The uppermost surface layer of the Earth, in which plants grow and on which,
directly or indirectly, almost all life on Earth depends. Soil consists,
in the upper layers, of organic material mixed with inorganic matter (see
humus) resultant from weathering. Soil
depth, where soil exists, may reach to many meters. Between the soil and
the bedrock is a layer called the subsoil.
|The composition and color of a soil identifies it
to a pedologist. Tundra soil (A) has a dark, peaty surface. Light-colored,
desert soil (B) is coarse and poor in organic matter. Chestnut-brown
soil (C) and chernozem (D) – Russian for "black earth" –
are humus-rich grassland soils typical of the steppes and the prairies
of North America. The reddish, leached latosol (E) of tropical savannas
as a very thin, but rich humus layer. Podzolic soils are typical of
northern climates, where rainfall is heavy and evaporation is slow.
They include the organically rich brown forest podzol (F), the gray-brown
podzol (H), and the gray-stony podzol (I) that supports mixed growth
of conifers and hardwoods. All are relatively acidic. The red-yellow
podzol (G) of pine forests is quite highly leached.
Mature soil may be described in terms of four soil horizons:
(1) the uppermost layer, containing organic matter, though most of the soluble
chemicals have been leached (washed out); (2) strongly leached and with
little or no organic matter; (3) the subsoil, a layer of weathered and shattered
rock; and (4) the bedrock.
Three main types of soil are commonly distinguished:
A soil profile is a vertical view of the layers of soil
from the surface down to the unaltered parent material, and is used in classifying
- pedalfers, associated with temperate, humid climates,
have a leached A-horizon but contain iron and aluminum salts with clay
in the B-horizon;
- pedocals, associated with low-rainfall regions, contain
soluble substances such as calcium carbonate (soluble in rainwater,
which contains carbon dioxide) and other salts;
- laterites, tropical red or yellow soils, heavily
leached and rich in iron and aluminum.
Soils may also be classified in terms of texture (e.g., clay, silt, sand).
Loams, with roughly equal proportions of sand, silt, and clay, together
with humus, are among the richest agricultural soils.
testing is the process of analyzing the chemical and mineral composition
and porosity of soil. It is carried out for various reasons, including to
determine the soil's ability to support plant life.
AND THE ENVIRONMENT