MANUAL OF AGPJCULTURE. 229 



being the technical name given to potassium in chemistry. This 

 oxide has a powerful affinity for water. The combination is 

 potash KHO . The change can be represented by a chemical 

 equation as follows : — KgO + H20=:2(KHO} . One molecule of 

 oxide of potassium and one of water form two molcules of potash. 

 It is one of the mort important compounds in the ash of plants, 

 forming from 20 to 50 per cent, of its weight. It is mainly 

 present in roots and tubers, seeds and grasses, and in the leaves 

 and branches of trees. 



Sodium. — Sodium is a metal closely resembling potassium in 

 all its features. These two, together with four other less import- 

 ant elements, which do not enter into living structures, are called 

 the metals of the alkalies. They decompose water at all tempe- 

 ratures, and combining violently with oxygen, they form jDOwer- 

 ful caustic and alkaline basic oxides, which possess a strong 

 affinity for water, which last cannot be expelled from them by 

 heat agency alone. The principal oxide of sodium is XagO — 

 Nairium = ^o^mn\. This combined with water is HXaO, or 

 soda. The compounds of sodium are widely distributed, and 

 along with those of potassium abound in the primary rocks, as 

 well as in sea-water. Soda is a less important constituent, and 

 forms a less proportion of the ash of plants than potash. It is 

 more largely present in the ash of marine than of land plants. 



Calciuni. — Calcium, when free, is a light yellow metal. 

 Eeadily combining \Yi\h the oxygen of the atmosphere, it be- 

 comes the oxide CaO, — lime. Lime has a strong affinity for 

 water, and decomposes it at any temperature. It forms with 

 water CaOH20, or slaked lime. Calcium compounds largely 

 from the rock-forming materials of the globe, in such varieties as, 

 e.fj., chalk and limestone. From the last, lime for ordinary use is 

 prepared, by driving off, by means of heat, tlie carbonic acid, — 

 limestone being a salt called carbonate of lime, composed of 

 carbonic acid and lime as a base. Lime discharges most import- 

 ant functions in tlie soil, in the way of breaking up compounds, 

 liberating their constituents in such a manner, as to render them 

 readily available for the purposes of plant life. Its percentage 

 in plant ash, varies as much as from 1 to 40. 



Mar/nesium is a silvery white metal. If strongly heated, 

 it takes lire in the air, burning with a dtizzling white 

 light, and forming the oxide ^IgO, known as magnesia. In dry 

 air it does not oxidise. It is slowly acted upon by cold water, 

 more rapidly by hot. As the carbonate of magnesia, it occurs, 

 together with carbonate of lime, in enormous quantity in the 

 species of limestone called dolomite. It most abounds in the 

 ash of grains, contributing 12 or 13 per cent, of the same. In 

 the ash of the remaining parts of cereals and of other plants, it 

 varies from 2 to 4 per cent. 



