atomic no. 19, atomic wt. 39.100, metal, row 5, col 1A, val. 1, orbits 1-8-8-1

{Merck Index - © 1952 by Merck & Co., Inc.}

Potassium. K,; at. wt. 39.100; at. no. 19; valence 1. First prepared in the free form by Davy in 1807 by electrolysis of fused potassium hydroxide. Found in nature in large quantities; constitutes 2.4% of the crust of the earth; occurs in the minerals orthoclase, microcline, anorthoclase, hyalophane. Prepn.: Hackspill, Helv. Chim. Acta 11, 1003 (1928). The naturally occurring element has isotopes of mass number 39, 40, 41; of these 40K is radioactive, half-life 2.4 X 108 years.

Soft, silvery-white metal; body-centered cubic structure; tarnishes on exposure to air; becomes brittle at low temperatures; m. 63.65° b. 774° d° 0.862. Soluble in liquid ammonia, ethylenediamine, aniline; soluble in several metals, forming alloys. One of the most active metals; reacts vigorously with oxygen; with water even at –100° with acids. Reacts vigorously with the halogens, igniting with bromine and iodine. The molten metal reacts with sulfur; with hydrogen sulfide. Reacts with hydrogen slowly at 200°, rapidly at 350 – 400°. Reacts slowly with anhydrous hydrogen halides at room temperature; when the molten metal is used it ignites in the reaction. Reduces silicates, sulfates, nitrates, carbonates, phosphates, oxides and hydroxides of the heavy metals, often with the separation of the metal. Reacts with organic compounds containing active groups. Inert to saturated aliphatic and to aromatic hydrocarbons. Keep under liquid containing no oxygen, e.g., liquid petrolatum, petroleum, etc.

Use: In synthesis of inorganic potassium compounds; in organic syntheses involving condensation, dehalogenation, reduction, and polymerization reactions.

{mineral deficiencies in plants}

Unlike all the other major elements, potassium does not enter into the composition of any of the important plant constituents, such as proteins, chlorophyll, fats and carbohydrates, concerned in plant metabolism. For this reason its role is more difficult to determine and in spite of much study it cannot be said that the functions of potassium are clearly understood.

The element is present in all parts of plants in large or fairly large proportions. It seems to be of special importance in leaves and at growing points, as these are especially rich in potassium. Probably the whole of the potassium in plants is present in soluble form and most of it seems to be contained in the cell sap and cytoplasm.

It is outstanding among the nutrient elements for its mobility and solubility within the plant tissues and these properties no doubt account for the ready way in which potassium can be re-utilized by young tissues when the element is in short supply.

Among the functions which have been attributed to potassium and the processes with which it may be concerned, the following may be mentioned: The formation of carbohydrates and proteins; the regulation of water conditions within the plant cell and of water loss by transpiration; as a catalyst and condensing agent of complex substances; as an accelerator of enzyme action (e.g. for diastase); as contributing to photosynthesis through its radio-active properties.

It has been shown in many instances that the potassium content of plants is frequently much higher than is necessary for healthy growth and it is generally considered that luxury (i.e. unnecessary) absorption of potassium often takes place.

The great mobility of potassium in plants, its special importance for and its re-utilization by young tissues and its apparent functions as a regulator of plant processes on a large scale are in harmony with the observation that, when potassium is moderately deficient, the effects are seen first in the older tissues and progress from these towards the growing points, but, when the deficiency is acute, growing points are severely affected and die-back and general collapse of the plants commonly occur.

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Special Note:

Commercial preparations of plant food have a somewhat misleading labeling system.

It is often said that the three main numbers listed is the amount of Nitrogen, Phosphorus, and Potassium in the product.   This is NOT the case.   You must real the label carefully.

If you read carefully, you will find that the Last number is the percentage of Soluble Potash – NOT Potassium – expressed as K2O.   This means that you need to multiply the number by 0.83 to obtain the actual amount of Potassium present in the product.   Therefore, if the middle number is 15, the actual amount of Potassium is only 12.45%.

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