506 MANURE of water, organic or combustible matters, and inorganic, incombustible, or mineral matters. These last are left as the ash when the plant is burnt. The elements forming the organic portion of plants are carbon, hydrogen, oxygen, and nitrogen ; the mineral portion, or ash, consists prin cipally of lime, magnesia, potash, soda, oxide of iron, phosphoric acid, sulphuric acid, chlorine, and silica. The atmosphere is the great storehouse of organic plant- food. The carbonic acid always present in the air is, as id well known, eagerly, taken up by the leaves of plants, all of which have the power of decomposing carbonic acid, giving off its oxygen, and assimilating its carbon. Roughly speaking, three-fourths of the dry substance of all plants is derived from the atmosphere. Under conditions of natural growth and decay, when no crops are gathered in, or consumed on the land by live stock, the herbage on dying down and decaying returns to the atmosphere and the soil the elements taken from them during life; but under cultivation a succession of crops deprives the land of the constituents which are essential to healthy and luxuriant growth. Without an adequate return to the land of the matters removed in the produce, its fertility cannot be maintained for many years. In newly-opened countries, where old forests have been cleared and the land brought into cultivation, the virgin soil often possesses at first a high degree of fertility, but gradually its productive power decreases from year to year. Gene- nlly speaking, it is more convenient to clear fresh forest land than to improve more or less exhausted virgin land by the application of manure, labour, and skill. In all densely peopled countries where such a mode of cultiva tion cannot be followed it is necessary to resort to arti ficial meaus to restore the natural fertility of the land and maintain and increase its productiveness. The researches of Liebig, Wiegmann, Polstorff, and others have proved beyond doubt the important functions of the mineral constituents of the soil in relation to plant life. The gradual removal of phosphate of lime in the tillage of dairy districts, or the removal of other mineral matters essential to the healthy growth of farm crops, certainly impoverishes the land. The exhaustion of the soil is caused in a much more marked way, however, by the rapid loss in available nitrogenous plant food which soils sustain when under cultivation without manure. Agricultural improvements manifest themselves in two different directions the mechanical and the chemical. Under mechanical improvement the physical condition of the soil is bettered and its latent stores of plant food brought into action by mechanical means, such as ploughing, sub- soiling, steam cultivation, <fec. The introduction of new and superior agricultural implements, good systems of drainage, and intelligent division of labour characterize the first stage of progress in agriculture. The second stage is marked by the application of chemical principles to practical agriculture, an application shown by the intro duction of a rational system of feeding, a proper rotation of crops, and chiefly the use of chemical, or artificial, manures for the purpose of restoring the natural fertility of the soil and increasing its productive powers. The aid which chemistry has rendered during the last twenty-five or thirty years to practical agriculture has greatly promoted agricultural improvements ; and farming, which is in a large measure dependent for success upon an economical use of manures, is now being carried on much more rationally than in former times. The proper appli cation of various kinds of manures is one of the most prominent features of successful modern farming. Iti considering the economical use of manures on the land, regard must be had to the following points : (1) the requirements of the crops intended to be cultivated ; (2) the physical condition of the soil ; (3) the composition of the soil ; and (4) the composition of the manure. Briefly stated, the guiding principle of manuring economically and profitably is to meet the requirements of the crops intended to be cultivated, by incorporating with the soil, in the most efficacious states of combination, the materials in which it is deficient, or which the various crops usually grown on the farm do not find in the land in a sufficiently avail able condition to ensure an abundant harvest. Soils vary greatly in composition, and lience it will be readily understood that in one locality or on one particular field a certain manure may be used with great benefit, while in another field the same manure has little or no effect upon the produce. Although increased attention has of recent years been paid to the chemical composition and properties of soils, there is still much room left for improve ment, for many farmers disregard almost altogether the composition of their fields in buying artificial manures. It has been pointed out by Sir John Lawes that in actual English farm-practice there is, speaking practically, a standard of natural produce which varies within certain limits, as influenced by seasons and management, and which cannot be permanently increased or reduced by cultivation ; and further, when land is spoken of as being in " good condition," reference is made only to the tem porary rise of fertility by means of the manures employed, while by land "out of condition" is signified the exhaus tion of the manures by the removal of crops, loss by drain age, &c., and that the soil has merely gone back to its standard of natural productiveness. Some soils, indeed, contain in their natural condition hardly a sufficient pro portion of available elements of plant food to yield remunerative crops ; such soils are naturally barren, and, although by the constant use of manures they may be improved and may attain some degree of fertility, they will, if left unrnanured, speedily revert to their former natural unproductive state. The principal constituents of manures are nitrogen, in the form of ammonia, nitrates, and nitrogenous organic matters; organic matters not containing nitrogen (humus); phosphoric acid, potash, soda, lime, magnesia, silica, sulphuric acid, and chlorine. Of these constituents by far the most important are the nitrogen, phosphoric acid, and potash, and these will be considered more in detail. 1. Nitrogen. Nothing so much affects the productive ness of soils as nitrogen, when it is supplied to plants in a form in which it can be assimilated by them, and nothing is more readily removed from the land, either in the crop grown or, in the form of nitrates, by drainage. The " good condition " of land, to which allusion has been made already, is, in a great measure, the fertility which has been imparted to the soil by the nitrogen supplied to it, while the " natural productiveness " may be taken as that due to the phos phoric acid, potash, and other mineral constituents of the soil. Supply of Nitrogenous Plant Food. In the case of a crop growing under natural conditions, and not removed from the land, the mineral constituents taken from the soil, and carbonaceous as well as some nitrogenous organic matter, principally derived from the atmosphere, are returned to the land, and a rich carbonaceous soil or humus is produced in the course of years. Such a soil is capable of supplying for a considerable number of years, when under cultivation, the nitrogen needed by the crops grown on the land. This supply of nitrogen, however, gradually becomes exhausted by repeated cropping. The atmosphere, in addition to free nitrogen gas, which consti tutes about four-fifths of its volume, contains but very little combined nitrogen in the forms of ammonia or nitric acid.
M. Boussingault s experiments clearly show that plants