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?