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.