MANURES (Fa., Engrais; GER., Diinger).
Every plant requires to be fed with certain substances, in order to attain perfection. The proportion of each substance needed by different plants varies considerably ; but the subataneea themselves are the same in kind in almost every case, and consist principally of nitrogen, potash, phosphoric acid, lime, sulphur, soda, magnesia, chlorine, and silica. A productive soil contains all these elements in sufficient abundance, and will go en bearing crops incessantly, provided that all the plants which grow upon it be returned to it, either in a rotten natural condition, or as the excreta and remains of the animals feeding upon them. Further, it is well known that in se-called "new countries," it is possible to grow and remove crops from the same soil for many years in succession, without artificial aid, a fact which is due to the presence in the " virgin soil " of more than enough of the various or principal elements for immediate needs. But when this store has been exhausted by constant cropping, the plants become at first sickly, and, after a time, altogether refuse to grow. Then arises a necessity for enriching the soil by the addition of the ingredients in which it is wanting. This operation is known as " manuring " ; while the materials thus supplied are called " manures." The " rotation " system of cropping; based upon the difference in proportion of the various elements required by certain crops, is very judicious ; but it cannot be made to obviate the necessity for adopting artificial measures for maintaining the fertility of the soil. In the present condition of agricultural science, no attempt is made to supply a manure containing all the ingredients required by the crop to be grown, nor, except iu rare instances, is an analysis made of the soil, in order to determine in what element it is deficient. The farming community are generally content to apply the article which the manufacturer chooses to send them, looking rather to its price than to its efficacy, and manifesting satisfaction according to the degree of its odour.
Manures may be generally divided into two kinds, natural and artificial.
Natural Manures.—Natural manures are farm-yard dung, sewage, seaweed, soot, nitrate of soda, and Stassfurt salts; these may be applied to the land without any previous preparation. Besides these, there are ether natural products, of great value in agriculture, which, used in a raw state, are not in such a fit condition to he assimilated by the plant as after undergoing the treatment presently to be described. Principally, they are bones, fish, flesh, blood, and guanea. The prepara tion of " poudrette," or sewage treated with sulphuric acid, and dried, will be described hereafter.
Bones.—Though very slow in undergoing decomposition, bones are a most valuable manure, especially on light soils. They contain about 2-4 per cent. of nitrogen, and 50-60 per cent. of phosphates. They are seldom applied without having been first boiled to eliminate the fat, which may be done in ordinary coppers of large size. When it is desired to extract their gelatine, for glue-making and similar purposes, the boiling is effected under pressure (see Bones, p. 521). To hasten the fertilizing action of bones, they are almost always ground in a,mill, after having become sufficiently dry from the boiling operation. The finer their state, the more rapidly will the crop he benefited by their applica tion. One of the best forms of mill for reducing bones is that shown in Fig. 930. This is a powerful and compact machine, with a cast-iron frame and foundation-plate, and when attached to a 10-H.P. steam-engine, or a water-wheel, it should crush and dress 15-20 tens daily. It weighs 71 tone, and the fly-wheel should make about 145 rev. a minute. It has two pairs of rollers, with cutters (made of cast-iron, and case-hardened) for crushing the bones, a revolving riddle for separating them into" rough," " half-inch," and " dust" or" quarter-inch," and a friction-sheave for preventing accidents to the cutters. Such a mill should cost approximately 250/.