Potash
A compound of oxygen with a metallic element, first separated, in the pure state, by Sir Humphrey Davy, in the beginning of this century, and named by him potassium, in allusion to its preparation from Potash. The latter was itself long regarded as an element, because of the difficulty of analysing it, and of separating the oxygen in it from the potassium. This latter is an essential element in the chemical composition of plants; as may be inferred from two facts, viz.: (1) it is always present in the ash that remains when well-dried plants are burned; and (2) plants grown in soils or fluids of known composition, from which potassium is entirely withheld, remain stunted. Among the chief commercial sources of potassium are wood-ashes. These are washed, and carbonate of potassium and various other substances are separated from them in this operation; the water in which they are dissolved is then heated until it is mostly driven off in the form of steam, and the less soluble substances can no longer be kept in solution, but fall to the bottom of the vessel. The carbonate of potassium remains dissolved after the other compounds have separated out. The solution is poured off, and, on being heated till all the water is driven off, the carbonate remains in a somewhat impure state, known as pearl-ash. From this are prepared, by appropriate chemical processes, the element potassium, and its various compounds. The ashes of leaves, and, in fact, of all parts of plants, yield a considerable proportion of pearl-ash; indeed, it is more abundant in the green parts than in the wood. It is plentiful also in seeds. Phosphate and chloride of potassium are also found in the ashes of plants. The compounds of potassium, taken together, amount very frequently to one-half, or even more, of the total weight of the ashes. Potassium probably exists in plants combined with organic acids, formed in the plants during growth, e.g., as potassic tartrate, potassic oxalate, &c.; but these compounds are broken up when the dried plants are burned; and they are replaced in the ashes by the compounds already mentioned. Plants obtain the potassium that they require from the soil, which is seldom, if ever, wholly devoid of the element. Its exact uses to plants are still somewhat uncertain, as the results of experiments do not entirely agree among themselves; but there is little doubt in regard to certain conclusions, which are founded alike on analyses of ash of plants and on experiments. It has been found that plants from which potassium is entirely withheld cease to form new food for themselves, although supplied with every other element required for their nutrition. The tissues and organs of the plants remain healthy for a time; but they do not increase in size. On supplying a solution of any compound of potassium to them, they begin to grow again, and they continue to do so if the supply is kept up. It has been ascertained, by experiment, that the compounds of potassium most useful to plants are the chloride and nitrate; the phosphates and sulphates being less effective in promoting increase in size. In reference to the mode of action of potassium on plants, its presence seems necessary to permit of starch being formed in the green tissues, by means of the chlorophyll. But, in addition to starch being formed in the green parts, it must be transferred from the tissues, where it is formed, to those in which it is to be made use of, or to be stored; and this seems to require the presence of chloride or of nitrate of potassium in the tissues. When the sulphate alone is supplied, the leaves become gorged with starch grains, and fleshy, and look sickly. The same result follows, though less markedly, when the phosphate is used instead of the sulphate.
