1296. Humic acid and carbonic acid gas, mixed with water, constituted, according to this theory, the chief food of plants; and every description of manure was only valuable in proportion as it contained these substances. Humic acid is found in abundance in putrescent manure when it is so far rotted as that it may be cut with a spade; and it has been proved that rotten dung contains much more carbonic acid gas and humic acid, weight for weight, than fresh dung. Fresh dung is injurious to vegetation in consequence of its ammonia, which, from its acridity, in practice, is found to burn plants. Ammonia becomes concentrated in stale liquid manures, and such manures are, therefore, mixed lightly with water, 'in order to dilute the ammonia, and allow the proper action of the humic acid, which exists in large quantity in them.' Covering a dunghill with soil in hot weather is also explained as preventing the escape of ammonia; the fresh earth absorbing and condensing the gases which would otherwise be dispersed in the air. Violent fermentation in the dung is checked by the earth partly excluding the atmospheric air and rain water; the oxygen in either of which is indispensable to continue the process of forming carbonic acid gas by uniting with the dung. In regard to composts, it is said that to mix lime with fresh or rotten dung is to waste it; because the lime takes up and renders useless the carbonic acid gas which it contains, and decomposes salts of ammonia. In like manner, a compost of fresh dung and weeds, green leaves, grass, turf, and green vegetables, without lime, is valuable, because all these substances supply abundance of humin. On the other hand, lime promotes the fermentation of peat earth, dry leaves, and every thing which contains hard woody fibre, and supplies humin in quantity.
