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? How may it affect excrementitious 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 classed among manures, if we consider its effects both chemical and mechanical in the soil. 1. By oxidizing or _rusting_ some of the constituents 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 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?] _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.