CHAPTER XX. 



OF SOIL. 



The word soil signifies that portion of the crust of the earth 

 in which plants grow ; what lies below it is rock. It is to a 

 great extent formed by the wearing down or decomposition of 

 rock, with the addition of matters derived from the action of 

 plants and animals, and from their decay. Soil, therefore, 

 consists of two kinds of matter arising from different sources : 

 one formed by the decay of rock is inorganic; the other 

 originating in living things is organic. Clay or loam, sand, 

 lime, and all the earthy or alkaline matters found associated 

 with them, are inorganic. Peat, mould, and every thing which 

 is convertible into these two substances, is organic. The 

 supposed action upon vegetation of these kinds of matter has 

 been scientifically investigated by all modern writers upon 

 rural chemistry, to whose most valuable labours the reader in 

 search of chemical facts will have recourse. In this place they 

 will be regarded merely from a practical point of view. 



Clay is a dense, plastic substance, pasty when wet, abounding 

 in iron, tenacious of water, hardening and cracking when long 

 exposed to dryness, and having the power of condensing 

 ammonia and other gaseous matters. Its peculiar properties 

 are chiefly owing to its containing alumina, a substance in- 

 capable, when pure, of supporting vegetation. In cultivated 

 land clay always occurs mixed with sand or chalk, or both, in 

 variable proportions. Hewvy clay contains about 30 per cent, 

 of sand ; loam 50 or 60 per cent. ; calcareous loam has a con- 

 siderable quantity of lime (chalk) in addition ; fibrous loam 



