NITRATES—SALT.

147

on vegetation is but little understood. Some plants,such as the sunflower, tobacco, lettuce, and many others,always contain more or less of these salts. Others donot contain them, but when supplied with nitrates, aresubsequently found to contain the base, without the acid.The soda, potash, or lime, is combined with some organicacid, whilst the nitric acid has disappeared. It is pro-bable that in these cases the nitrogen of the acid is assi-milated by the plant, or that it assists in the formationof gluten and albumen.

459. Nitrates can have but very little value asmanures on the soils which naturally contain salts ofnitric acid, or which, in consequence of the substancesthey contain, are constantly forming nitrates. On soilsneither containing nitrates nor other alkaline salts, theyappear to produce very beneficial results. It has beenfound that wheat manured with alkaline nitrates containsmore gluten and albumen, than that grown in land notso manured.

460. These remarks apply equally to nitrate of sodaand nitrate of potash ; at least, similar effects are pro-duced by the two salts, as far as regards the increasedformation of gluten and albumen.

461. Common salt, and a number of other saline sub-stances, have been proposed and employed as manure.Many experiments might be quoted to show the beneficialeffects produced by this or that substance in particularsoils; but this is foreign to our object, and would merelybe collecting isolated facts. Nearly all that can at