The _neutrals_, silica, chlorine, oxide of iron, and oxide of manganese, deserve a careful examination. Silica exists in the soil usually in the form of _sand,_ in which it is, as is well known, perfectly insoluble; and, before it can be used by plants, which often require it in large quantities, it must be made soluble, which is done by combining it with an alkali. For instance, if the silica in the soil is insoluble, we must make an application of an alkali, such as potash, which will unite with the silica, and form the silicate of potash, which is in the exact condition to be dissolved and carried into the roots of plants. Chlorine in the soil is probably always in an available condition. Oxide of iron exists, as has been previously stated, usually in the form of the _per_oxide (or red oxide). Sometimes, however, it exists in the form of the _prot_oxide (or black oxide), which is poisonous to plants, and renders the soil unfertile. By loosening the soil in such a manner as to admit air and water, this compound takes up more oxygen, which renders it a peroxide, and makes it available for plants. The oxide of manganese is probably of little consequence. The usefulness of all of these matters in the soil depends on their _exposure_; if they are in the _interior_ of particles, they cannot be made use of; while, if the particles are so pulverized that their constituents are exposed, they become available, because water can immediately attack to dissolve, and carry them into roots. [What is one of the chief offices of plowing and hoeing? Is the subsoil usually different from the surface soil? What circumstances have occasioned the difference? In what way?] This is one of the great offices of plowing and hoeing; the _lumps_ of soil being thereby more broken up and exposed to the action of atmospheric influences, which are often necessary to produce a fertile condition of soil, while the trituration of particles reduces them in size. SUBSOIL. [May the subsoil be made to resemble the surface soil? May all soils be brought to the highest state of fertility? On what examination must improvement be based? What is the difference between the soil of some parts of Massachusetts and that of the Miami valley?]