Agriculture Reference
In-Depth Information
a  thin veneer of our planet where rock meets
life. The critical zone is 'the heterogeneous,
near-surface environment in which complex
interactions involving rock, soil, water, air,
and living organisms regulate the natural
habitat and determine the availability of life-
sustaining resources' (National Research Coun-
cil, 2001; Figs 6.1 and 6.2 ) .
In order to discuss soil formation in this
CZ context, we adopt the definitions of the
CZ system of Brantley et al . (2011), noting
that other definitions may exist. The base of
the CZ is the parent material - earth mater-
ial that was present prior to soil formation,
be it bedrock or unconsolidated sediment.
Alteration of bedrock leads to regolith ; the
mantle of unconsolidated and altered mater-
ial derived from the parent material. Rego-
lith is the transportable material at Earth's
surface. Unconsolidated parent material and
regolith can be transported great distances
and deposited, where soil formation subse-
quently proceeds, potentially far from the
location of the initial exposure of the parent
material to Earth's surface. Saprolite is the
zone within the regolith where bedrock
alters in place, generally retaining evidence
of the parent material texture and fabric.
Weathering refers to processes that turn
parent material into regolith. In general,
weathering is a consequence of the exposure
of earth materials, formed at elevated tem-
perature and pressure, to near-surface condi-
tions at which they are thermodynamically
unstable. The regolith-parent material inter-
face is the weathering front .
The most important consequence of
weathering is to form soil , which is the layer
capping the regolith that has been altered
extensively by chemical, physical and bio-
logical processes, often leading to the devel-
opment of horizons (unique layers within
the soil). There is substantial complexity in
how these horizons develop, given the wide
variety in the interaction of the state factors
of soil formation (described below). Like-
wise, the terminology to describe soil layers
is complex and differs around the globe.
However, in simplified form, the layers from
top to bottom commonly consist of: the O
horizon, a layer of plant residues; the A hori-
zon, a layer of mineral soil depleted in iron,
clay, and aluminum and soluble constituents;
the B horizon, which has accumulated iron,
clay and aluminium; and the C horizon, the
parent material. Typically, soil organic car-
bon is most elevated near the surface (O and
A horizons) compared to deeper layers.
With this conceptual model in mind,
we can summarize soil formation, by a mass
balance approach, as primarily the product of
weathering losses of mineral elements from
Erosion, W
Fig. 6.1. Earth's weathering engine and the
critical zone. Uplift of continents and exposure
of rock provides a flux of reactive soil parent
material at Earth's surface. This weathering
engine is an enormous, planetary-scale,
biogeochemical reactor. The other inputs to form
soil are the fluxes of infiltrating precipitation
and solutes, the addition of organic carbon
that has been fixed by plant photosynthesis
above the surface and the diffusion of
atmospheric gases. As soil layers form, they
are also removed by ongoing physical erosion
and chemical denudation of elements from the
soil profile - by aeolian and hydrological
transport to depositional environments and to
groundwater. (From Brantley et al ., 2007.)
Mineral
movement
through the
engine
Weathering
advance, w
Parent material
 
 
Search WWH ::




Custom Search