This section is from "The Horticulturist, And Journal Of Rural Art And Rural Taste", by P. Barry, A. J. Downing, J. Jay Smith, Peter B. Mead, F. W. Woodward, Henry T. Williams. Also available from Amazon: Horticulturist and Journal of Rural Art and Rural Taste.
The day has long passed when it was disputed whether saline bodies are promotive of vegetable growth. It is now determined that some plants will not even live without the means of procuring certain salts. Borage, the Kettle, and Parietaria, will not exist except where nitrate of potash is in the soil; Turnips, Lucerne, and some other plants, will not succeed where there is no sulphate of lime. These are facts that have silenced disputation. Still there are found persons who maintain that salts are not essential parts of a plant's structure; they assert that such bodies are beneficial to a plant by absorbing moisture to the vicinity of its roots; or by improving the staple of the soil; or by some other secondary mode. This, however, is refuted by the fact that Baits enter as intimately into the constitution of plants as do phosphate of lime into that of bones, and carbonate of lime into that of egg-shells. They are part of their very fabric, universally present, remaining after the longest washing, and to be found in the ashes of all and any of their parts, when subjected to incineration.
Thus Saussure observes, that the phosphate of lime is universally present in plants. - (Sur la Veget., c. 8, s. 4.) The sap of all trees contains acetate of potash; beet root contains malate and oxalate of potash, ammonia, and lime; rhubarb, oxalate of potash and lime; horseradish, sulphur; asparagus, super-malates, chlorides, acetates, and phosphates of potash and lime; potatoes, magnesia, citrates, and phosphates of potash and lime; Jerusalem artichoke, citrate, malate, sulphate, chloride, and phosphate of potash; garlic, sulphate of potash, magnesia, and phosphate of lime; geraniums, tartrate of lime, phosphates of lime, and magnesia; peas, phosphate of lime; kidney beans, phosphate of lime and potash; oranges, carbonate, sulphate, and muriate of potash; apples and pears, malate of potash; grapes, tartrate of lime; capsicums, citrate, muriate, and phosphate of potash; oak, carbonate of potash; and the lilac, nitrate of potash. Let no one fancy that the salts are a very trivial portion of the fabric of plants.
In the capsicum, they constitute one-tenth of its fruit: of carrot juice, one-hundredth; of rhubarb, one-eleventh; of potatoes, one-twentieth; while of the seed of the Lithospermum officinale they actually form more than one-half.