Saline Manures
These are contrasted with farmyard Manures and others that contain abundant organic matter, i.e., remains of animals and of plants. The Saline Manures, consisting of mineral substances, are compounds or "salts" of various metals. They differ widely in composition, some being made up of one mineral substance alone, others being compounds of several, mixed naturally or artificially. The artificial Manures are largely employed on farms, and, to some extent, in gardens, to improve the crops either in quantity or in quality. They differ much in their mode of action: some of the substances supply to the plants those minerals that are required by all of them, as shown by the chemical analysis of their ashes; e.g., Potassium Chloride supplies to plants the elements Potassium and Chlorine, both of which, but especially the former, are required by them. Ammonium Sulphate, which is often made use of as a Manure, supplies Nitrogen in a form readily taken up by plants. Other Saline Manures serve as food for the plants not only directly, but even still more efficiently by rendering certain injurious acids and other substances harmless, or by acting on various substances already in the soil, changing them from an insolube to a soluble condition. Plants are able to absorb the substances thus changed, and to employ them as food. As examples of such Manures, Carbonates of Potash and of Soda, and Gypsum or Sulphate of Lime, may be mentioned.
The views generally entertained in regard to the uses of the various elements in Saline Manures may be summed up as follows:
Potassium is concerned in the formation of starch, as is shown by the absence of the latter from plants from which the element is completely withheld. Such plants soon cease to grow; but growth is resumed on Potassium being again given. Potassium Chloride is the best source of this element for plants, and Potassium Nitrate (saltpetre) is the next best. Potassium Sulphates and Phosphates are less efficacious.
Sodium compounds are abundant in many plants near the seashore; but Sodium does not seem essential; at least, plants from which it is withheld often grow equally well with others to which it is supplied.
Calcium and Magnesium are always present, though varying much in amount. The absence of Calcium checks growth after a time; but the exact use of each element is uncertain. Possibly, both benefit plants, "partly in serving as a vehicle for Sulphuric and Phosphoric acids in the absorption of food materials, and partly in fixing the Oxalic Acid, which is poisonous to the plant, and in rendering it harmless" (Sachs).
Iron is required, though only in minute quantity, to permit of chlorophyll becoming green, and doing its work in the nutrition of plants; but most soils contain a sufficient amount of Iron, and even a small excess is apt to be injurious.
The Saline constituents required by plants may be supplied to any soil in which they are deficient, either by adding such Manures as contain them, or by subjecting the soil to such treatment as will convert insoluble into soluble salts. One of the most important Manures is Kainite, a mineral brought from various localities, including Greenland. It occurs in rock masses, and consists of Potassic Sulphate, Magnesic Sulphate, and Magnesic Chloride, thus supplying several important elements. Other valuable Manures are mentioned above.