Agriculture Reference
In-Depth Information
depend on the structure of the soil matrix as
well as the development of preferential flow
macropore structures such as root holes,
fractures and earthworm burrows. The com-
bination of the inherent characteristics of
soil matrix (homogeneous versus mixed
permeability) and the macropore structure
are primary factors affecting the hydraulic
retention of water in the soil (Gerke and van
Genuchten, 1993a,b; Kohne et al ., 2009a,b).
A critical element for the conceptual
model is the role of soil in water filtration
and transformation. The foundations of this
are the soil-forming processes (Jenny, 1941,
1994; Drever, 1997; Langmuir, 1997). Soils
are formed from parent rock as a function of
climate conditions, chemical and physical
weathering processes and microbial and
plant action, leading to minerals with spe-
cific chemical composition and physical
properties. The parent material can be classi-
fied broadly into igneous, metamorphic and
sedimentary rock types containing primary
minerals that determine the rate and chem-
ical composition of soils. Climate conditions
modulate physical erosion and chemical
weathering that through time reduces particle
sizes, leading to increased mineral reactive
surface area. The most important minerals
are feldspars, quartz, olivine, pyroxenes,
amphibole (hornblend), micas (biotite and
moskovite) and clays (montmorillonite,
kaolinite, etc.). Other minerals found in na-
ture are those composed of soluble salts
(halite, gypsum, pyrite), carbonates (calcite
and dolomite), phosphate rocks (apatite),
aluminium (Al) oxides (gibbsite and dia-
spore) and iron (Fe) oxides (goethite and
hematite). Understanding the soil-forming
processes lays the foundation for soil char-
acterization and model development.
Figure 8.2 presents a schematic of the
biogeochemical reactions affecting solute
transport in the soil. Soil and sediments have
the capability to remove ions from water
(adapted from Kohne et al ., 2009a,b). The
water-soil interface plays a significant role
in water filtration and transformation. Clays
due to the isomorphic substitution of their
central metal ion have a permanent negative
charge. In aquatic environments, the nega-
tive surface charge is balanced with cations,
which are exchanged by other cations of higher
charge. Ion exchange is a reversible reaction
that can affect clay and non-clay minerals,
as well as organic and oxide coatings.
Photosynthesis
C/N/P/K cycling
Erosion and nutrient leaching
Volatilization
Soil aggregate formation and
C/N/P/K sequestration
Ion exchange
Solute leaching
Biodegradation
Chemical weathering
Redox reactions
Oxide adsorption
Precipitation-dissolution
Groundwater
recharge
SOM adsorption
Complexation
Fig. 8.2. Biogeochemical reactions affecting solute transport. (From Kohne et al ., 2009a,b.)
 
 
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