Nature of Sorts. 569 
adhesive svuil 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 hums, and which imparts a dark colour to the soil, 
is more or lessrich 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 or stiff soils. 
Lime or calcareous matter results from the attrition of 
marble and limestone rocks (carbonates of lime). Ina stare 
