Answer:
Soil in this text is a natural body comprised of solids
(minerals and organic matter), liquid, and gases that occurs
on the land surface, occupies space, and is characterized by
one or both of the following: horizons, or layers, that are
distinguishable from the initial material as a result of additions,
losses, transfers, and transformations of energy and matter or
the ability to support rooted plants in a natural environment
(Soil Survey Staff, 1999). This definition is expanded from the
previous version of Soil Taxonomy (Soil Survey Staff, 1975) to
include soils in areas of Antarctica where pedogenesis occurs
but where the climate is too harsh to support the higher plant
forms.
The upper limit of soil is the boundary between soil and
either air, shallow water, live plants, or plant materials that
have not begun to decompose. Areas are not considered to have
soil if the surface is permanently covered by water too deep
(typically more than about 2.5 m) for the growth of rooted
plants. The horizontal boundaries of soil are areas where the soil
grades to deep water, barren areas, rock, or ice. In some places
the separation between soil and non soil is so gradual that clear
distinctions cannot be made.
The lower boundary that separates soil from the non soil
underneath is most difficult to define. Soil consists of the
horizons near the earth’s surface that, in contrast to the
underlying parent material, have been altered by the interactions
of climate, relief, and living organisms over time. Commonly,
soil grades at its lower boundary to hard rock or to earthy
materials virtually devoid of animals, roots, or other marks
of biological activity. The lowest depth of biological activity,
however, is difficult to discern and is often gradual. For
the practicality of soil survey, the lower boundary of soil is
arbitrarily set at 200 cm. In soils where either biological activity
or current pedogenic processes extend to depths greater than
200 cm, the lower limit of the soil for classification purposes
is still 200 cm. In some instances the more weakly cemented
bedrocks (paralithic materials, defined later) and uncemented
bedrocks (some densic materials, defined later) have been
described below the lower boundary of soil and used to
differentiate soil series (series control section, defined in chapter
17). This is permissible even though the paralithic materials
below a paralithic contact are not considered soil in the true
sense. In areas where soil has thin, pedagogically cemented
horizons that are impermeable to roots, the soil extends as
deep as the deepest cemented horizon, but not below 200 cm.
For certain management goals, layers deeper than the lower
boundary of the soil that is classified (200 cm) must also be
described if they affect the content and movement of water and
air or other interpretative concerns.
In the humid tropics, earthy materials may extend to a depth
of many meters with no obvious changes below the upper 1 or
2 m, except for an occasional stone line. In many wet soils,
gleyed soil material may begin a few centimeters below the
surface and, in some areas, continue down for several meters
apparently unchanged with increasing depth. The latter
condition can arise through the gradual filling of a wet basin
in which the A horizon is gradually added to the surface and
becomes gleyed beneath. Finally, the A horizon rests on a thick
mass of gleyed material that may be relatively uniform. In both
of these situations, there is no alternative but to set the lower
limit of soil at the arbitrary limit of 200 cm.
Soil, as defined in this text, does not need to have discernible
genetic horizons, although the presence or absence of genetic
horizons and their nature are of extreme importance in soil
classification. Plants can be grown under glass in pots filled
with earthy materials, such as peat or sand, or even in water.
Under proper conditions all these media are productive for
plants, but they are non soil here in the sense that they cannot
be classified in the same system that is used for the soils of a
survey area, county, or even nation. Plants even grow on trees
or in cracks of exposed bedrock (i.e., rock outcrop), but trees
and rock outcrop are regarded as non soil.
Soil has many temporal properties that fluctuate hourly,
daily, and seasonally. It may be alternately cold, warm, dry,
or moist. Biological activity is slowed or stopped if the soil
becomes too cold or too dry. The soil receives additions of
fresh, undecomposed organic matter when leaves fall or grasses
die. Soil is not static. The pH, soluble salts, amount of organic
matter and carbon-nitrogen ratio, numbers of microorganisms,
soil fauna, temperature, and moisture status all change with
the seasons as well as with more extended periods of time.
Soil must be viewed from both the short-term and long-term
perspective.
