THEIR USE.

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soil; and the excess of carbonic acid is even hurtful toplants themselves.

430. The principal vegetable substances employed asmanure are straw of all kinds, leaves, sawdust, bran,oil-cake, seaweed, and green manures, or crops whichare merely sown to be ploughed in, and thus afford foodto a second crop, of some more valuable plant.

431. All these manures when mixed with soil slowlydecay, and yield carbonic acid and small quantities ofsaline and earthy matters. They are most advantageouslyused when employed in combination with some kind ofanimal manure ; this is the case in farm-yard manure.Straw alone decays but slowly, but when mixed withthe dung and urine of cattle it soon begins to change,and in a short time the whole is brought into a state ofdecomposition.

432. The decay of vegetable manures may also befacilitated by the addition of lime; for the objectionwhich applies to the mixture of lime with animalmanures is not applicable to vegetable substances. Thelatter for the most part contain but little nitrogen, theirvalue principally depending on their mechanical action,and on the formation of carbonic acid.

433. Vegetable manures decay more or less rapidlyin proportion to the quantity of nitrogen which theycontain; green manures contain a notable quantity ofgluten and albumen, and accordingly decompose rapidly,whilst sawdust, which consists principally of woodyfibre,and contains hardly any nitrogen, decomposes slowly.