10 POTASSIUM SODIUM 



consists mainly of sulphates and chlorides of magnesium, potassium, 

 sodium and calcium. This deposit has been extensively worked for 

 some years, largely for supplies of potash salts for agricultural 

 purposes. The element itself is of little interest from an agricultural 

 standpoint, as its great affinity for oxygen and other electro-negative 

 elements renders its preparation and preservation difficult. Its com- 

 pounds, however, are of the utmost importance, indeed potash seems very 

 intimately connected with the processes of plant growth and is always 

 most abundant in the growing portions, the young shoots or twigs. 

 The maintenance of a supply of its compounds is essential to a plant's 

 welfare. In the plant it is combined with various acids nitric, sul- 

 phuric, hydrochloric, and very often with organic acids oxalic, 

 malic, citric, or tartaric. In the ashes of plants it is usually found as 

 carbonate, this being formed by the destruction of the organic potas- 

 sium salts by heat. The ashes of the twigs and leaves of trees, 

 indeed, formerly furnished almost the whole of the potash used in the 

 arts. The earlier chemists distinguished potash by calling it the 

 " vegetable alkali " in contradistinction to the " mineral alkali," by 

 which they meant soda, and the "volatile alkali" or ammonia. 



Potash compounds are remarkable in the property which they 

 possess of being retained by clay, and especially by the mixture of clay 

 and organic matter found in nearly all fertile soils. In this respect 

 potash differs greatly from soda, for whose compounds soil possesses 

 little or no retentive power. This retention of potash by soil probably 

 explains the fact that in sea-water there is so much more of sodium 

 compounds than of potassium ones, notwithstanding the fact that the 

 primary rocks of the earth's crust contain about equal amounts of 

 these substances. Denudation carries off to the sea, large quantities of 

 soluble sodium compounds, but comparatively little potassium salts, 

 owing to the retention of the latter by the clay, simultaneously formed 

 by the decay of the felspar, mica, etc. 



Sodium occurs in many silicates, replacing potash. It is ex- 

 tremely widely diffused throughout nature, and, in the form of common 

 salt, plays an important part in animal nutrition. It is asserted by 

 many authorities to be a merely accidental constituent of plants, and 

 in most instances it is found that the exclusion of sodium from a 

 plant's food produces no ill effects ; on the other hand, many marine 

 plants and plants growing near the coast contain large quantities of 

 sodium compounds and a due supply appears essential to their 

 welfare. Although sodium is chemically very like potassium, forming 

 compounds whose properties are very similar to those of that element, 

 its compounds are not retained by the clay or organic matter of a soil, 

 and if applied to the land, soon find their way into the drains and 

 thencs by streams and rivers to the sea. 



Certain sodium salts are used in agriculture, e.g., sodium nitrate 

 and sodium borate, but in most cases it is the acid constituent which 

 is of most value, and rarely that the sodium itself plays any important 

 part, unless it be in rendering more available the potash or other 

 valuable constituents of the soil. 



