powder and by repeated digestion in water. The
magnesia and lime are separated and determined in the usual manner. If
the quantity of gypsum present be sufficient to warrant it the sulfuric
acid may be separated and weighed in the manner already described.
Common salt when present in the soil in proportions greater than
one-tenth per cent is injurious to vegetation.
=288. Green Vitriol.=—When iron is used as a fertilizer it is
usually applied as ferrous sulfate. The value of iron in a soil is
incontestable and by reason of the fact that fertile soils are always
well aerated the iron present in the arable layer is found in the
ferric state. When green vitriol is applied to the soil it undergoes
gradual oxidation and appears finally in a more highly oxidized
form. Iron acts directly on the plant in promoting the development
of the chlorophyll cells, and is also found in almost all parts of
the vegetable organism. A too great quantity of ferrous sulfate is
destructive of plant growth in which respect it resembles common salt.
It should therefore be applied with due regard to the dangers which
might arise from an excessive quantity. It is not likely, however, that
when applied in a finely powdered state at the rate of from two to four
hundred pounds per acre it would ever prove poisonous to vegetation.
In the analysis of a sample of green vitriol it will be sufficient to
determine the moisture, water of crystallization, iron, and sulfuric
acid. The moisture may be ascertained by drying the finely powdered
sample over sulfuric acid for a few hours. The water of crystallization
is separated by exposing the sample to a temperature of 285° for two
hours. The iron may be determined by oxidizing to the ferrous state
by boiling with nitric acid and then precipitating with ammonia,
and proceeding as directed for iron analysis. The sulfuric acid is
separated as barium sulfate and determined as already directed.
=289. Stall Manures.=—There are no definite methods to be described for
the analyses of that large class of valuable fertilizer produced in the
stable and pen, and which collectively may be called stall manures. The
methods of sampling have already been described,[245] but only patience
and tact will enable the collector to get a fair representation of the
whole mass. These manures are a mixture of urine, excrement, waste
fragments of fodder, and the bedding used for the animals. With them
may also be included the night soil and waste from human habitations
and the garbage from cities. All of these bodies contain valuable plant
foods and the phosphoric acid, potash, and nitrogen therein are to be
determined by the methods already given for these bodies when they
occur in, or are mixed with, organic matter. In general, stall manures
are found to have a higher manurial value than is indicated by the
amount of phosphorus, potash, and nitrogen which they contain. Through
them there is introduced into the soil large quantities of humus bodies
whereby the physical state of the soil is profoundly modified and its
adaptability to the growth of crops, as a rule increased. The addition
of active nitrifying ferments in stall manure is also advantageous.
Stall manures, however, may in many cases prove to be injurious to a
crop, as for instance, when they are applied in a poorly decomposed
state and in a season deficient in moisture.
It is essential therefore that the bedding of animals be in a finely
divided state, whereby not only are the absorptive powers of the
organic matter increased but also the conditions for their speedy decay
favored. To avoid the loss of ammonia arising from decomposing urine it
is advisable to compost the stall manure with gypsum or to sprinkle it
from time to time with oil of vitriol.
In the analysis the moisture may be estimated by drying a weighed
portion of the sample to constant weight at 100° or at a lower
temperature in a vacuum. The potash and phosphoric acid are determined
as usual, with previous careful incineration, and the nitrogen secured
by the moist combustion process.
=290. Hen Manure.=—This fertilizing substance is a mixture of the
excrement of the fowl yard with feathers, dust, and other débris.
Measured by the standard applied to commercial fertilizers hen manure
has a low value. As in the case of other farm manures, however, it
produces effects quite out of proportion to the amount of ordinary
plant foods which it contains. In a sample examined at the Connecticut
station the percentages of fertilizing constituents were found to be
the following[246]:
Water 51.84
Organic and volatile matters 24.27
Ash 23.89
The organic matter contained 0.61 per cent of nitrogen as ammonia and
the ash 0.97 per cent of phosphoric acid, and 0.59 per cent of potash,
all calculated to the original weight of the sample. The percentage of
water in this sample is undoubtedly higher than the average, so that it
can hardly be taken to represent the true composition of this manure.
The potash, phosphoric acid, and nitrogen are to be determined by some
one of the standard methods already described, the two former after
careful incineration.
=291. Guanos and Cave Deposits.=—The principal constituents of value
in these deposits are nitrogen and phosphoric acid. The other organic
matters are also of some value but have no commercial rating. The
nitrogen may be present in all its forms; _viz._, organic, ammoniacal,
amid, and nitric, and for this reason is well suited not only to supply
nourishment to the plant in the earlier stages of its growth but also
to cater to its later wants. In guano deposits in caves, due usually to
the presence of bats, similar forms of fertilizers are found and the
soluble constituents due to decay and nitrification are protected from
the leaching to which they would be subjected in the open air.
In many localities in the United States these deposits are found, but
the humidity of our climate has prevented the immense open deposits of
guano that characterize some of the arid islands of the Pacific Ocean.
Many bat guanos examined in this laboratory have also been found to
contain potash, in one case 1.78 per cent. It is suggested therefore
that the analyst do not omit to examine each sample qualitatively for
this substance and to determine its amount when indications point to
its presence in weighable proportions. In the many samples of bat guano
of American origin which have been analyzed in this laboratory in the
last few years some very rich in plant food have been found. In one
instance the total percentage of nitrogen present, was 10.11 per cent.
In some cases the phosphoric acid is high but rarely in conjunction
with a high content of nitrogen. In one instance where the total
phosphoric acid reached 14.53 per cent, the content of nitrogen was
4.87 per cent.
In respect of the processes of analysis there are no especial
directions to be given. The phosphoric acid, as given below, and the
potash are to be determined by the usual methods, the total phosphoric
acid and potash after the destruction of the organic matter.
In old cave deposits the processes of decay and nitrification seem
to have long been completed and we have found very little power of
inducing nitrification in culture solutions seeded from these samples.
=292. French Official Method for Total Phosphoric Acid in Guanos.=—To
determine the phosphoric acid in guanos, the method officially adopted
by the French agricultural chemists may be used.[247]
Two grams of the sample are rubbed up in a porcelain crucible with
a decigram of slaked lime to prevent the possible reduction of the
phosphoric acid by the organic matter. The mixture is slightly
moistened with a few drops of water, dried on a sand-bath, and
afterwards heated to redness, best in a muffle, until organic matter is
destroyed. The contents of the crucible are detached and placed in a
flask of 200 cubic centimeters capacity. The crucible is well digested
twice with some hydrochloric acid to dissolve any adhering fragments,
and finally washed with hot water, the acid and water being added to
the flask. The contents of the flask are boiled for fifteen minutes and
then poured into a flat-bottomed dish, the flask well rinsed three or
four times with small quantities of water, and the liquor and washings
are evaporated to dryness to render the silica insoluble. The residue
is taken up by a mixture of ten cubic centimeters each of hydrochloric
acid and water, heated for a few minutes and filtered, and the dish
well washed with successive small portions of water, but the total
volume of the filtrate and washings should not exceed eighty cubic
centimeters. In this filtrate the phosphoric acid may be determined by
any one of the approved methods.
=293. Waste Leather.=—This material belongs probably to that class
of nitrogenous substances which has already been considered in
paragraph =149=. The chief manurial value of the waste is found in its
nitrogenous content. The value of this for available plant food has
been investigated by Lindsey.[248] A complete resumé of the literature
of the subject is also given by him.
The best way of identifying leather waste is by the process proposed
by Dabney.[249] It depends on the color produced in a solution of iron
phosphate by the tannin compounds derived from the leather. The reagent
is prepared by dissolving a freshly made precipitate of iron phosphate
from ten grams of ferric chlorid in 400 cubic centimeters of an aqueous
solution of forty grams of glacial phosphoric acid. A gentle heat
promotes the solution of the phosphate.
In the case of a fertilizer supposed to contain leather, about one
gram of the material is treated with thirty cubic centimeters of water
and a few drops of sulfuric acid. The mixture is boiled and poured
on a filter. To a portion of the filtrate some of the solution of
iron phosphate is added, and the mixture made alkaline with ammonia.
If leather be present in the sample, a purple or wine color will be
developed. Lindsey could easily detect the leather when it was added in
ten per cent quantities by the above method, and he regards this method
as superior to the microscope which is unreliable in the case of finely
ground material.
While leather, as such, decays slowly, and therefore is not at once
available for the nourishment of plants it acquires greater utility
after digestion in sulfuric acid. Artificial digestion experiments with
leather previously treated with sulfuric acid show that, approximately,
seventy per cent of the nitrogen pass into solution. Such a prepared
leather has, therefore, a digestive coefficient in respect of nitrogen
not much inferior to most organic bodies.
In comparative trials with sodium nitrate it was demonstrated that
nitrogen in leather, previously dissolved in sulfuric acid, has a rank
of about sixty when it is rated at one hundred in the soda salt.
For the estimation of the nitrogen in leather the moist combustion
process is to be preferred.
=294. Analysis of Wood Ashes.=—The only kinds of ashes used extensively
for manurial purposes are those derived from the burning of hard woods.
The ash of soft woods, such as the pine, is too poor in plant foods to
warrant its transportation to any great distance for manurial purposes.
The methods of incineration of organic bodies for the purpose of
obtaining and estimating their mineral contents will be fully discussed
in the third volume of this work.
It is important in ash analysis to know whether there be enough of iron
present to combine with all the phosphoric acid. For manurial purposes
it will be found sufficient to determine the percentages of potash and
phosphoric acid alone. For hygienic purposes it is advisable to examine
the ash qualitatively and, if necessary, quantitatively for zinc, lead,
copper, boric acid, and other bodies of a similar character which
may be naturally present in the ash, or may have been added to the
organic substance from which it was prepared for preservation or other
purposes. The methods of making these special investigations will be
discussed in the succeeding volume. At present will be given, however,
not only the methods for detecting phosphoric acid and potash, but also
for a complete analysis of an ash in so far as its usual constituents
are concerned.
=295. Carbon, Sand, and Silica.=—The official agricultural chemists
have recommended the following procedure for the determination of the
unburned carbon, and the sand and silica.[250]
Five grams of the ash are treated in a beaker, covered with a
watch glass with fifty cubic centimeters of hydrochloric acid of
1.115 specific gravity, and digested on the water-bath until all
effervescence has ceased. The cover is then removed and the liquid
evaporated to complete dryness to render the silica insoluble.
The residue is moistened with two or three cubic centimeters of
hydrochloric acid and taken up with about fifty cubic centimeters of
water, allowed to stand on the water-bath a few minutes, filtered,
and thoroughly washed. The filtrate and washings are made up to a
quarter of a liter for analysis. The residue is washed from the
filter into a platinum dish and boiled about five minutes with twenty
cubic centimeters of a saturated solution of pure sodium carbonate;
afterwards a few drops of pure sodium hydroxid solution are added
and the liquid allowed to settle, and it is then decanted through a
tared gooch. The residue is boiled with sodium carbonate solution and
decanted as before, a second and a third time, and finally brought
upon the felt and thoroughly washed, first with hot water, then with
a little dilute hydrochloric acid, and finally with hot water until
free of chlorids. The residue in the gooch is dried at 110° to constant
weight, giving the carbon and sand. It is then incinerated and the
weight of the sand determined, the difference giving the carbon. It
is advisable to examine the sand with a microscope to determine if it
be pure. The alkaline filtrate and washings from the carbon and sand
are acidified with hydrochloric, evaporated to dryness, and the silica
separated and determined in the usual way.
Instead of determining soluble silica directly from the sodium
carbonate solution, as above, another portion of the ash may be treated
with hydrochloric acid and evaporated to dryness as before described,
filtered on an ordinary filter, washed, burned, and weighed, giving the
weight of silica plus sand, from which the weight of sand is deducted
to obtain soluble silica. It is inadmissible to separate the soluble
silica from the residue after it has been ignited.
Instead of limiting the quantity of hydrochloric acid used for
moistening the dried residue, as suggested above by the official
chemists, enough should be employed to fully saturate the mass. The
weight of pure ash is obtained by subtracting from the weight of the
sample taken the sum of the weights of carbon, sand, and carbon dioxid.
=296. Ferric Phosphate and the Alkaline Earths.=—The ferric phosphate,
lime, magnesia, and manganese are determined in an aliquot part of the
first hydrochloric acid solution and washings obtained above. Fifty
or one hundred cubic centimeters may be used, corresponding to one or
two grams of the original ash. The accurately measured quantity of
the solution is carefully treated with ammonia until the precipitate
formed on its addition becomes permanent on shaking. Ammonium acetate
and acetic acid are then added until the mixture has assumed a strongly
acid reaction. The separation of the ferric phosphate precipitate is
promoted by gentle warming, and it is separated by filtration without
unnecessary delay. If the precipitate be not large the sample contains
no manganese and alumina in weighable quantities, and if the filtrate
be not red the precipitate be washed with hot water containing a
little ammonium nitrate. It is then ignited and weighed as Fe₂P₂O₈
and the quantity of ferric oxid computed therefrom. If, however, the
precipitate be large it is well washed as above and then dissolved in
as small a quantity as possible of hydrochloric acid, and the solution
is again precipitated as above by the addition of ammonia, ammonium
acetate, and acetic acid. The ferric phosphate obtained by the second
precipitation is treated exactly as above described.
In case, however, any weighable quantities of manganese or alumina are
present it will not do to weigh the precipitate of ferric phosphate
directly even after a second precipitation. Also if the filtrate at
first obtained have a red color the precipitate may contain basic
ferric phosphate. In this latter case it should be ignited and weighed,
then dissolved in hydrochloric acid and the ferric oxid estimated
in the solution and from the difference the quantity of combined
phosphoric acid calculated.
The separation of the iron from the phosphoric acid may be accomplished
by adding tartaric acid to the hydrochloric acid solution of the iron
phosphate above obtained and then ammonium chlorid and ammonia. The
mixture is placed in a flask and ammonium sulfid added. The flask
is closed, placed in a warm place, and allowed to stand until the
supernatant liquid is clear and of a pure yellow color without a trace
of green. The iron is separated by filtration, washed, dissolved, and
estimated in the usual way.
If manganese and alumina be present the iron and manganese are
separated from the phosphoric acid and alumina by the processes
just given for the separation of iron from phosphoric acid. In the
filtrate the alumina and phosphoric acid are separated as follows:
The filtrate is evaporated in a platinum dish after the addition
of an excess of pure sodium carbonate until no ammonia is set free
by a further addition of the carbonate. Some nitric acid is then
added and the evaporation continued to dryness. The residue is fused
and after cooling softened with water, washed into a small beaker,
some hydrochloric acid added, warmed, and filtered. Ammonia is next
added until the reaction is alkaline. If no precipitate be produced
no alumina is present. In this case more nitric acid is added, the
solution again evaporated, and the phosphoric acid determined by the
usual methods.
In case a precipitate is formed, showing the presence of alumina,
nitric acid is added until the precipitate is dissolved, and then in
slight excess and after evaporation the phosphoric acid separated
by molybdic solution and determined as usual. From the filtrate the
excess of molybdic acid is removed by hydrogen sulfid and the alumina
determined in the filtrate: Or the alumina may be determined directly
in the hydrochloric acid solution of the melt above obtained as
aluminum phosphate by adding sodium phosphate, ammonia and acetic acid.
The aluminum phosphate is separated by filtration and determined in the
usual manner. The phosphoric acid is then determined in another aliquot
part of the original filtrate from the first solution of the ash.
Since most ashes contain an excess of phosphoric acid above the
quantity required to combine with the iron it is preferable to proceed
on that basis as described in the next paragraph.
=297. Method Used in this Laboratory.=—The principle of the method
rests on the assumption that all the phosphoric acid may be removed
from the solution by the careful addition of iron chlorid. Any excess
of iron is then removed by ammonium acetate and the manganese, lime,
and magnesia, are separated in the filtrate. The percentage of iron is
determined by reduction of the iron in another portion of the solution
and titration with potassium permanganate. The process as conducted by
McElroy, is as follows:[251]
_Moisture._—If the ash contain much carbon the water is best determined
by drying in vacuo to avoid oxidation.
_Sand, Silica, and Carbon._—Place a portion of the ash in a weighed
platinum dish, weigh, and cover the sample with hydrochloric acid of
1.115 specific gravity. Evaporate to dryness on the water-bath, and
then heat for fifteen minutes at 105° to 110° in an air-bath. Repeat
the treatment with acid and drying: Finally cover with a third portion
of acid and digest on the water-bath for an hour or two. Filter into a
weighed gooch and wash the residue free of chlorids. The gooch is best
weighed with the dish to avoid the necessity of transferring the silica
which may adhere to the sides of the former. Dry at a few degrees above
the boiling temperature of water and weigh. Where the ash contains
much charcoal the drying is best done in a vacuum at 60° to 70°. The
increase in weight found represents sand, silica, and carbon: Burn and
reweigh. The loss is carbon.
Another portion of ash is treated as before except that it is filtered
through a paper filter. The filtrate is united with that of the
previous sample in a graduated flask. When the washing is completed
the filter is placed in the dish, a weak solution of caustic soda
added, and the mixture heated on the water-bath for some time. Decant
while hot through a fresh filter and retreat the residue in the dish
with another portion of alkali. Finally wash with hot water till the
alkaline reaction disappears, then with weak hydrochloric acid, then
with water until chlorids disappear. The washed mass on the filter
is transferred to a platinum dish and ignited. The weight obtained
represents sand.
_Separation of Phosphoric Acid._—The united filtrates from the two
determinations are placed in a graduated flask and made up to the
mark. An aliquot portion of this solution representing half a gram of
the original ash or any other convenient quantity is transferred to a
beaker and a solution of ferric chlorid added until ammonia produces
a brown precipitate in the mixture. Neutralize with ammonia and
hydrochloric acid alternately until the liquid is as little acid as it
can be and still remain clear. Add from ten to twenty cubic centimeters
of a solution of sodium acetate (1:10) and bring to a boil. The liquid
should be quite dilute. Filter and wash free of chlorids with boiling
water containing some sodium acetate.
_Manganese._—Make the filtrate faintly alkaline with ammonia and add
ammonium sulfid. Any manganese sulfid which may form is separated by
filtration, treated with dilute acetic acid and the resulting solution,
which should be clear, heated to boiling, nearly neutralized with
caustic soda, and mixed with bromin water. The resultant manganese
dioxid is to be filtered into a gooch, ignited and weighed as Mn₃O₄.
_Lime._—Reacidify the filtrate from the manganese sulfid with acetic
acid, heat to boiling and add ammonium oxalate: Allow to stand over
night, filter through a gooch and wash with water containing acetic
acid. The calcium oxalate can be weighed as such, but it is preferable
to dry thoroughly and then heat in a small bunsen flame until a change
can be noted passing over the precipitate. If this is carefully done
the residue will be calcium carbonate. In any case the result is to be
checked by igniting over the blast lamp to constant weight and weighing
the lime thus obtained.
_Magnesia._—In the filtrate the magnesia can be determined by sodium
phosphate in the usual manner. In very accurate work the calcium
oxalate obtained as directed above can be dissolved and reprecipitated,
and the magnesia in the filtrate added to that in the first filtrate.
_Iron._—For iron another aliquot portion of the original solution is
taken, acidified with sulfuric evaporated to drive off hydrochloric
acid, rediluted and passed through the Jones reductor described in
paragraph =112=. The filtrate is titrated with potassium permanganate
solution in the usual manner.
_Alkalies._—For the alkalies another aliquot portion is taken and
precipitated while hot, with barium chlorid and barium hydrate,
filtered, and ammonia and ammonium carbonate added to remove the
excess of barium salt. Refilter, evaporate to dryness in a platinum
dish, and ignite gently to expel all ammonia salts, repeat this
operation after taking up with water and finally heat to constant
weight. The weight obtained represents a mixture of potassium and
sodium chlorids, with usually carbon derived from impurities in
the ammonia. A little magnesia is often present. The potassium is
estimated by means of platinum solution, and the potassium chlorid
found deducted from the total weight gives the sodium chlorid. The
carbon is usually unweighable, though it often looks as if present in
considerable quantity. It may be estimated, however, by dissolving the
mixed chlorids in weak hydrochloric acid and filtering through a gooch
before making the potassium estimation. The estimation of the magnesia
remaining with the mixed chlorids may be effected by evaporating the
alcoholic solution remaining after the precipitation of the potassium
to dryness, redissolving in water, placing the solution in a flask
provided with gas tubulures, introducing hydrogen, and placing in the
sunlight. The platinum is soon reduced, leaving the liquid colorless.
Heating facilitates the reaction. Displace the hydrogen by a current
of carbon dioxid, filter, concentrate the solution and precipitate the
magnesia by sodium phosphate in the usual manner.
_Phosphoric Acid._—It is best to determine the phosphoric acid directly
in an aliquot part of the first filtrate from the hydrochloric acid
solution of the ash obtained as described under the determinations of
sand, silica and carbon. When there is not enough of the material for
this, the precipitate of ferric phosphate may be dissolved and the
phosphoric acid determined after separation with ammonium molybdate.
_Sulfuric Acid._—Fifty cubic centimeters of the original hydrochloric
acid filtrate, obtained as described under the determinations of sand,
silica and carbon, are heated to boiling, and the sulfuric acid thrown
out by the gradual addition of barium chlorid. During the precipitation
the mixture is kept at the boiling temperature, but taken from the
lamp and the precipitate allowed to settle from time to time until
it is seen that an additional drop of the reagent causes no further
precipitate. The barium sulfate is collected, dried, and weighed in the
usual manner.
_Chlorin._—Dissolve from one to five grams of the ash in nitric acid
in very slight excess, or in water. If the solution be made in nitric
acid the excess must be neutralized if the chlorin be determined
volumetrically; and if the solution be in water, nitric acid must be
added if the determination be gravimetric.
The volumetric determination is accomplished in the usual manner
with a standard silver nitrate solution, using potassium chromate as
indicator. The gravimetric determination is effected by precipitation
with silver nitrate, collecting, washing, and drying at 150° the silver
chlorid obtained.
_Carbon Dioxid._—The carbon dioxid is most conveniently estimated
in from one to five grams of the ash, according to its richness in
carbonates, by the apparatus described in volume first, or some similar
device.[252]
=298. Official Method for Determinations of the Alkalies.=—Evaporate
the filtrate and washings from the sulfuric acid determination,
paragraph =297=, in a porcelain dish to dryness, redissolve in about
fifty cubic centimeters of water and add milk of lime, or barium
hydroxid solution, which must be perfectly free from alkalies, until
no further precipitation is produced, and it is evident there is
an excess of calcium hydroxid or barium hydroxid present; boil for
two or three minutes, filter hot, and wash thoroughly with boiling
water, precipitate the lime and baryta from the filtrate with ammonia
and ammonium carbonate, filter, evaporate the filtrate to dryness
in a porcelain dish, and drive off the ammonia salts by heat below
redness.[253] When cold, redissolve in fifteen or twenty cubic
centimeters of water, precipitate again with a few drops of ammonia and
ammonium carbonate solution, let stand a few minutes on the water-bath
and filter into a tared platinum dish and evaporate to dryness, expel
the ammonia salts by heating to just perceptible dull redness, weigh
the potassium and sodium chlorids obtained and determine the potassium
chlorid with platinic chlorid as usual.
The potassium may also be determined by the perchlorate method, or
the total chlorin be determined volumetrically, and the relative
percentages of potassium and sodium chlorids calculated by the usual
formula: Or multiply the weight of chlorin in the mixture by 2.1035,
deduct from the product the total weight of the chlorids and multiply
the remainder by 3.6358. The product expresses the weight of the sodium
chlorid contained in the mixed salts. The indirect method is only
applicable when there are considerable quantities of alkalies present
and where they exist in approximately molecular proportions. It is
therefore a process rarely to be recommended in ash analysis.
=299. Statement of Results.=—The bases which are found present in the
ashes of wood and other vegetable tissues exist without doubt before
incineration, chiefly in combination with inorganic acids. Even the
phosphorus and sulfur which after ignition appear as phosphates and
sulfates, have previous thereto existed in an organic form to a large
extent. The silica itself is profoundly modified in the organism of
the growing plant and doubtless does not exist there in the purely
mineral form in which it is found in the ash. During the progress of
incineration, with proper precautions, all the phosphorus and sulfur
are oxidized and appear as phosphoric and sulfuric acids. The silica is
reduced to a mineral state, and if a high heat be employed silicates
are formed. The organic salts of lime, magnesia and other bases at
a low temperature are converted into carbonates, and if a higher
temperature be used, may appear as oxids. The organic compounds of
alkalies will be found in the ash as carbonates. It would be useless,
therefore, to try to state the results of ash analysis in forms of
combination similar to those existing in the original vegetable
tissues. It is not certain even that we can in all cases judge of the
form of combination in which the different constituents exist in the
ash itself. It is therefore to be preferred in a statement of ash
analysis to give the bases in the form of oxids, and the sulfur and
phosphorus in the form of anhydrids, and the chlorin in its elementary
state. In this case an equivalent amount of oxygen to the chlorin found
must be subtracted from the total. If an attempt be made to combine
the acid and basic elements the chlorin should first be united with
sodium, and any excess thereof with potassium, and the amount of base
so combined calculated to oxid and deducted from the total of such
base or bases present. The carbonic acid present should be combined
first with alkalies after the chlorin has been supplied. The phosphoric
acid should be combined first with the iron and afterwards with lime
or magnesia. In all cases the percentages should be based upon the
ash, after the carbon and sand have been deducted, or it is also
convenient at times to throw out of the results the carbon dioxid and
to calculate the other constituents to the ash free of that substance.
In determining the quantities of mineral matters removed from soil by
crops, the ash should be determined with great care, freed of carbon
and sand, and the calculations made on the percentage thus secured. In
all statements of percentages of the essential constituents of ash,
as regards fertilizing materials, it should be specified whether the
percentage is calculated on a crude ash, the pure ash, that is free
of carbon and sand, or upon a basis excluding the carbon dioxid. For
the purpose of fertilizer control, the analyst and dealer will be
satisfied, as a rule, with the determination of the percentages of
phosphoric acid and potash alone. To the other constituents of an ash
is not assigned any commercial value.