POTASH MANURES 277 



potassium chloride contains 63 per cent, of potash. In 

 reality it contains none at all, but 63 parts of the oxide 

 contain the same amount of potassium as 100 parts of 

 the chloride, and are therefore equal to that amount. 

 Potassium sulphate and other oxysalts may be said to 

 contain the oxide, K a O, in combination -with the acid oxides, 

 S0 3 , etc. 



SOURCES OF POTASSIUM COMPOUNDS. 



The principal sources of potassium compounds are 

 potassic minerals, plant ashes, and the natural deposits of 

 potash salts. 



Potash-bearing Minerals. The most important potash- 

 bearing minerals are the potash felspar, orthoclase, and 

 the white or potash mica. The former contains from 

 12 to 17 per cent, of potash, and the latter from 8 to 13 

 per cent. They enter largely into the composition of 

 some of the crystalline rocks, and are the ultimate source 

 of the potash in soils. They are both silicates (p. 21), 

 decompose very slowly, and cannot be used directly as 

 manures though both have been tried. The use of mi- 

 caceous sand has been previously referred to. Felspar 

 has been tried in a finely-ground condition, but not with 

 much success. More recently, attempts have been made 

 to utilise it by fusing it with sodium carbonate and 

 phosphorite (p. 235). The treatment is said to render 

 the potash more readily available, but the products are 

 regarded more as phosphatic than as potash manures. 



Wood Ashes. This was formerly the principal, and _is 

 still a considerable source of potassium compounds. The 

 potassic minerals of the rocks from which the soil is 

 derived are gradually decomposed ; the potash becomes 

 available, is taken up by plants, and forms a large 

 proportion of the ash which remains when the organic 



