ATMOSPHERIC FERTILIZERS.
[Are the gases in the atmosphere manures?
What would be the result if they were not so?]
It is not common to look on the gases in the atmosphere in the light of
manures, but they are decidedly so. Indeed, they are almost the only
organic manure ever received by the uncultivated parts of the earth, as
well as a large portion of that which is occupied in the production of
food for man.
If these gases were not manures; if there were no means by which they
could be used by plants, the fertility of the soil would long since have
ceased, and the earth would now be in an unfertile condition. That this
must be true, will be proved by a few moments' reflection on the facts
stated in the first part of this book. The fertilizing gases in the
atmosphere being composed of the constituents of decayed plants and
animals, it is as necessary that they should be again returned to the
form of organized matter, as it is that constituents taken from the
_soil_ should not be put out of existence.
AMMONIA.
[How is ammonia used by plants?
How may it be carried to the soil?
How may the value of organic manures be estimated?
What effects has ammonia beside supplying food to plants?]
The _ammonia_ in the atmosphere probably cannot be appropriated by the
leaves of plants, and must, therefore, enter the soil to be assimilated
by roots. It reaches the soil in two ways. It is either arrested from
the air circulating through the soil, or it is absorbed by rains in the
atmosphere, and thus carried to the earth, where it is retained by clay
and carbon, for the uses of plants. In the soil, ammonia is the most
important of all organic manures. In fact, the value of organic manure
may be estimated, either by the amount of ammonia which it will yield,
or by its power of absorbing ammonia from other sources.
The most important action of ammonia in the soil is the supply of
_nitrogen_ to plants; but it has other offices which are of consequence.
It assists in some of the chemical changes necessary to prepare the
matters in the soil for assimilation. Some argue that ammonia
_stimulates_ the roots of plants, and causes them to take up increased
quantities of inorganic matter. The discussion of this question would be
out of place here, and we will simply say, that it gives them such vigor
that they require increased amounts of ashy matter, and enables them to
take this from the soil.
[To how great a degree can the farmer control atmospheric
fertilizers?
What should be the condition of the soil?
What substances are good absorbents in the soil?
How may sandy soils be made retentive of ammonia?]
Although, in the course of nature, the atmospheric fertilizers are
plentifully supplied to the soil, without the immediate attention of the
farmer, it is not beyond his power to manage them in such a manner as
to arrest a greater quantity. The precautions necessary have been
repeatedly given in the preceding pages, but it may be well to name them
again in this chapter.
The condition of the soil is the main point to be considered. It must be
such as to absorb and retain ammonia--to allow water to pass _through_
it, and be discharged _below_ the point to which the roots of crops are
searching for food--and to admit of a free circulation of air.
The power of absorbing and retaining ammonia is not possessed by sand,
but it is a prominent property of clay, charcoal, and some other matters
named as absorbents. Hence, if the soil consists of nearly pure sand, it
will not make use of the ammonia brought to it from the atmosphere, but
will allow it to evaporate immediately after a shower. Soils in this
condition require additions of absorbent matters, to enable them to use
the ammonia received from the atmosphere. Soils already containing a
sufficient amount of clay or charcoal, are thus far prepared to receive
benefit from this source.
[Why does under-draining increase the absorptive power of the
soil?
How do plants obtain their carbonic acid?
How does carbonic acid affect caustic lime in the soil?]
The next point is to cause the water of rains to pass _through_ the
soil. If it lies on the surface, or runs off without entering the soil,
or even if it only enters to a slight depth, and comes in contact with
but a small quantity of the absorbents, it is not probable that the
fertilizing matters which it contains will all be abstracted. Some of
them will undoubtedly return to the atmosphere on the evaporation of the
water; but, if the soil contains a sufficient supply of absorbents, and
will allow all rain water to pass through it, the fertilizing gases will
all be retained. They will be filtered (or raked) out of the water.
This subject will be more fully treated in Section IV. in connection
with under-draining.
Besides the properties just described, the soil must possess the power
of admitting a free circulation of air. To effect this, it is necessary
that the soil should be well pulverized to a great depth. If, in
addition to this, the soil be such as to admit water to pass through, it
will allow that circulation of air necessary to the greatest supply of
ammonia.
CARBONIC ACID.
[What power does it give to water?
What condition of the soil is necessary for the reception of the largest
quantity of carbonic acid?
May oxygen be considered a manure?
What is the effect of the oxidation of the constituents of the soil?]
Carbonic acid is received from the atmosphere, both by the leaves and
roots of plants.
If there is caustic lime in the soil, it unites with it, and makes it
milder and finer. It is absorbed by the water in the soil, and gives it
the power of dissolving many more substances than it would do without
the carbonic acid. This use is one of very great importance, as it is
equivalent to making the minerals themselves more soluble. Water
dissolves carbonate of lime, etc., exactly in proportion to the amount
of carbonic acid which it contains. We should, therefore, strive to have
as much carbonic acid as possible in the water in the soil; and one way,
in which to effect this, is to admit to the soil the largest possible
quantity of atmospheric air which contains this gas.
The condition of soil necessary for this, is the same as is required for
the deposit of ammonia by the same circulation of air.
OXYGEN.
[How does it affect the protoxide of iron?
How does it neutralize the acids in the soil?
How does it affect its organic parts?
How does it form nitric acid?
How may it affect excrementitious matter of plants?
What effect has it on the mechanical condition of the soil?]
_Oxygen_, though not taken up by plants in its pure form, may justly be
classed among manures, if we consider its effects both chemical and
mechanical in the soil.
1. By oxidizing or _rusting_ some of the constituents of the soil, it
prepares them for the uses of plants.
2. It unites with the _prot_oxide of iron, and changes it to the
_per_oxide.
3. If there are _acids_ in the soil, which make it sour and unfertile,
it may be opened to the circulation of the air, and the oxygen will
prepare some of the mineral matters contained in the soil to unite with
the acids and neutralize them.
4. Oxygen combines with the carbon of organic matters in the soil, and
causes them to decay. The combination produces carbonic acid.
5. It combines with the nitrogen of decaying substances and forms
_nitric acid_, which is serviceable as food for plants.
6. It undoubtedly affects in some way the matter which is thrown out
from the roots of plants. This, if allowed to accumulate, and remain
unchanged, is often very injurious to plants; but, probably, the oxygen
and carbonic acid of the air in the soil change it to a form to be
inoffensive, or even make it again useful to the plant.
7. It may also improve the _mechanical_ condition of the soil, as it
causes its particles to crumble, thus making it finer; and it roughens
the surfaces of particles, making them less easy to move among each
other.
These properties of oxygen claim for it a high place among the
atmospheric fertilizers.
WATER.
[Why may water be considered an atmospheric manure?
What classes of action have manures?
What are chemical manures? Mechanical?]
_Water_ may be considered an atmospheric manure, as its chief supply to
vegetation is received from the air in the form of rain or dew. Its many
effects are already too well known to need farther comment.
The means of supplying water to the soil by the deposit of _dew_ will be
fully explained in Section IV.