_Drainage promotes absorption of fertilizing substances from the air._ The atmosphere bears upon its bosom, not only the oxygen essential to the vitality of plants, not only water in the form of vapor, to quench their thirst in Summer droughts, but also various substances, which rise in exhalations from the sea, from decomposing animals and vegetables, from the breathing of all living creatures, from combustion, and a thousand other causes. These would be sufficient to corrupt the very air, and render it unfit for respiration, did not Nature, with her wondrous laws of compensation, provide for its purification. It has already been stated, how the atmosphere returns to the hills, in clouds and vapor, condensed at last to rain, all the water which the rivers carry to the sea; and how the well-drained soil derives moisture, in severest time of need, from its contact with the vapor-loaded air. But the rain and dew return not their waters to the earth without treasures of fertility. Ammonia, which is one of the most valuable substances found in farm-yard manures, and which is a constant result of decomposition, is absorbed in almost incredible quantities by water. About 780 times its own bulk of ammonia is readily absorbed by water at the common temperature and pressure of the atmosphere; and, freighted thus with treasures for the fields, the moisture of the atmosphere descends upon the earth. The rain cleanses the air of its impurities, and conveys them to the plants. The vapors of the marshes, and of the exposed manure heaps of the thriftless farmer, are gently wafted to the well-drained fields of his neighbor, and there, amidst the roots of the well-tilled crops, deposit, at the same time, their moisture and fertilizing wealth. Of the wonderful power of the soil to absorb moisture, both from the heavens above and the earth beneath--by the deposition of dew, as well as by attraction--we shall treat more fully in another chapter. It will be found to be intimately connected with the present topic. _Thorough drainage supplies air to the roots._ Plants, if they do not breathe like animals, require for their life almost the same constant supply of air. "All plants," says Liebig, "die in soils and water destitute of oxygen; absence of air acts exactly in the same manner as an excess of carbonic acid. Stagnant water on a marshy soil excludes air, but a renewal of water has the same effect as a renewal of air, because water contains it in solution. When the water is withdrawn from a marsh, free access is given to the air, and the marsh is changed into a fruitful meadow." Animal and vegetable matter do not decay, or decompose, so as to furnish food for plants, unless freely supplied with oxygen, which they must obtain from air. A slight quantity of air, however, is sufficient for putrefaction, which is a powerful deoxydizing process that extracts oxygen even from the roots of plants.