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?