Nature of Soils. 569 



adhesive soil retains an excess of moisture for many plants in 

 wet seasons, and during a period of drought contracts and 

 hardens as evaporation goes on, and opening in broad deep 

 fissures permits the escape of moisture to a still greater depth 

 than a loose soil. 



Ordinary alluvial soil contains, according to its quality, 

 more or less of the materials consumed by growing plants, 

 such as alkaline oxides (lime, aluminium, potash, &c.), almost 

 always in combination, as neutral salts, with carbonic acid 

 (carbonates), silicic acid (silicates), and sulphuric acid (sul- 

 phates). Phosphate of lime and carbonates of iron, copper and 

 other metals, and soda occur, besides many others, in small 

 quantities, little influencing the cultural operations. Besides 

 the foregoing inorganic constituents, the soil embodies more 

 or less decayed remains of vegetables and animals that have 

 lived upon or in it. This detritus, which has received the 

 name of humus, and which imparts a dark colour to the soil, 

 is more or less rich in phosphate of lime, as well as nitrogenous 

 and carbonaceous substances, and their presence in abundance 

 indicates a fertile soil. 



Soils are usually divided into three primary groups, namely, 

 argillaceous, calcareous, and silicious, according to the pre- 

 dominance of clay, lime, or silica the latter in the form of 

 sand, resulting from the disintegration of granite rocks or 

 sandstone. These substances are rarely found in a state of 

 absolute purity, being usually mixed in some proportion. But 

 the preponderance of one over the others is sufficient to give 

 the name to a soil, and indicate its particular qualities. 



Pure, or nearly pure clay is a compact, heavy earth, soft to 

 the touch, and impervious to water. It dries with difficulty, 

 but will acquire excessive hardness from long exposure to 

 the sun. In this state of purity it defies all attempts at 

 cultivation, and even with from 12 to 15 per cent, of silicious 

 and calcareous matter it is equally intractable. A soil into 

 whose composition clay enters to the extent of 40 per cent, 

 would be termed argillaceous, but in this proportion it would 

 be suitable for the cultivation of many things. Argillaceous 

 soils are naturally more tenacious and difficult to work in pro- 

 portion to the quantity of clay they contain. The soils belong- 

 ing to this group are commonly designated heavy pr stiff soils. 



Lime or calcareous matter results from the attrition of 

 marble and limestone rocks (carbonates of lime). In a state 



