=143. Kinds of Nitrogen in Fertilizers.=—Nitrogen is the most costly
of the essential plant foods. It has been shown in the first volume,
paragraph =23=, that the popular notion regarding the relatively great
abundance of nitrogen is erroneous. It forms only 0.02 per cent of the
matter forming and pertaining to the earth’s crust. The great mass of
nitrogen forming the bulk of the atmosphere is inert and useless in
respect of its adaptation to plant food. It is not until it becomes
oxidized by combustion, electrical discharges, or the action of certain
microorganisms that it assumes an agricultural value.
Having already, in the first volume, described the relation of nitrogen
to the soil it remains the sole province of the present part to
study it as aggregated in a form suited to plant fertilization. In
this function nitrogen may claim the attention of the analyst in the
following forms:
1. In organic combination in animal or vegetable substances, forming a
large class of bodies, of which protein may be taken as the type. Dried
blood or cottonseed-meal illustrates this form of combination.
2. In the form of ammonia or combinations thereof, especially as
ammonium sulfate, or as amid nitrogen.
3. In a more highly oxidized form as nitrous or nitric acid usually
united with a base of which Chile saltpeter may be taken as a type.
The analyst has often to deal with single forms of nitrogenous
compounds, but in many instances may also find all the typical forms in
a single sample. Among the possible cases which may arise the following
are types:
_a._ The sample under examination may contain nitrogen in all three
forms mentioned above.
_b._ There may be present nitrogen in the organic form mixed with
nitric nitrogen.
_c._ Ammoniacal nitrogen may replace the nitric in the above
combination.
_d._ The sample may contain no organic but only nitric and ammoniacal
nitrogen.
_e._ Only nitric or ammoniacal nitrogen may be present.
=144. Determination of the State of Combination.=—Some of the sample
is mixed with a little powdered soda-lime. If ammoniacal nitrogen be
present free ammonia is evolved even in the cold and may be detected
either by its odor or by testing the escaping gas with litmus or
turmeric paper. A glass rod moistened with strong hydrochloric acid
will produce white fumes of ammonium chlorid when brought near the
escaping ammonia.
If the sample contain any notable amount of nitric acid it will
be revealed by treating an aqueous solution of it with a crystal
of ferrous sulfate and strong sulfuric acid. The iron salt should
be placed in a test-tube with a few drops of the solution of the
fertilizer and the sulfuric acid poured down the sides of the tube in
such a way as not to mix with the other liquids. The tube must be kept
cold. A dark brown ring will mark the disk of separation between the
sulfuric acid and the aqueous solution in case nitric acid be present.
If water produce a solution of the sample too highly colored to be used
as above, alcohol of eighty per cent strength may be substituted. The
coloration produced in this case is of a rose or purple tint.
Nitric nitrogen may also be detected by means of brucin. If a few drops
of an aqueous solution of brucin be mixed with the same quantity of an
aqueous extract of the sample under examination and strong sulfuric
acid be added, as described above, there will be developed at the disk
of contact between the acid and the mixed solutions a persistent rose
tint varying to yellow.
To detect the presence of organic albuminoid nitrogen the residue
insoluble in water, when heated with soda-lime, will give rise to
ammonia which may be detected as described above.
=145. Microscopic Examination.=—If the chemical test reveal the
presence of organic nitrogen the next point to be determined is the
nature of the substance containing it. Often this is revealed by simple
inspection, as in the case of cottonseed-meal. Frequently, however,
especially in cases of fine-ground mixed goods, the microscope must
be employed to determine the character of the organic matter. It is
important to know whether hair, horn, hoof, and other less valuable
forms of nitrogenous compounds have been substituted for dried blood,
tankage, and more valuable forms. In most cases the qualitative
chemical, and microscopic examination will be sufficient. There may be
cases, however, where the analyst will be under the necessity of using
other means of identification suggested by his skill and experience or
the circumstances connected with any particular instance. In such cases
the general appearance, odor, and consistence of the sample may afford
valuable indications which will aid in discovering the origin of the
nitrogenous materials.