PHOSPHATES AND NITROGEN
“Phosphorus, which is a dangerous poison, as we have seen, is
nevertheless found in abundance in the bodies of all animals. It occurs
in the urine, whence Brandt was the first to extract it; it is found
still more plentifully in the bones, and from thence it is now
obtained. There is some in meat, in milk, and in cheese; also in
plants, notably cereals; hence flour and bread contain it. But do not
be alarmed: we shall not die of poison like the rats that have nibbled
crusts smeared with grease and phosphorus.”
“But why not,” asked Emile, “if we eat it as the rats do?”
“I will try to explain,” replied his uncle. “When two or more
substances are mixed together, they lose their original properties,
while the new substance obtained by their combination is found to
possess new properties having nothing in common with the old ones. Thus
carbon, when combined with the air that we breathe, becomes an
invisible gas, subtle, and unfit for breathing. In like manner lime,
burning to the taste, is converted by union with carbonic acid gas into
chalk, a calcareous stone void of taste. Furthermore, poisonous
substances, deadly in a very small dose, may become harmless and even
enter into the composition of our food when they are combined with
other substances. Thus it is with phosphorus. What, then, is united
with phosphorus in the form in which it ceases to be poisonous and
enters into the composition of meat and flour? That is what we will now
consider.
“When phosphorus is burned it produces a thick white smoke, of which
you can get some idea by striking a number of matches all at once. This
white smoke with the slightest trace of humidity is reducible to an
extraordinarily acid liquid called phosphoric acid. Since this compound
results from the combustion of phosphorus, just as carbonic acid is the
result of the combustion of carbon, it must and in fact does contain
the air without which no combustion can take place. Phosphoric acid is
no longer inflammable, however much it may be heated; being itself the
product of combustion, it cannot burn again. But if there is no danger
of its catching fire, phosphoric acid is nevertheless dangerous on
account of its intense acidity, which makes it violently corrosive in
its action on flesh. If mixed with lime, however, this formidable
compound loses its injurious properties and is changed into a white
substance without the least taste or the slightest poisonous effect.
This substance is called phosphate of lime. Burnt phosphorus and lime,
thus united, furnish the greater part of the mineral matter found in
bones. Put a bone into the fire: the grease and juices that permeate
its substance will be burnt up and the bone will lose a part of its
weight and become friable and perfectly white. Well, this bone,
calcined in the fire for a long time, is composed chiefly of phosphate
of lime. It contains phosphorus, the most combustible of substances,
and yet is itself absolutely incombustible; it contains one of the most
poisonous substances, and yet is itself quite harmless; into its
composition there has entered an ingredient possessing atrocious
acidity, and yet the compound itself has no taste. Similarly combined
and equally harmless, phosphorus is found in meat, milk, cereals, in
flour and bread.
“A cow can furnish each week about 70 liters of milk containing 460
grams of phosphate. This phosphate comes from hay, which obtains it
from the soil. But as the soil contains only a moderate quantity of it,
and the hay continually takes it away, the supply will at last become
exhausted and the milk will become poorer and less abundant. If a
kilogram of powdered bones, containing about the same quantity of
phosphate as the 70 liters of milk, is spread over the pasture, it will
make good the weekly loss in phosphate that the soil undergoes in the
production of the cow’s milk. Hence the efficacy of powdered bones on
exhausted pasture land.
“Phosphoric acid combined with other substances is found in all our
agricultural products, and hence the phosphate from bones has a very
marked effect on our crops. Harvests have been doubled as if by magic
through the use of powdered bones. A kilogram of this powder contains
enough phosphoric acid for the growth of a hundred kilograms of wheat.
Despite their great value as a fertilizer bones will never be thus used
except to a limited extent, because they are not abundant enough and
also because they are much in demand in various arts and manufactures.
Fortunately in some localities phosphate of lime is found in certain
coarse pebbles called nodules or coprolites. These precious stones are
carefully collected and ground to powder in a mill. Then, in order to
make the substance more soluble in damp soil, and thus better fitted
for the nutrition of plants, it is sprayed with an extremely corrosive
liquid called sulphuric acid or, more commonly, oil of vitriol. In this
way is obtained the superphosphate of lime which manufacture gives to
agriculture as one of the most powerful of fertilizers, especially for
the raising of grain.
“We were wondering a little while ago what substances could be
contained in the ashes of a burnt plant, and we have now found potash
to be one of them. Moreover, since all vegetation must have phosphate
in order to thrive, this also ought to be found in the ashes, phosphate
being indestructible by heat. And, in fact, after the incineration of
any vegetable matter whatever, as a bundle of hay or a handful of
grain, the delicate processes of science can always recover this
compound of phosphorus; and they further find lime, iron in the form of
rust, the silicious component of pebbles, and divers other substances
of less interest.
“To finish this difficult but very important subject of the nutrition
of plants, I must say a few words about ammonia. This word does not
tell you anything since it is a new word to you. But I will make its
meaning clear to you by a familiar illustration.
“You must have noticed the strong, penetrating odor prevalent in
ill-kept water-closets; and you have also perceived the same odor when
soiled garments are cleaned with a certain liquid that looks like clear
water. Well, this odor, so pungent that it almost produces the effect
of fine needles thrust up into the nostrils and brings tears to the
eyes, is the odor of ammonia.
“Ammonia is an invisible gas capable of being taken up in large
quantities by water, the mixture being known as aqua ammoniæ, or water
of ammonia. Combined with other substances ammonia loses its pungent
odor and forms compounds which are among the most effective
fertilizers. These compounds furnish vegetation with one of its
essential ingredients called nitrogen. By itself nitrogen is an
odorless and colorless gas. In this state it forms four-fifths of the
volume of ordinary air, the air we breathe. The other fifth is composed
of a second gas called oxygen, also colorless and odorless. It is
oxygen that our lungs demand when we breathe, and it is oxygen that is
necessary when we wish to burn anything. It is this alone that plays
its invaluable part in the combustion of certain substances in our
blood and in the generation of natural heat; it is this that in the
process of combustion releases carbon, phosphorus, sulphur, and other
combustibles, to combine with them and produce a compound known as
carbonic acid gas in the case of burnt carbon, phosphoric acid in the
case of phosphorus. In fact, to it belong the properties that we have
until now attributed to the atmosphere as a whole. As for nitrogen, it
has no other purpose in the atmosphere than to moderate by its presence
the too violent energies of oxygen; it plays there the part of the
water that we put into too strong wine.
“All vegetation requires nitrogen. Wheat, for example, must have it to
develop the grain in the ear; peas, beans, lentils demand it in order
to fill out their pods; the pasture and the hay-field need it if they
are to furnish the nutriment that the sheep and the cow will transform
into milk. But plants cannot take this nitrogen from the air, where it
is so abundant; it must be served up to them after a certain necessary
preparation. We ourselves need phosphorus, since it enters into the
composition of our bones; we need carbon still more, the principal fuel
used in maintaining the heat of the body. But are we to eat the
charcoal that the charcoal-burner manufactures in his furnace, and the
phosphorus used in the making of matches? Certainly not. The first
would be a frightful mouthful, the second an atrocious poison. We must
have them prepared in a suitable way, such as they are found in bread,
milk, meat, fruits, vegetables. In the same manner plant-life requires
nitrogen, not as it occurs in the atmosphere, but as it exists in
certain combinations, of which the most notable are the compounds of
ammonia. This explains to us the highly beneficial effect of manure on
our crops. Manure is composed of the bedding used in stables and the
animal excrement with which it has become mixed and impregnated. Now
this excrementitious matter, especially urine, yields ammonia in
decomposing, as is proved by the odor arising from latrines in hot
weather and so powerfully affecting the eyes and nose. Thus manure may
be said to hold ammonia compounds in storage, and from them plants
derive their nitrogen, as also many other ingredients.
“Let us summarize these details. In the nutrition of plants four
substances are of prime importance. First, carbonic acid gas, which
yields carbon, the most widely diffused of all the elements (but which
we need not dwell upon here), since plants take it chiefly from the
atmosphere, to which it is supplied unceasingly. After carbonic acid
come potash, phosphoric acid, and nitrogen, all of which the roots
extract from the soil, where it occurs in some compound or other. These
are the ingredients that the soil, if it is to remain fertile, must
have given back to it as fast as they are exhausted by the crops. Such
is the part played by fertilizers, without which the soil becomes
exhausted and ceases to produce.”