22 GAKDEN MANAGEMENT. 



doubt that humus supplies plants with an essential part of their food ; but it 

 acts in various ways, which, as we have said, are not very clearly ascertained. 



56. All fertile soils thus contain, besides organic matter, a determinate 

 quantity of ten or eleven chemical substances ; namely, — 1. Potash. 2. Soda. 

 3. Lime, 4. Magnesia. 5. Alumina. 6. Iron, 7. Manganese. 8, Silica. 

 9. Sulphur. 10, Phosphorus. 11, Chlorine. 



57. Potash is obtained from bm-ning- wood, small bmnches, or leaves, the 

 ash being washed in water, and evaporated in an iron pot and calcined. Add 

 a small quantity of water, decant the liquid, and evaporate to dryness, and 

 pearl-ash is obtained, which is an impure form of potash in combination with 

 carbonic acid, or crude cai'bonate of potash. When this is boiled with newly- 

 slaked quick-lime, it is deprived of carbonic acid, which enters into combina- 

 tion with the lime, and the carboiiate of potash is thus converted into pure or 

 caustic potash, which can be separated into a silvery-white, soft, metallic 

 substance, — potassium, and a gaseous element, — oxygen. Many plants require 

 a lai-ge amount of potash for their food, the only source from which it can be 

 obtained being the soil. This accounts for the fact that wood ashes, which 

 contain carbonate of potash, are so conducive to the healthy gi-owth of clover, 

 beans, peas, potatoes, and other plants whose ashes yield potash in retium. 

 The combination in which potash is found in soils is chiefly as silicates of potash. 

 Some kinds of felspar, mica, and granite, contain large proportions, as much 

 as 15 to 20 per cent. It also enters into the composition of trap-rock, basalt, 

 and svhinstone, though in smaller proportions. As the rock crumbles, silicates 

 of potash are set free, and rendered available for the plants. Clay, which is 

 chiefl}'' derived from felspar, invaiiably contains it ; and it is partly for this 

 reason that light land, in which potash is usually deficient, is benefited by 

 claying. 



58. Soda is obtained by burning seaweed ; and plants growing on the sea- 

 shore, are rendered caustic by the same process. Its most common form, 

 however, is sea-salt, or chloride of sodium, Seakale, asparagus, and similar 

 plants, are benefited by its use, 



59. Lime. — Chalk, marble, and limestone, are carbonates of lime. Under 

 heat, the carbonic acid is driven out, and pure or caustic lime remains. la 

 its effects on animal and vegetable matters it resembles potash and soda, is 

 slower in action, and is used most beneficially on peat land : its excess of 

 organic matter is thus gradually destroyed, and converted into nutritious food 

 for plants. Quick-lime sprinkled with water absorbs it ; heat is evolved, and 

 it falls to powder, or is slaked. Slaked lime is a white powder, dry to aj^pear- 

 ance, but contains, in reality, water in an invisible form, chemicallj' combined 

 with lime. If exposed to the air, it attracts carbonic acid from the atmo- 

 sphere, and becomes pai-tially changed into carbonate of lime. Salts of lime 

 are found in all ashes of plants ; soils, therefore, capable of sustaining vegetable 

 lite, must contain lime in some form or other, 



60. Marjnesia is never wanting in fertile soils, Magnesian limestone, which 

 is a natm-al compound of the carbonates of lime and magnesia, contains 30 to 

 40 per cent, ; and in this form it exists in all dolomite and many other solid 

 rocks. Soils containing much carbonate of magnesia absorb ^ moisture with 

 great avidity, and are generally cold soils. Silicate of mag-nesia enters largely 

 into the composition of serpentine rocks. Soapstone and limestone frequently 

 contain it. Compounds of sulphuric acid and muriatic acid with magnesia 

 are also found in many mineral waters. Sulphate of magnesia, which is the 

 name of the familiar Epsom salts, is formed from the decomposition of 

 dolomitic rocks. 



61. Alumina is the compound of a metal, — aluminum, with oxygen, which 

 occiu-s very abundantly in the mineral kingdom, both free and in combination 

 with acids. In its crystallized state it forms the precious stones, sapphires and 



