Agricultural Chemistry, the study of the chemical relations of substances concerned in agricultural production. The whole natural science of vegetable and animal production is usually called agricultural chemistry, although it includes much of physics, meteorology, vegetable and animal physiology, and geology. It is impossible to separate these subjects, without grave errors; and hence those works which give the justest view of the chemistry of agriculture are not strictly treatises on agricultural chemistry. The object of agriculture is to develop from the soil as large a quantity as possible of useful vegetable products; or indirectly, of animal products. To assist in this, agricultural chemistry must inquire into the composition of the plant and animal. It finds that all vegetable and animal substances contain a variable, usually large proportion of water, which is essential to their life, but may be separated from them by heat without otherwise affecting their chemical composition. At a high temperature, dry animal or vegetable tissues are resolved into two portions; one passes into the air as volatile gases or vapors; another, indestructible by heat, remains as ashes.

In most vegetable and animal substances, the combustible or organic part forms 90 to 99 per cent, of the whole dry matter; the proportion of inorganic substances (ash) being small. The organic matter mainly consists of four elements, viz.: carbon, oxygen, hydrogen, and nitrogen. These simple bodies are united in the plant and animal into thousands of combinations, the extended study of which belongs to organic chemistry. Most agricultural products, however, consist chiefly of but a few of these combinations or proximate elements. These may be specified under four classes: 1. The oils and resins, including wax. 2. Cellulose (cell tissue, woody fibre); starch; the sugars, cane and grape; the gums, arabine, bassorine, dextrine (starch gums). 3. Pectose (the pulp of green fruits) and its derivatives. 4. The nitrogenous or sanguigenous* principles, viz.: albumen, case-ine (legumine, avenine), emulsine, and fibrine (gluten). The first three groups are composed exclusively of carbon, hydrogen, and oxygen (some of the oils, of carbon and hydrogen only), while all the members of the fourth group contain 15 to 18 per cent, of nitrogen, most of them small quantities of sulphur, and phosphorus also, in addition to the three elements above named.

The whole growing part of the plant is a porous substance, as easily penetrable by air as a sieve, and a hygroscopic substance, absorbing and retaining the vapor of water from the air or soil with great force and obstinacy. When a vegetable is destroyed by burning, it is mostly resolved into air. On the other hand, when it is formed by growth, its substance is mostly derived from air. The atmosphere which perpetually bathes and penetrates the leaves of plants supplies them with carbon, hydrogen, nitrogen, and oxygen. The atmospheric source of carbon is carbonic acid. This gas is a constant ingredient of the atmosphere to the extent of 1/2500 of the volume of the latter. It is rapidly absorbed by the leaves of growing plants under the influence of sunlight, and undergoes decomposition in the vegetable cells, carbon being retained and assimilated, while the oxygen is set free wholly or in part, and exhales from the leaves. Water, which always exists in the atmosphere in the state of vapor, is an abundant source of both oxygen and hydrogen. Ammonia, a compound of hydrogen and nitrogen, is the chief source of nitrogen to the plant. It is ever present in the atmosphere in the form of carbonate, though in exceedingly small quantity.

Nitric acid, which is formed by the oxidation of ammonia, is also a source of nitrogen. The plant being fixed and at rest, its food must necessarily be in perpetual motion around the organs destined to take it up. The atmospheric food is kept in motion, not only by the winds, but more effectually by the osmotic force (exosmose and endosmose). When two or more gases of unequal density are brought in contact in a confined space, they will gradually diffuse into each other, until they form a homogeneous mixture. If into a mixture of gases any solid or liquid body be introduced, which can combine with and remove one of the gases, it first takes up those particles of this gas which are in its immediate vicinity; but as fast as the uniformity of the mixture is thus disturbed, the absorbable gas diffuses into the space which has become void of it; and as new portions are removed, other new ones are presented, until the whole is absorbed. All the forms of plant food are soluble in water. In virtue of these physical laws, it is plain that the tissues of a growing plant must be constantly surrounded with water, and with carbonic acid and ammonia dissolved in this water; and as these are removed by the assimilating processes of the vegetable, they are restored by osmotic diffusion, so long as the atmospheric supply suffices. - The ash of agricultural plants consists of the phosphates, sulphates, silicates, and carbonates of potash, soda, lime, and magnesia, with small quantities of oxide of iron and manganese, and alkaline chlorides.

Other bodies, as alumina, copper, and zinc, are found in some kinds of land plants. The living plant contains sulphur (and perhaps phosphorus) in a state of organic combination, in the various nitrogenous principles, or in sulphurized oils. On burning these compounds, sulphuric and phosphoric acid result. Portions of the potash, soda, lime, and magnesia are combined with vegetable acids (oxalic, tartaric, malic) in the living plants, but these compounds are converted into carbonates by burning. Silica exists probably in the un-combined state in many cases, as in the bam-boo (tabasheer), stalks of grasses, and scouring rush; but in burning it combines with potash, lime, etc, so that it is found as a silicate in the ash. That these ingredients of the ash are indispensable to the development of vegetation, is proved not only by their invariable occurrence in normally developed plants, but by direct experiment. The cereal grains, for example, will not mature in a soil which is deficient in any one of the following substances, viz.: potash, soda, lime, magnesia, oxide of iron, oxide of manganese, silica, sulphuric acid, phosphoric acid, chlorine. These kinds of plant food are all derived from the soil, and enter the plant through its roots.