POTASSIUM. 13 



rock, though the average amount present in the soils and rocks 

 of the earth's crust probably does not exceed one or two parts 

 in 10,000. From the soil plants obtain their phosphates ; 

 these in turn pass into the bodies of animals, where they often 

 accumulate in large quantities, thus the bones or shells of 

 animals always contain relatively large amounts of phosphate 

 of lime. 



Some minerals, too, consist mainly of phosphates, e.g., apatite 

 consists of 3Ca ;! P 2 O 8 .CaCl 2 or 3Ca 8 P 2 H .CaF 2 ; vivianite is essen- 

 tially Fe 8 P 2 H 8H 2 0. More abundant are deposits of impure 

 phosphate of lime in the various forms of phosphorite, osteo- 

 lite, coprolites, &c., &c. These are often used as fertilisers. 



Potassium occurs in many silicates : some contain a rela- 

 tively small quantity, while in others, e.g., orthoclase or potasli 

 felspar, Al 2 O 3 .K 2 O.6Si0 2 , the proportion of this element is con- 

 siderable. It also occurs largely in sea -water, from which 

 sea-weeds often accumulate large quantities of potassium com- 

 pounds. Another very important source of potassium is the 

 huge saline deposit at Stassfurth ; this is supposed to be the 

 result of the evaporation of a large inland sea, and 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 compounds, however, are of the 

 utmost importance, indeed potash seems very intimately con- 

 nected 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, sulphuric, hydrochloric, and very often with 

 organic acids, e.g., 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 potassium salts by 

 heat. The ashes of the twigs and leaves of trees, indeed, 

 formerly furnished almost the whole of the potash used in the 



