SOILS, CHARACTER OF. 



474 SOILS, CHARACTER OF. 



besides organic matter, a determinate 

 quantity of eleven chemical substances ; 

 namely (i) Potash; (2) Soda; (3) Lime; 

 (4) Magnesia ; (5) Alumina ; (6) Iron ; 

 (7) Manganese ; (8) Silica ; (9) Sulphur ; 

 (10) Phosphorus ; and (ll) Chlorine. 



POTASH. This substance is obtained 

 from burning wood, small branches, or 

 leaves, the ashed 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 carbonate of potash. When 

 this is boiled with newly slaked quicklime, 

 it is deprived of carbonic acid, which 

 enters into combination with the lime, and 

 the carbonate 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 ele- 

 ment, oxygen. Many plants require a 

 large 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 

 growth of clover, beans, peas, potatoes, 

 and other plants whose ashes yield potash 

 in return. The combination in which 

 potash is found in soils is chiefly as sili- 

 cates of potash. Some kinds of felspar, 

 mica, and granite contain large propor- 

 tions, as much as 15 to 20 per cent. It 

 also enters into the composition of trap- 

 rock, basalt, and whinstonc, though in 

 smaller proportions. As the rock crumbles, 

 silicates of potash are set free, and rendered 

 available for the plants. Clay, which is 

 chiefly derived from felspar, invariably 

 contains it ; and it is partly for this reason 

 that light land, in which potash is usually 

 deficient, is benefited by claying. 



SODA. This 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. 



LIME. Chalk, marble, and limestone 

 are carbonates of lime. Under heat, the 

 carbonic acid is driven out, and pure or 

 caustic lime remains. In 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. Quicklime sprinkled with 

 water absorbs it ; heat is evolved, and it 

 falls to powder, or is slaked. Slaked lime 

 is a white powder, dry to appearance, but 

 contains, in reality, water in an invisible 

 form, chemically combined with lime. If 

 exposed to the air, it attracts carbonic acid 

 from the atmosphere, and becomes partially 

 changed into carbonate of lime. Salts of 

 lime are found in all ashes of plants ; soils, 

 therefore, capable of sustaining vegetable 

 life, must contain lime in some form or 

 other. 



MAGNESIA. This ingredient is never 

 wanting in fertile soils. Magnesian lime- 

 stone, which is a natural 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. Sili- 

 cate of magnesia enters largely into the 

 composition of serpentine rocks. Soap- 

 stone 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- 



ALUMINA. This is the compound of 



