The condition of the soil is the main point to be considered. It must be such as to absorb and retain ammonia--to allow water to pass _through_ it, and be discharged _below_ the point to which the roots of crops are searching for food--and to admit of a free circulation of air. The power of absorbing and retaining ammonia is not possessed by sand, but it is a prominent property of clay, charcoal, and some other matters named as absorbents. Hence, if the soil consists of nearly pure sand, it will not make use of the ammonia brought to it from the atmosphere, but will allow it to evaporate immediately after a shower. Soils in this condition require additions of absorbent matters, to enable them to use the ammonia received from the atmosphere. Soils already containing a sufficient amount of clay or charcoal, are thus far prepared to receive benefit from this source. [Why does under-draining increase the absorptive power of the soil? How do plants obtain their carbonic acid? How does carbonic acid affect caustic lime in 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.