Principles of terrestrial ecosystem ecology.pdf

leached material from above, it is relatively
unaffected by soil-forming processes. C horizons
typically include a significant portion of
unweathered parent material. Finally, at some
depth, there is an unweathered bedrock layer,
the regolith (R).
Despite the large variation among the
world’s soils, they can be classed into major
groups that share many of the same properties
because they have formed in response to
similar soil-forming factors and processes. Soil
classification systems rely on the diagnostic
characteristics of specific horizons and on
organic matter content, base saturation, and
properties that indicate wetness or dryness. The
U.S. Soil Taxonomy recognizes 12 major soil
groupings, called soil orders (Table 3.3) (Brady
and Weil 1999). Most agronomic and ecosystem
studies classify soils to the level of a soil series,
a group of soil profiles with similar profile
characteristics such as type, thickness, and properties
of the soil horizons. Soil series can be
further subdivided into types, based on the
texture of the A horizon, and into phases, based
on information such as landscape position,
Soil Horizons and Soil Classification 59
stoniness, and salinity.A comparison of soil profiles
from the major soil orders illustrates the
effect of different climatic regimes on soil
development (Figs. 3.10 and 3.11).
Entisols are soils with minimal soil development.
They occur either because the soils are
recent or processes that disrupt soil development
dominate over processes that form soils.
This is the most widespread soil type in the
world, making up 16% of the ice-free surface.
Inceptisols, in which the soil profile has only
begun to develop, occupy an additional 10% of
the ice-free surface. Rock and shifting sand
account for another 14% of the ice-free surface.
Thus about 40% of the ice-free surface of Earth
shows minimal soil development.
Histosols are highly organic soils that
develop in any climate zone under conditions
in which poor drainage restricts oxygen diffusion
into the soil, leading to slow rates of
decomposition and accumulation of organic
matter. There is a well-developed O horizon
of undecomposed organic material where most
plants are rooted.The high water table prevents
the vertical leaching required for soil devel-
Table 3.3. Names of the soil orders in the United States soil taxonomy and their characteristics and typical
locations.
Area (% of
Soil Order ice-free land) Major Characteristics Typical Occurrence
Entisols 16.3 no well-developed horizons sand deposits, plowed fields
Inceptisols 9.9 weakly developed soils young or eroded soils
Histosols 1.2 highly organic; low oxygen peatland, bog
Gelisols 8.6 presence of permafrost tundra, boreal forest
Andisols 0.7 from volcanic ejecta; moderately developed recent volcanic areas
horizons
Aridisols 12.1 dry soils with little leaching arid areas
Mollisols 6.9 deep dark-colored A horizon with >50% base grasslands, some deciduous forests
saturation
Vertisols 2.4 high content (>30%) of swelling clays; crack grassland with distinct wet and dry
deeply when dry seasons
Alfisols 9.7 sufficient precipitation to leach clays into a B humid forests; shrublands
horizon; >50% base saturation
Spodosols 2.6 sandy leached (E) horizon; acidic B horizon; cold wet climates, usually beneath
surface organic accumulation conifer forests
Ultisols 8.5 clay-rich B horizon, low base saturation wet tropical or subtropical climate;
forest or savanna
Oxisols 7.6 highly leached horizon with low clay; highly hot humid tropics beneath forests
weathered on old landforms
Rock and sand 14.1
Data from Miller and Donahue (1990) and Brady and Weil (2001).