Manure. Any material, whether of animal, vegetable, or mineral origin, which adds to the fertility of the soil has been generally regarded as manure. The application of stable and farmyard manure, as also the ashes of plants, &c., to the soil has been practised probably in all ages; but the scientific principles involved in this ancient practice were but little understood until more recent times, when chemists, botanists, and physiologists set themselves the task of explaining to the agriculturist the changes which are ever taking place in the soil and in the plant itself. On virgin soils crops may be grown for years without much evident diminution in quantity or quality; but a period must come when there will be an exhaustion of one or more of the constituents of plants, and the soil can then be no longer regarded as fertile. That is to say, soils contain certain proportions of certain ingredients; and when these are abstracted by the plant and carried away in the form of crops, the soil must in time become exhausted. It then becomes necessary to add to the soil in the form of manure such constituents as the crops have removed in order that the land may regain fertility. When we consider that Soils (q.v.) are formed mainly from the weathering of rocks, it will at once be understood how it is generally unnecessary that manures should contain such things as magnesia, iron, alumina, &c. Speaking generally, the constituents which are removed by plants from soils, the loss of which brings about that condition of 'exhaustion,' are compounds of nitrogen, phosphoric acid, and potash; and hence it is, in part at least, that farmyard manure is so universally regarded as the 'stand-by' of the agriculturist, for that material contains all those ingredients, and in a form easily assimilated by plants. It must not be overlooked, however, that possibly the chief advantages derived from the use of farmyard manure are that it makes the soil porous, and that the conditions which result from the decomposition of the organic matter are favourable to the development of those micro-organisms which bacteriologists are endeavouring to prove are of as much importance as the manure itself (see NITRIFICATION). Manures containing large proportions of organic matter, such as stable manure, wrack or seaweed, fish offal, &c., have value as plant-food; but the heat developed during their decomposition, or rotting, and the fact that the carbonic acid resulting from that change acts as a solvent on the mineral constituents of the soil and otherwise, are of still greater moment.
The first artificial manure systematically used was probably bones, applied in the earlier periods, either in an unground condition or simply bruised. About the beginning of the 19th century, however, it was proved that fineness of division rendered bone more easily assimilated by plants; and further progress still was made when Liebig introduced the treatment of bone with sulphuric acid, whereby chemical division was realised. There are about 50,000 tons of bone imported into Britain annually, while of home-collected bone not less than 60,000 tons are employed in the manufacture of manure, and of this latter quantity London alone produces fully 23,000 tons annually.
Guano.—Peruvian guano was at one time imported from the Chincha Islands in enormous quantities; but the old deposits are practically exhausted, and the quality of the now available supply is comparatively poor (see GUANO). Ichaboe (off south-west Africa) guano is still imported, though in somewhat limited quantity; but it is much the richest available guano. The value of these guanos lies in the percentages of nitrogenous organic matter, ammonia salts, phosphates of the alkalies and of lime, and the potash salts which they contain. Some of the islands in the South Pacific yield supplies of guano, but these are almost purely phosphatic, owing to the abundant rainfall of that region having washed out all the ammoniacal salts. Liebig's Guano and Liebig's Meat Meal are by-products from the preparation of Liebig's extract of meat. They are in a fine state of mechanical division, and are valuable sources of nitrogen and phosphates. Fish-guano is largely produced in Norway and the north of Scotland from fish offal (see GUANO). The process employed is essentially steaming to remove the oil, which is run off with the water; the solid residue is pressed and dried. The manurial constituents of this material are nitrogen and phosphates.
Dried Blood is another valuable source of nitrogen. Horn powder, shoddy and wool waste, leather raspings, &c. are also employed for the same purpose, but they are of much less value.
Phosphatic Substances.—The coprolites of Cambridge, Norfolk, and Suffolk come under this classification, as does also the land phosphate from the Ashley Basin of South Carolina, and the dredged phosphate from the Ashley and Cooper rivers. In addition to these we have Canadian apatite, Sombrero, Navassa, Somme, Belgian, Florida, Spanish, Curacao, and numerous other phosphates. These are of little value as manure in their natural and ground state, and are almost wholly converted into superphosphate by the action of sulphuric acid, whereby the natural or tricalcic phosphate—which is insoluble in water—is converted into the monocalcic phosphate, which is soluble, and therefore readily available to plants.
Sulphate of Ammonia is principally derived from the destructive distillation of coal and shale (see GAS, Vol. V. p. 215, and PARAFFIN). Its value depends upon the percentage of ammonia which it contains. This salt sometimes contains sulphocyanide of ammonium, a substance which is inimical to plant-life. Some experimentalists assert with all confidence that ammonia salts must undergo nitrification before they can enter the plant; but that contention is scarcely now tenable, because nitrification is possible in some cases within the plant itself.