=8. Mixed Fertilizers.=—In fertilizing materials in bulk, the first
requisite is that they shall be thoroughly mixed so that a given volume
of the material may represent, practically, definite quantities of the
materials sampled. The finer the material is, in the original state,
and the more thoroughly it has been mixed, the better the sample will
be. If the sample be already in sacks it will be sufficient to take
portions by means of the ordinary trier, such as is used for sampling
sugar and other substances. This consists of a long metal implement
such as would be formed by a longitudinal section of a tube. The end
is pointed and suited for penetrating into the sack and the materials
contained therein. On withdrawing it, the semi-circular concavity is
found filled with the material sampled. Samples in this way should
be taken from various parts of the sack and these samples well mixed
together and a subsample of the amount necessary to be taken to the
laboratory can then be obtained.
=9. Method of the French Experiment Stations.=—In the method employed
by the French Experiment Stations it is directed that in no case should
stones or other foreign particles be removed from the fertilizer
sampled, but they should enter into the sample taken in, as nearly as
possible, the same proportions as they exist in the whole mass.
In the case of stones or other solid masses which are to be sampled, as
many samples as possible should be taken from all parts of the heap and
these should be reduced to a coarse powder, thoroughly mixed together
and sampled.
In case the material is in the form of a paste, if it is homogeneous,
it will be sufficient to mix it well and take the sample directly; but
in case there is a tendency for the pasty mass to separate into two
parts, of which the one is a liquid and the other, more of a solid
consistence, it may be well to take samples from each in case they can
not be thoroughly incorporated by stirring.
=10. Method of the French Association of Sugar Chemists.=—The method
adopted by the French sugar chemists directs that the sample should be
taken from the fertilizer in bulk or from a portion used for industrial
purposes.[4] The sample for analysis is to be taken from the above
sample after it has been sent to the laboratory. The method of taking
should be varied according to the condition of the substances to be
analyzed.
The large sample selected from the goods delivered to commerce having
been delivered at the laboratory, the analytical sample is taken as
follows:
When the industrial sample, more or less voluminous, reaches the
laboratory, the chemist is to begin by taking a note of the marks,
labels, and descriptions found thereon, and of the nature and state of
the package which contains it, and the date of its arrival. All this
information should be entered upon the laboratory book and afterwards
transcribed on the paper containing the results of the analysis, as
well as the name of the person sending it. This having been done, the
sample is to be properly prepared in order that a portion may be taken
representing exactly the mean composition of the whole.
If it is in a state of fine powder, such as ground phosphates and
certain other fertilizers, it is sufficient to pass it two or three
times through a sieve with meshes one millimeter in diameter, taking
care to break up the material each time in order to mix it and
to pulverize the fragments which the sieve retains. The whole is
afterwards spread in a thin layer upon a large sheet of paper, and a
portion is taken here and there upon the point of a knife until about
twenty grains are removed, and from this the portion subjected to
analysis is afterwards taken.
If the sample comes in fragments, more or less voluminous, such as
phosphatic rocks or coarsely pulverized guanos containing agglomerated
particles, it is necessary first to reduce the whole to powder by
rubbing it in a mortar or in a small drug mill. It is next passed
through a sieve of the size mentioned above and that which remains upon
the sieve pulverized anew until all has passed through. This precaution
is very important, since the parts which resist the action of the
pestle the most have often a composition different from those which are
easily broken.
When the products to be analyzed contain organic materials, such as
horn, flesh, dry blood, etc., the pulverization is often a long and
difficult process, and results in a certain degree of heating which
drives off some of the moisture in such a way that the pulverized
product is at the last drier, and, consequently, richer than the
primitive sample. It is important to take account of this desiccation,
and since the pulverization of a mass so voluminous can not be made
without loss, the determination of the total weight of the sample
before and after pulverization does not give exact results.
In such a case it is indispensable to determine the moisture, both
before and after pulverizing, and to calculate the analytical results
obtained upon the pulverized sample back to the original sample.
In order to escape this necessity, as well as the difficulties
resulting from the variations in moisture during transportation, some
chemists have thought it better to always dry the commercial products
before submitting them to analysis, and to report their results in the
dry state, accompanied by a determination of the moisture, leaving
thus to the one interested the labor of calculating the richness in
the normal state, that is to say, in the real state in which the
merchandise was delivered.
In addition to the fact that this method allows numerous chances
of errors, many substances undergoing important changes in their
composition by drying alone, it has been productive of the most serious
consequences. The sellers have placed their wares on the market with
the analysis of the material in a dry state, and a great number of
purchasers have not perceived the fraud concealed under this expression
so innocent in appearance. It is thus that there has been met with in
the markets guano containing twenty-five per cent of water, which was
guaranteed to contain twelve per cent of phosphoric acid, when, in
reality, it contained only eight per cent in the moist state.
=11. Barn-Yard Manures.=—The sampling of stall and barnyard manures
is more difficult on account of the fact that the materials are not
homogeneous and that they are usually mixed with straw and other débris
from the feed trough, and only the greatest care and patience will
enable the operator to secure a fair sample.
In the case of liquid manures the liquid should be thoroughly stirred
before the sample is taken.
Frear points out the difficulty of securing representative samples of
stall manure and describes methods of removing it.[5] The stall manure
sampled had been piled in the cattle-yard for a time and the cattle
were allowed to run over the heaps for an hour or two each day. Pigs
were also allowed free access to the heaps in order to insure a more
perfect mixture of the ingredients.
Twenty-nine loads of 3,000 pounds each were taken from the exposed heap
and thirty-four loads of 2,000 pounds each were taken from the covered
heap. From each load were removed two carefully selected portions of
ten pounds each, which were placed in separate covered boxes numbered A
and B. When the sampling was completed these boxes were covered. After
being removed to the laboratory the boxes were weighed and the contents
thoroughly mixed. Two samples of twelve liters volume each, were drawn
from each box. One-third of this was chopped in a large meat chopper
and the other two-thirds taken into the laboratory without being cut.
These samples, on entering the laboratory, were weighed and dried at a
temperature of 60°. Smaller samples were then drawn from each of these
and ground in a drug mill for analysis. Duplicate samples taken in this
way, while they did not give absolutely concordant results, showed a
near approximation. A more careful sampling on the line proposed would,
in all probability, secure absolutely agreeing results in duplicate
samples.
=12. Preparation of Sample in Laboratory.=—The method of preparing
mineral fertilizers for analysis has been given under directions for
sampling. Many difficulties attend the proper preparation of other
samples, and the best approved methods of procedure are given below:
According to the directions given by the Association of Official
Agricultural Chemists the sample should be well intermixed, finely
ground, and passed through a sieve having circular perforations one
millimeter in diameter.[6] The processes of grinding and sifting should
take place as rapidly as possible so that there may be no gain or loss
of moisture during the operation.
=13. Method of the French Agricultural Stations.=—The manner of
proceeding recommended by the French stations varies with the
fertilizer.[7] If it is not already in the form of a powder it is
necessary to pulverize it as finely as possible by rubbing it up in a
mortar. In certain cases, as with superphosphates, the material should
be passed through a sieve having apertures of one millimeter diameter,
all the larger parts being pulverized until they will pass this sieve.
When the matters are too pasty to be divided in the mortar they should
be divided by means of a knife or a spatula. They should then be
incorporated with a known weight of inert, pulverulent matter such as
fine sand, with which they should be thoroughly mixed and in subsequent
calculations the quantity of sand or other inert matter added must be
taken into consideration. Usually a pasty state of a fertilizer is due
to the humidity of the mixture. In this case a considerable volume of
the sample is taken and dried and then reduced to a pulverulent state.
In the subsequent calculations, however, the percentage of moisture
lost must be taken into consideration.
Before drying a sample it is necessary to take into consideration
whether or not the product will be modified by desiccation as would
be the case, for instance, with superphosphates. With these, which
are often in a state more or less agglomerated, it is recommended to
introduce into them, in order to divide them, a certain quantity of
calcium sulfate in order to obtain them in a pulverulent state.
In the case of animal débris they should be divided as finely as
possible with the aid of scissors and then passed through a drug mill
if dry enough. They are then mixed by hand and may finally be obtained
in a state of considerable homogeneity.
When fertilizers are in a pasty state more or less liquid, they are
dried at 100°, first introducing a little oxalic acid in case they
contain any volatile ammoniacal compounds. The product of desiccation
is then passed through a mill. Before treating in this way it is
necessary to be sure that the composition will not be altered by
drying. In the case of a mixture containing superphosphates and
nitrate, for instance, drying would eliminate the nitric acid. In
such a case the free phosphoric acid should be neutralized with a
base like lime. In the case of fertilizers containing both nitrates
and volatile ammoniacal compounds the addition of oxalic acid might
also set free nitric acid during the desiccation. In such a case it
is necessary to dry two samples; one with the addition of oxalic acid
for the purpose of estimating the ammonia, and the other without the
acid for the purpose of estimating the nitrate. A qualitative analysis
should precede all the operations so as to determine the nature of the
material to be operated on.
=14. German Method.=—In the method pursued by the German experiment
stations it is directed:[8]
(1) Dry samples of fertilizers must be passed through a sieve and
afterwards well mixed.
(2) With moist fertilizers, which can not be subjected to the above
process, the preparation should consist in a careful and thorough
mixing, without sieving.
(3) On the arrival of the samples in the laboratory their weight should
be determined. The half of the sample is prepared for analysis and
the other part, to the amount of a kilo, should be placed in a glass
vessel, closed air-tight, and placed in a cool place for at least a
quarter of a year from the time of its reception, in order that it may
be subjected to any subsequent investigations which may be demanded.
(4) In the case of raw phosphates and bone-black the amount of water
which they contain should be determined at from 105° to 110°. Samples
which in drying lose ammonia in any way, should have this ammonia
determined.
(5) Samples which are sent to other laboratories for control analyses,
should be sent securely packed in air-tight glass bottles.
(6) The weight of the samples sent should be entered in the
certificates of analysis.
(7) Samples which, on pulverizing, change their content of water,
must have the water content estimated in both the coarse and powdered
condition and the results of the analysis must be calculated to the
water content of the original coarse substance.
=15. Special Cases.=—Many cases arise of such a nature as to make it
impossible to lay down any rule which can be followed with success. As
in almost every other process in agricultural chemistry the analyst in
such cases must be guided by his judgment and experience. Keeping in
view the main object, _viz._, to secure in a few grams of material a
fair representation of large masses he will generally be able to reach
the required result by following the broad principles already outlined.
In many cases the details of the work and the adaptations necessary to
success must be left to his own determination.
=16. Drying Samples of Fertilizers.=—The determination of the
uncombined moisture in a sample of fertilizer is not an easy task.
In some cases, as in powdered minerals, drying to constant weight at
the temperature of boiling water is sufficient. In organic matters
containing volatile nitrogenous compounds, these must first be fixed
by oxalic or sulfuric acid, before the desiccation begins. If any
excess of sulfuric acid be added, however, drying at 100° becomes
almost impossible. Particular precautions must be observed in drying
superphosphates. In drying samples preparatory to grinding for
analysis, it is best to stop the process as soon as the materials can
be pulverized. In general, samples should be dried only to determine
water, and the analytical processes should be performed on the
undried portions. It is not necessary, as a rule, to dry samples of
fertilizers in an inert atmosphere, such as hydrogen or carbon dioxid.
Drying in vacuo may be practiced when it is desired to secure a speedy
desiccation or one at a low temperature.
=17. Official Methods.=—The Official Agricultural Chemists direct, in
the case of potash salts, sodium nitrate, and ammonium sulfate, to
heat from one to five grams in a flat platinum or aluminum dish at
130° until the weight is constant.[9] The loss in weight is taken to
represent the water. In all other cases heat two grams, or five grams
if the sample be very coarse, for five hours in a steam-bath.
In the German stations in the case of untreated phosphates and
bone-black the moisture is estimated at from 105° to 110°. Samples
which lose ammonia should have the weight of ammonia given off at that
temperature, determined separately.
For purposes of comparison it would be far better to have all contents
of moisture determined at the boiling-point of water. While this varies
with the altitude and barometric pressure yet it is quite certain that
the loss on drying to constant weight at all altitudes is practically
the same. Where the atmospheric pressure is diminished for any cause
the water escapes all the more easily. This, practically, is a complete
compensation for the diminished temperature at which water boils.
Where the samples contain no ingredient capable of attacking aluminum,
they can be conveniently dried, in circular dishes of this metal about
seven centimeters in diameter and one centimeter deep, to constant
weight, at the temperature of boiling water.
=18. Moisture in Monocalcium Phosphates.=—In certain fertilizers,
especially superphosphates, containing the monocalcium salt, the
estimation of water is a matter of extreme difficulty on account of the
presence of free acids and of progressive changes in the sample due to
different degrees of heat.
Stoklasa has studied these changes and reaches the following
results[10]:
A chemically pure monocalcium phosphate of the following composition,
_viz._,
CaO 22.36 per cent.
P₂O₅ 56.67 “ “
H₂O 21.53 “ “
was subjected to progressive dryings. The loss of water after ten hours
was 1.83 per cent; after twenty hours, 2.46 per cent; after thirty
hours, 5.21 per cent; after forty hours, 6.32 per cent; after fifty
hours, 6.43 per cent. This loss of water remained constant at 6.43 per
cent. This loss represents one molecule of water as compared with the
total molecular magnitude of the mass treated. A calcium phosphate,
therefore, of the following composition, CaH₄(PO₄)₂·H₂O loses, after
forty hours, drying at 100°, its water of crystallization. The calcium
phosphate produced by this method forms opaque crystals which are not
hygroscopic and which give, on analysis, the following numbers:
CaO 24.02 per cent.
P₂O₅ 16.74 “ “
H₂O 15.09 “ “
The temperature can be raised to 105° without marked change. If the
temperature be raised to 200° the decomposition of the molecule is
hastened according to the following formula: