Term
|
Definition
| the h-o-h angle of 105 degrees results in an asymmetrical arrangement. one side of the water molecule (that with the two hydrogens) is electropositive; the other is electronegative. this accounts for the polarity of water |
|
|
Term
|
Definition
| cohesion - between water molecules; adhesion - between water and solid surface; the forces are largely a result of h-bonding shown as broken lines. the adhesive or adsorptive force diminishes rapidy with distance from solid surface. the cohesion of one water molecule to another results in water molecules forming temporary clusters that are constantly changing in size and shape as individual water molecules break free or join up with others. the cohesion between water molecules also allows the soild to indirectly restrict the freedom of water for some distance beyond the solid-liquid interface. |
|
|
Term
| Within a temperate region watershed... |
|
Definition
the majority of
annual precipitation infiltrates through the soil,
eventually recharging ground water and leaving as
surface water discharge. |
|
|
Term
A combination of cohesion and
surface tension forces lead to |
|
Definition
water “beading up” on a
freshly sealed surface. |
|
|
Term
|
Definition
in sandy soils is rapid but low;
capillary rise in finer textures soils is slower
but higher over time. the finer the soil texture, the greater the proportion of small-sized pores and, hence, the higher the ultimate rise of water above a free-water table. however, because of the much greater frictional forces in the smaller pores, the capilarry rise is much slower in the finer textured soil than in the sand. |
|
|
Term
| unsaturated flow by capillarity |
|
Definition
| the capillary equation can be graphed to show that the height of rise h doubles when the tube inside radius is halved. the same relationship can be demonstrated using glass tubes of different bore size. the same principle also relates pore sizes in a soil and height of capillary rise, but the rise of water in a soil is rather jerky and irregular because of the torturous shape and variability in size of the soil pores, as well as because of pockets of trapped air. |
|
|
Term
Soil water is held in pore spaces due to capillary forces of
attraction. |
|
Definition
This force is also known as “matric potential”
which will be described later. However, water that is in
the centers of larger macro-pores is far enough away
from the charged soil surfaces so that it is not held up
against gravity, and drains downward. |
|
|
Term
| Soil Water Energy Concepts |
|
Definition
Water movement and behavior in soils
are described based upon the
differences in energy level of water.
• Water will always move from areas of
higher energy to areas of lower energy,
e.g. water usually moves downhill! |
|
|
Term
| Soil Water Energy Concepts |
|
Definition
So, if I have some perfectly pure water
(no salts) in a huge container without
any matric effects that was also exactly
at sea level or whatever my reference
water table is, then I have a soil with a
net free energy or total soil water
potential equal to 0. |
|
|
Term
| Soil Water Energy Concepts |
|
Definition
• The “free energy” of water is
decreased by it’s attraction to soil
surfaces. These are matric forces.
• The free energy of water is also
decreased by it’s attraction to salts
in soil solution – osmotic forces. |
|
|
Term
| Total Soil Water Potential |
|
Definition
The total water potential (Ψt) at
any point in the soil is given by:
Ψt = ψm + ψo + ψg
Ψm is the matric force, ψo is
osmotic and ψg is gravitational. |
|
|
Term
| Soil Water Energy Concepts |
|
Definition
However, when water is
above the water table, the
force of gravity actually
adds to the energy state of
water in a positive manner. |
|
|
Term
| Ranges of Water Potential |
|
Definition
• In unsaturated soils, matric forces dominate
total water potential and range from –0.1
bar down to –15 bars or lower at plant
wilting.
• The gravitational force in unsaturated soils
is quite small; |
|
|
Term
| Ranges of Water Potential |
|
Definition
The force of gravity is only important in
saturated soils where the net pull of gravity
downward moves water rapidly through
large pores.
• The osmotic force in “normal soils” is
typically less than – 0.2 bars or so. In very
salty soils it can be as high as several bars. |
|
|
Term
|
Definition
Soil water can also be expressed as percent
water by volume (θ), and you will note that
your book uses both gravimetric and
volumetric labels on figures.
Volumetric Water = Gravimetric Water X
Soil Bulk Density,
or Grav. Water = Vol. Water / B.D.
When you want to convert them! |
|
|
Term
A field tensiometer
which directly
measures the
“suction” or matric
potential that soil
exerts on water
through a porous
ceramic cup. |
|
Definition
This method is
good for telling
when a soil is quite
wet vs. becoming
unsaturated, but is
not accurate in dry
soils. |
|
|
Term
|
Definition
Water can move in three ways in soils:
1. As saturated flow, which is relatively
rapid movement through macropores.
2. As unsaturated flow, which is much
slower as water moves in thin adhered
films on soil surfaces.
3. As water vapor which can be rapid, but
doesn’t account for much mass movement. |
|
|
Term
|
Definition
Saturated flow follows the Darcy Equation as
defined in the book. Basically, the larger the
macropores (>0.05 mm) and the higher the
potential pressure head driving water
downward, the faster the rate of flow. So,
water can rip through a sand, but barely
moves in a clayey soil. |
|
|
Term
|
Definition
especially high clay
subsoils like this one is
strongly influenced by
the presence of
macropores like this
continuous ped face or
like vertical root
channels and worm
burrows. Water flow
will concentrate in
these larger pores
causing a phenomenon
known as |
|
|
Term
| As the soil becomes unsaturated, |
|
Definition
the water remaining is
held with varying degrees of suction (or tension) which
keeps it from moving as freely (or as quickly) as does water
that is moving in the centers of larger macropores under
saturated conditions. This water is shown here as
“capillary water”. |
|
|
Term
|
Definition
is the
process whereby
water first enters
the soil surface and
is controlled
primarily by the
aggregation and
texture of the A
horizon. As
macropores
increase, so do
infiltration rates. |
|
|
Term
|
Definition
Once the soil macropores empty via saturated
flow downward, the remaining water is
affected by strong matric potential forces.
So, the rate of water movement drops
precipitously! Essentially, the water has to
crawl around the soil surfaces to move.
Unsaturated flow rates are proportional to
the difference in matric potential between
soil zones. |
|
|
Term
| The rate of infiltration is enhanced |
|
Definition
by large aggregates and
macropores which tends to be increased by OM, but decreased by
tillage over time. |
|
|
Term
Effects of contrasting textural layers on water
movement. |
|
Definition
Rule of thumb: anytime the textures varies by two
texture classes or more (e.g. loamy sand over a clay loam),
water will “back up” and saturate at the contact for some
period of time. This phenomenon is also called “perching”. |
|
|
Term
|
Definition
Once the soil loses its’ gravitational water
downward (usually in minutes to hours),
water that is held up the soil against
leaching is bound there by matric forces
which range from –0.1 to –0.3 bars in the
thicker portions of water films extending
into macropores. The soil is now at field
capacity. |
|
|
Term
|
Definition
As water continues to be evapotranspired
away from the soil, the films of water
around the soil surfaces become much
thinner, so the matric forces holding water
get much stronger (more negative). Finally,
at about –15 bar potential (very thin water
films), plants wilt because they can’t pull
water off the soil. This is the wilting point. |
|
|
Term
|
Definition
• When the soil is saturated, all macropores
are filled, but gravitational water rapidly
percolates downward from macropores.
• This state of saturation is also called
maximum retentive capacity and is the
maximum amount of water the soil can
hold. |
|
|
Term
|
Definition
So, overall, the most important
concept here is that “plant
available water” in a soil is
taken as the difference
between water held at Field
Capacity and Wilting. |
|
|
Term
|
Definition
| the percentage of water remaining in a soil two or three days after its having been saturated and after free drainage has practically ceased. |
|
|
Term
|
Definition
| the moisture content of soil, on an oven-dry basis, at which plants wilt and fail to recover their turgidity when placed in a dark, humid atmosphere |
|
|
Term
| Nursery/Greenhouse Applic. |
|
Definition
• Similar concepts and terms are used to
describe the bulk water holding of soil-less
media in greehousses or nursery pots.
• “Container Capacity” usually means field
capacity expressed as volumetric water.
• Note: Just like in soils, water perches and
saturates a zone at the bottom of a pot, even
in very sandy media. |
|
|
Term
| Factors Affecting Availability |
|
Definition
• Texture – see Fig. 5.35
• Organic Matter – See Fig. 5.36
• Compaction – Decreases water availability
through lack of pore space interconnection.
• Weakening structure – less macropores
• Layering – Perches water at contacts |
|
|
Term
| Topsoil Substitute Selection |
|
Definition
The selected strata should generate a spoil
that contains > 20% soil sized (< 2mm)
material and few rocks > 0.5 m in size.
• Perhaps most importantly, the selected
strata must (1) be thick enough to generate
at least 0.5 m of final mine soil cover, and
(2) occur in the mining column in a
position which allows it to be readily
utilized. |
|
|
Term
Regardless of
their overall
acidity and
fertility status, |
|
Definition
the number one
limitation to
plant growth in
mine soils
worldwide is
severe
compaction. |
|
|
Term
|
Definition
Water behaves as a polar compound; it sticks
to things and to itself.
• The free energy of water is decreased (made
more negative) by its attraction to soil surfaces
and ions (salts) in solution.
• The free energy of water is increased by the
downward pull of gravity (made more positive)
when it is above the water table, sea level, or a
similar point of reference. |
|
|
Term
|
Definition
Whenever it is economically feasible,
native topsoils should be salvaged and
re-applied to final reclamation
surfaces.
In general, native soil materials will be
much higher in organic matter,
available N and P, and perhaps most
importantly, beneficial microbial
populations than any topsoil substitute
materials. |
|
|
Term
|
Definition
• Water will always move from areas of high net
water potential towards areas of lower (more
negative) water potential.
• Under saturated conditions, water moves
relatively rapidly through the macropores in
soils. Micropores slow down saturated flow.
• Water movement under unsaturated conditions
is much slower due to matric interactions and
forces. (e.g. the water sticks to soil!) |
|
|
Term
|
Definition
Net water movement in unsaturated soils
is due almost entirely to differences in
matric potential. Osmotic and
gravitational forces are quite small in
comparison.
• The water holding capacity of a given soil
is a mixed function of its texture, density
and aggregation. In general, loamy soils
with good levels of macroporosity hold
more plant available water. |
|
|
Term
|
Definition
Plant available water is always taken as
the difference between water held at field
capacity (- 0.1 to – 0.3 bars net water
potential) and water held at the wilting
point (- 15 bars).
• Water content is soils can be expressed as
% gravimetric water (g water per 100 g
dry soil) or as % volumetric water
(Grav. Water % X Db). |
|
|
Term
|
Definition
• Sandy soils hold relatively low amounts
(< 10%) of total water at field capacity,
but the vast majority of that water is
plant available.
• Clayey soils hold relatively high amounts
(>40%) of total water at field capacity,
but the majority of that water is held at
suctions below the wilting point, making
it unavailable. |
|
|
Term
|
Definition
• Soil and soil-landscape properties
directly influence runoff/infiltration
partitioning
• The soil is the major reservoir for
water released back to the
atmosphere via evapotranspiration
• The chemical quality of
groundwater is directly controlled
by soil chemistry |
|
|
Term
|
Definition
• The water budget for a soil or even an
entire watershed can be accounted for
via a simple summation approach to the
inputs and outputs.
• While this is really very simple and
elegant in theory, it is very difficult to
actually measure all these variables
accurately! |
|
|
Term
|
Definition
• Precipitation falling on a soil landscape
will first be subject to interception
losses of anywhere from 10 to 50%.
• When the rate of rainfall exceeds the
infiltration rate of the soil, net runoff
results.
• The infiltration rate is a direct function of
the degree of macroporosity of the
surface soil. |
|
|
Term
Soil Plant Atmosphere
Continuum (SPAC) |
|
Definition
• The flow of water from the atmosphere
to the soil, to the root, and then back
through the plant to the atmosphere is a
major component of the hydrologic
cycle
• Water always moves from zones of
higher to lower relative potential
throughout the SPAC. |
|
|
Term
|
Definition
• Once water infiltrates the soil, water in
large macropores will continue to move
downward due to gravity via drainage.
This is the same concept as
gravitational water.
• Water held up against leaching
(remember field capacity?) is referred to
as “soil storage” available for
evaporation (E) and transpiration (T) by
plants. |
|
|
Term
|
Definition
• Water moves throughout the soil-plantatmospheric
system due to differences in free
energy – always towards a more negative
potential.
• During the growing season in Virginia,
particularly once we get plant canopy
developed, it is very difficult to drive a wetting
front all the way through the solum due to net
ET demands of the vegetation. |
|
|
Term
|
Definition
• Once the crop is harvested from the soil in the fall, or
the trees form abscission layers and drop their
leaves, net ET demand plummets and the subsoils
begin to “wet up”.
• By late fall, rainfall that infiltrates the surface soil is
able to percolate rapidly via macropore flow down
vertical prism faces and via a more uniform wetting
front as the profile moistens. Subsequently, during
the winter, we see repeated events where saturated
flow occurs down through the soil (particularly the
macropores) driving wetting fronts and solutes (like
nitrate-N) completely through the solum and
effectively recharging groundwater. |
|
|
Term
|
Definition
• First and foremost, you must avoid
impacts to jurisdictional wetlands
whenever possible.
• Where impacts are unavoidable, you
must minimize those impacts by rerouting
the bypass, developing
around drainage, etc. |
|
|
Term
|
Definition
• Where possible, any local wetland
conditions must be restored on site.
• Impacts that cannot be remedied via
on-site restoration are generally
mitigated for via off-site creation of
new wetlands. |
|
|
Term
So, the most
commonly employed
method to “create”
new wetlands is |
|
Definition
to
take areas with soils
like this, and enough
of the upper soil to
bring the wet zone
(with redox features)
close enough to the
surface to be wet
during the growing
season. |
|
|
Term
|
Definition
• Off site compensation supposedly
should be type-for-type replacement.
• Due to uncertainties of success,
forested wetlands must be created at
a 2:1 mitigation ratio (2 created: 1
disturbed. |
|
|
Term
|
Definition
• Root limiting (or critical) bulk densities
in soils range from around 1.4 for silty
clays and clays to around 1.75 for sands.
• So, we are routinely at or above
theoretical root limiting bulk density at
many (most?) mitigation sites,
particularly in the zone between – 20 and
– 50 cm. |
|
|
Term
| What Mitigates High B.D.? |
|
Definition
• Moisture content: in moist to wet soils,
soil strength (rooting impedance) is
lessened, so during the winter and spring,
high B.D. may be less of a limitation.
• Soil structure: macropore development
associated with soil structure allows root
tips to penetrate otherwise massive and high
strength soils. |
|
|
Term
|
Definition
Aeration - ventilation (gas exchange) of the
soil
Plants and microbes require oxygen
Redox - electropotential of soils
Influences availability of nutrients and toxicity
and mobility of contaminants |
|
|
Term
|
Definition
Atmospheric air
N2 = 79%; O2 = 20.9%; CO2 = 0.03%
Water vapor (relative humidity): 20 - 90%
Surface soil air
O2 = 20.6 - 14%; CO2 = 0.50 - 6.0%
Water vapor (relative humidity): 95 -99%
Subsoils
O2 = 18 - 7%; CO2 = 3.0 - 10.0%
Water vapor (relative humidity): 98 -99.5% |
|
|
Term
|
Definition
O2 availability
Soil macroporosity
○ Texture and structure
Soil H2O content
O2 consumption
Poor soil aeration - low O2 availability
80 - 90 % pore filled with water
Low solubility and diffusion of O2 in H2O |
|
|
Term
|
Definition
Gaseous exchange
Mass flow - minor
○ Changes in soils moisture
○ Barometric pressure changes
○ Wind
Diffusion - major
○ Differences in partial pressure (e.g concentration)
○ Partial pressure gradients
○ 10,000 times faster in air vs. H2O |
|
|
Term
|
Definition
Directly related to aeration
A substance is oxidized when it loses
electrons (Fe2+ = Fe3+ + e-)
A substance is reduced when it gains
electrons (Fe3+ + e- = Fe2+)
A substance that accepts e- is a oxidizing
agent; a substance that supplies e- is a
reducing agent |
|
|
Term
| Examples of Oxidized Species |
|
Definition
FeOOH
SO4
2-
Fe2O3
MnOOH
O2
CO2
CH4 |
|
|
Term
|
Definition
Examples of everyday REDOX reactions
Corrosion (Fe0 --> Fe3+)
Biological (02 --> H20 and CO2)
Photosynthesis (H20 --> O2 )
Nitrogen fixation ( N2 --> NH4)
Batteries
Many physiological functions |
|
|
Term
|
Definition
• Redox reactions generally control the
chemistry of wetland soils and are strongly
affected/controlled by biological processes.
• Redox reactions involve the transfer of
electrons.
• Reduction occurs as atoms gain electrons.
• Microbial metabolism of carbohydrates
generates the e- , and they need to go
somewhere! |
|
|
Term
| Factors Leading to Reduction |
|
Definition
• Saturation leads to poor gas exchange
and oxygen diffusion from surface.
• Palatable/oxidizable organics must be
present.
• Microbial population must be active;
warmth, pH, etc., important
• Water must be “stagnant” long enough
for oxygen depletion. |
|
|
Term
Redoximorphic Features
Formation Processes |
|
Definition
• Anaerobic conditions
– soil is saturated so almost all pores are
filled with water; absence of oxygen
• Reduction of Fe and Mn oxides
– results in distinct soil morphological
characteristics
• most are readily observable changes in soil
color |
|
|
Term
Soil Color and
Oxidation/Reduction 1 |
|
Definition
In subsoil horizons, Fe and Mn oxides
give soils their characteristic brown, red,
and yellow colors |
|
|
Term
Soil Color and
Oxidation/Reduction 2 |
|
Definition
• When reduced, Fe and Mn are mobile
and can be stripped from the soil particles |
|
|
Term
Soil Color and
Oxidation/Reduction 3 |
|
Definition
• The characteristic mineral grain color is usually
a neutral gray (value > 4, chroma < 2)
Coating of Fe2O3
Mineral grain (gray)
Remove Fe
Red Soil Gray Soil
NOTE: gray doesn’t mean reduced Fe is present,
but that oxidized Fe is absent |
|
|
Term
|
Definition
Bodies of
apparent
accumulation of
Fe/Mn oxides
• Masses
• Pore linings
– ped faces
– root channels
• Nodules and
concretions |
|
|
Term
|
Definition
• soft bodies
• frequently in the
soil matrix
• variable in shape
• can often be
removed from the
soil intact |
|
|
Term
|
Definition
• coatings on a
pore surface
• impregnations of
the matrix
adjacent to the
pore |
|
|
Term
|
Definition
• Firm to extremely
firm bodies are often
relict
• should be irregular
in shape
• Currently active
have diffuse
boundary
– “halo” or “corona” |
|
|
Term
|
Definition
Bodies of low chroma
where Fe/Mn oxides
have been reduced
and moved away
• generally value greater than or equal to 4
• chroma less than or equal 2
• formerly called
“gray mottles” |
|
|
Term
|
Definition
Factors Affecting soil aeration
The “growing season” -- soils must be warm
enough for microbial respiration for O2 to be
consumed.
Oxidized Rhizospheres – Somehow, many
plants are capable of specifically oxidizing Fe in
their rhizosphere.
Surface Compaction – Tillage, timber
harvesting, wheel traffic, etc. compact the A and
aeration. |
|
|
Term
| Ecological Importance of Soil Aeration |
|
Definition
OM degradation
Fastest under oxidized conditions
Toxic by-products may accumulate (reduced)
○ Ethylene gas, alcohols, and organic acids
Redox of elements
Nutrients (Fe3+ vs. Fe2+, SO4
2- vs. S2-)
Toxic elements
Soil colors
CH4 production
Plant growth |
|
|
Term
|
Definition
Coloring Agents
Fe, Mn, SOM, S
Redox state or aeration state
Time for Development
Relict features from parent material,
rock fragments, or old drainage regimes |
|
|
Term
Aeration and Soil and Plant
Management |
|
Definition
Soil structure and cultivation
Presence of macropores
Maintenance of organic matter
Limit cultivation
Container-grown plants
Tree and lawn management |
|
|
Term
|
Definition
Definition?? - matter of controversy
Brady - soils that are water-saturated near the
surface for prolonged periods when the soil
temperature is high enough to result in
anaerobic conditions
Constitute 14% of the world’s land area
Wetland delineation (3 charcteristics)
○ Wetland hydrology or water regime
○ Hydric soils
○ Hydrophytic plants |
|
|
Term
|
Definition
Wetland hydrology
Water balance (inflow vs. outflow)
Hydroperiod -temporal pattern of water table
changes
Residence time
Indicators - “hydric soils” and “hydrophytic
vegetation” |
|
|
Term
|
Definition
Soils that are wet within or immediately
below the A horizon to such an extent that
they become reduced and exhibit significant
redoximorphic features |
|
|
Term
|
Definition
O2 depleted microbes turn to other terminal
electron
Low chroma colors (gleyed) as Fe is
reduced
Manganese nodules
Mottling
Must occur near surface |
|
|
Term
|
Definition
Vegetation that has adapted to low O2
conditions
Grasses with aerenchyma tissues
Trees with adventitious roots |
|
|
Term
|
Definition
Depends on the redox state of the system
○ Availability of nutrients and trace elements
(increase and decrease)
○ Neutralization of acid drainage
○ Removal of toxic forms of trace elements
Cr and Se |
|
|
Term
|
Definition
Water
Figure 7.17: Representative redox potentials within a profile
Of a wetland soils
9
Wetlands
Importance of wetlands
Species habitat
Water filtration
Flooding reduction
Shoreline protection
Commerical/recreational activities
Natural products
Contaminant reduction and immobilization |
|
|
Term
|
Definition
• Processes affected - rates of physical,
chemical, and biological processes in soils
affected by temperature
– Plant processes - Plants have evolved to
grow at different temps
• Seed germination
• Root functions - nutrient and water uptake
• Vernalization of flowering bulbs |
|
|
Term
|
Definition
• Temp of soil directly or indirectly related to
– Net amount of heat energy soil absorbs
– Heat energy required to bring about given
change in temp of the soil
– Energy for processes such as evaporation
that occur at soil surface |
|
|
Term
|
Definition
• Specific heat or heat capacity
– The amount of heat required to raise the
temperature of 1.00 gram of a substance by
one degree celsius (cal/g or cal/kg)
– Soil 0.20 cal/g
– Water 1.00 cal/g
– Compare two soils: 10 kg H2O/kg and 30 kg
H2O/kg (See Box 7.2) |
|
|
Term
|
Definition
• Heat of vaporization
– 540 kcal/kg of H2O
• Energy comes from soil or solar radiation
• Tremendous cooling effect
• Thermal conductivity
– Analogous to water movement
– Qh = K T
Qh = thermal flux
K = thermal conductivity
T= temp
X= distance |
|
|
Term
| Thermal Properties of Soils |
|
Definition
Thermal Conductivity – Heat/cold is
transferred through soils primarily
via conduction through soil
solids and water. Heat/cold
transfers very slowly via soil air.
Thus, as the soil dries down, it’s
ability to transmit heat or cold drops
drastically. In fact, dry soils are very
good thermal insulators! |
|
|
Term
|
Definition
• Seasonally, the soil is heated by the
sun during the summer and then loses
heat to the atmosphere during the
winter
• In summer, the surface horizons are
always warmer than the subsurface
• In winter, because the subsoil is
warmer than the atmosphere and
buffers heat losses, the surface soils
are cooler than the subsoil |
|
|
Term
|
Definition
Since the warming of the subsoil occurs
via conduction from the surface, and
conduction is slow, the peak summer
temperature in the subsoil is timelagged
by weeks to months behind the
surface.
The deeper you go into the soil, the
seasonal differences in temperature
decrease. |
|
|
Term
|
Definition
• Soil Temperature Management
– Surface Cover
• Organic and synthetic mulches
– Buffer soil temperatures
– Moisture control
• Drainage systems
• Ridge tillage |
|
|
Term
Effects of Mulch or Litter
Layers |
|
Definition
• An organic mulch or litter layer will have
profound effects on both seasonal and
diurnal soil temperature regimes!
• A mulched soil will be warmer in winter
and cooler in summer than a bare soil
• A mulch or litter layer dampens the
diurnal temperature cycle compared
to a bare soil |
|
|
Term
|
Definition
• Mulches and litter layers also greatly
enhance infiltration of rain, especially
after heavy rains
• Mulches and litter layers also limit
direct evaporative losses of water,
keeping the surface soil moist
• Plastic mulches are routinely used to
accelerate soil heating and retain
water, but they shed rain! |
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