HUMUS is a generic term applied to a group of closely allied substances, which col lectively form the organic matter of the soil. These substances may be divided into three great classes: 1. Such as are soluble in water—crenic, apocrenic, and uhnic acids; 2. Such as are soluble in alkaline solutions, but not in pure water—humic and geic acids; 3. Such as are insoluble in all menstrua—humin and ulmin.
All of these are amorphous, ranging in color from a brownish yellow to a blackish brown; and non-volatile; they are probably all composed of carbon, hydrogen, and oxy gen, and they are all remarkable for their power of fixing ammonia. " They are all products of the decomposition of vegetable matters in the soil, and are formed during their decay by a succession of changes, which may be easily traced by observing the course of events, when a piece of wood, or any other vegetable substance, is exposed for a length of time to air and moisture. It is then found gradually to disintegrate with the evolution of carbonic acid, acquiring first a brown, and finally a black color. At one particular stage of the process, it is converted into one or other of two substances, called humin and ulmin, both insoluble in alkalies, and apparently identical with the insoluble humus of the soil; but when the decomposition is more advanced, the products become soluble in alkalies, and then contain humic, ulmic, and geic acids; and finally, by a still further progress, crenic and apocrenic acids are formed, as the result of an oxidation occurring at certain periods of the decay."—Anderson's Agricultural Chords • try, 1860, p. 22.
The roots and other vegetable matters remaining in the soil gradually undergo the changes which have been described in the preceding extract, and are thus converted into humus, which is found only in the surface soil, in which its quantity varies with the activity and profusion of the vegetation.
, Numerous analyses of the humus compounds have been made, but, as might be expected from the gradual passage of one substance into another, they present con siderable discrepancies. According to Mulder, who is perhaps the highest authority on this subject, geic acid is represented by C4JI12014; humic acid, by C4oHig012;_ulmic apid, by C4,,H,40,2; crenic acid, by 3HO,C2411,20,6; apocrenic acid, by 2HO,Cali2024. Crenic and apocrenic acids (which derive their names from cren0, the Greek word for a spring) not only occur in combination with ammonia, in the organic matter of the soil, but are likewise found in many mineral waters, and in the ochry deposits that accumulate round the margins of chalybeate springs.
All the above-named substances closely resemble in their composition the woody fiber or cellulose (C,4119,09,), from which they are derived by a slow process of oxidation.
Chemists hold very different opinions regarding the physiological value of humus, The earlier chemists, and Mulder at the present day, regard it as the almost (It not quite) exclusive source of the organic constituents of plants; while Liebeg and the great major ity of the chemists of the pre,§ent day the'atmosphcre (which consists of a mixture of nitrogen and oxygen gases, watery vapor, carbonic and nitric acids, and ammonia) as capable of affording an abundant supply of all these The latter is proba bly the more correct view; but although humus is not a direct source of the organic constituents of plants, and is not absorbed by their roots, as was formerly supposed, it is so indirectly in at least two modes—viz., by evolving (luring its decomposition a ci rtain quantity of carbonic acid which can be absorbed, and by its power of absorbing and combining with ammonia and with certain soluble inorganic constituents of plaids. Its power of absorbing ammonia is readily shown by pouring some ammoniacal solution on peat (which contains the humus compounds in great abundance); the pungent smell at once disappears, which is an evidence that combination has taken place. It possesses similar but less marked power in reference to potash, soda, lime, and magnesia, and thus plays an important part in preventing these substances from being washed out of the soil. The physical properties of humus are also of great importance in relation to the fertility of the soil. Humus is one of the most highly hygrometical substances known. While siliceous sand absorbs only one-fourth of its weight of water, and again gives off, in the course of four hours, four-fifths of its water, humus imbibes nearly twice its weight of water, and retains nine-tenths of it after four hours' exposure. It thus confers on the soil the power of absorbing and retaining water, and thus diminishes its tenacity, and allows of its being more easily worked; and additionally, from its dark color, it causes the more rapid absorption of heat from the sun's rays. Hence, although not contribut ing directly to the food of plants, it is in moderate quantity an indispensable constituent of fertile soil. The best wheat-bearing soils contain 9 or 10 per cent of humus compound.