20 ELEMENTARY AGRICULTURAL CHEMISTRY 



very high temperature upon the carbonate or hydroxide of 

 potassium. It. is a bright, lustrous metal, very soft, and so 

 susceptible to oxidation that it has to be kept from contact 

 with air or moisture; this is often effected by means of 

 naphtha, a hydrocarbon. By contact with water it yields 

 potassium hydroxide and liberates hydrogen : 



K, + 211,0 = 2K0H + H,. 



The heat evolved by this reaction is so great that the metal 

 melts to a globule, which floats on the surface of the water 

 with a hissing sound ; the hydrogen is ignited, and burns with 

 a flame coloured violet by the vapour of the potassium. 



Potassium compounds are of great importance in agriculture 

 and essential constituents of all fertile soils. They appear to 

 be closely associated with the growth and increase of plants, 

 and are always found in greatest abundance in the twigs, 

 young leaves, and other rapidly growing portions. In the 

 plant the potassium is in combination with various acids, 

 nitric, hydrochljric, and very often with organic acids — 

 e.g.f oxalic, Jlfifi^^ citric, HgCgH^Oy, tartaric, Hfififi^, 

 and malic, HfiJJ^O^, In the ash of plants, however, it 

 generally occurs as carbonate, and this is the chief constituent 

 in the ash of the twigs and leaves of trees. Potassium salts 

 are very soluble in water, but are absorbed and retained by 

 certain constituents of the soil, so that their loss by drainage 

 from soil is little to be feared. 



Sodium is very widely distributed in nature, and is a 

 constituent of many silicates. In the form of chloride, NaCl, 

 it is very plentiful as rock-salt and as the largest saline in- 

 gredient in sea-water. The element is prepared by the action 

 of carbon upon the carbonate or hydroxide, or by electrolysis 

 of common salt. Its properties resemble those of potassium. 

 Sodium compounds are very largely used in the arts, and the 

 preparation of sodium carbonate is one of the largest and most 

 important of the chemical industries. 



