Elements of agricultural chemistry and geology

by Jas. F. W. Johnston

The general nature and mode of operation of such mineral substances as are capable of acting as manures, will be in some measure understood from what has already been so fully stated in regard to the necessity of inorganic food to living plants, and to the kinds of such food which they specially require. A slight notice, therefore, of the more important of these manures now in use will here be sufficient.

1. Nitrates of Potash and Soda.—Saltpetre and nitrate of soda have been deservedly commended for their beneficial action, especially upon young vegetation. They are distinguished by imparting to the leaves a beautiful dark green colour, and are applied with advantage to grass and young corn, at the rate of 1 cwt. to ½ cwt. per acre. The nitric acid they contain yields nitrogen to the plant, while potash and soda are also put within reach of its roots, and no doubt serve many beneficial purposes.

Sulphate of Soda, or Glauber’s salt, has lately been recommended in this country for clovers, grasses, and green crops. Mixed with nitrate of soda it produces remarkable crops of potatoes.[19]

Sulphate of Magnesia, or Epsom salts, might also be beneficially applied in agriculture, probably to clovers and corn crops. As it can be had in pure crystals at 10s. a cwt., and in an impure state at a much less price, from the alum works, it might readily be submitted to trial.

Sulphate of Lime, or Gypsum, is in Germany applied to grass lands with great success, over large tracts of country. In the United States it is used for every kind of crop. It is especially adapted to clovers and legumes.

These three substances all afford sulphur to the growing plant, while the lime, soda, and magnesia are themselves in part directly appropriated by it, and in part employed in preparing other kinds of food, and in conveying them into the ascending sap.

Though there can be no question that these and similar substances are really useful to vegetation, yet the intelligent reader will not be surprised to find, or to hear, that this or that mineral substance has not succeeded in benefitting the land in this or that district. If he has already bricks enough at hand, you must carry the builder mortar, or he will be unable to go on with his work: so, if the soil contain gypsum or sulphate of magnesia in sufficient natural abundance, it is at once a needless and a foolish waste to attempt to improve the land by adding more; it is still more foolish to conclude that these same saline compounds are unlikely to reward the patient experimenter in other localities.

Common Salt has undoubtedly, in very many districts, a fertilizing influence upon the soil. The theoretical agriculturist knows that a small quantity of it is absolutely necessary to the healthy growth of all our cultivated crops, and he will therefore, early try by a preliminary experiment upon one of his fields, whether or not they require the addition of this species of vegetable food. It is in inland and sheltered situations, and on high lands often washed by the rains, that the effect of common salt is likely to be most appreciable. The spray of the sea, borne to great distances by the winds, is in many districts, where prevailing sea winds are known, sufficient to supply an ample annual dressing of common salt to the land.

Kelp.—Among mineral substances kelp ought not properly to be included, since it is the ash left by the burning of sea-weed. It, however, partakes of the nature of mineral substances, and may, therefore, be properly considered in this place. It contains potash, soda, silica, sulphur, chlorine, and several other of the inorganic constituents of plants required by them for food. It is nearly the same also—with the exception of the organic matter which is burned away—with the sea-weed which produces such remarkably beneficial effects upon the soil. In the Western Isles a method is practised of half burning or charring sea-weed, by which it is prevented from melting together, and is readily obtained in the form of a fine black powder. The use of this variety ought to combine the beneficial action of the ordinary saline constituents of kelp, with the remarkable properties observed in animal and vegetable charcoals.

Wood-ash, among other compounds, contains a portion of common pearl-ash in an impure form, with sulphate also, and silicate of potash. These are all valuable in feeding and in preparing the food of plants, and hence the extensive use of wood-ash as a manure in every country where it can readily be procured.