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 pa.s.s _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 quant.i.ty 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 pa.s.s 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 pa.s.s 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 quant.i.ty of carbonic acid?

May oxygen be considered a manure?

What is the effect of the oxidation of the const.i.tuents 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 quant.i.ty 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 excrement.i.tious 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 cla.s.sed among manures, if we consider its effects both chemical and mechanical in the soil.

1. By oxidizing or _rusting_ some of the const.i.tuents 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 acc.u.mulate, 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 cla.s.ses 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.

CHAPTER XI.

RECAPITULATION.

Manures have two distinct cla.s.ses of action in the soil, namely, _chemical_ and _mechanical_.

_Chemical_ manures are those which enter into the construction of plants, or produce such chemical effects on matters in the soil as shall prepare them for use.

_Mechanical_ manures are those which improve the mechanical condition of the soil, such as loosening stiff clays, compacting light sands, pulverizing large particles, etc.

[What are the three kinds of manures?

What are organic manures, and what are their uses? Mineral?

Atmospheric?]

Manures are of three distinct kinds, namely, _Organic_, _mineral_, and _atmospheric_.

_Organic_ manures comprise all vegetable and animal matters (except ashes) which are used to fertilize the soil. Vegetable manures supply carbonic acid, and inorganic matter to plants. Animal manures supply the same substances and ammonia.

_Mineral_ manures comprise ashes, salt, phosphate of lime, plaster, etc.

They supply plants with inorganic matter. Their usefulness depends on their solubility.