Exam #2 Flashcards
what are the fundamental properties of water?
-water is a polar molecule (O is electronegative)
-cohesion: H20 to H20 attraction; due to hydrogen bonding (attraction between electronegative O and hydrogen)
-cation hydration (water molecules surround cations (+)
-adhesion (H20 to surface attraction)
_________ materials attracts water (quartz, salt) and _________ materials repels water (oil, wax, PFAS (“forever chemicals”)
hydrophilic (water loving)
hydrophobic (water hating)
_______ ______ is a property of a cohesive liquid that allows it to resist external force
surface tension
what is capillarity?
tendency of liquids to flow into narrow spaces. capillary action can occur in any direction.
adhesion + cohesion –> capillarity
capillary movement = water flow towards __________ spaces/pores due to capillarity
smaller
the smaller the pore of a soil, the ______ the water is sucked up
more
what is capillary fringe?
occurs when water moves up higher from groundwater and is held stronger due to smaller particle sizes (micropores).
what is water potential (Ψ)?
the work (energy) needed to move water from a defined reference state to current state in soil
what is the reference state?
water has a water potential that equals zero (Ψ=0). (arbitrarily chosen).
usually is an unconfined pool of pure water at the soil surface or sea level.
soil water potential at a point of interest can be defined as the ….
energy needed to push water from a pool of pure H20 on the soil surface to the point of interest in the soil
water always moves from areas of ______ potential energy to areas of _____ potential area (think waterfalls)
high
low
free energy = the ability to do _____
work
because of adhesion and cohesive forces, soil water has _____ free energy than water at the ________. this is why soil water potential is almost always _________
less
surface
negative
what are the units of Ψ?
units of pressure (kPa; kilopascals)
what is the equation for total water potential (Ψt)?
Ψt = Ψg + Ψm + Ψo + Ψh
what is Ψg?
gravitational potential (water potential due to gravity, proportional to change in elevation). Ψg is positive when above the reference point and it is negative when below the reference point.
what is Ψm?
matric potential; attraction of water to soil surfaces due to adhesion and cohesion (capillarity). usually in soil, Ψm is negative. water is more strongly held in micropores than macropores. (smaller pores will have a lower matric potential than larger pores)
what is Ψo?
osmotic potential; water potential due to gradient in solute concentration. with an increase in salt concentration = lower osmotic potential. water moves from high to low osmotic potential. water moves from low to high solute (salt) concentration
what is Ψh?
hydrostatic potential; water potential due to the downward force exerted by water above the area of interest. (only applies to areas with water overhead: flooded soils (histosols) or saturated zones below the water table). thicker water on top = increase in hydrostatic potential
what are the units of gravimetric water content (θm) and what does it equal?
g/g or kg/kg
θm = mass of water/mass of dry soil = water mass/soil particle mass = Mw/Mp
how do you calculate the mass of water?
wet soil mass - dry soil mass
what are the units of volumetric water content (θv) and what does it equal?
mL/mL or m^3/m^3 or cm^3/cm^3
θv = volume of water/volume of soil = water volume/particle + pore volume = Vw/Vs
how is θv calculated if you know the bulk density of the soil?
θv = Db (bulk density) x Dw (density of water) x θm (gravimetric water content)
(units are cm^3 of water / cm^3 of soil)
what are the three important soil water potentials?
- saturation (Ψm = 0 kPa)
- field capacity (Ψm ~ -33kPa)
- permanent wilting point (Ψm = -1500 kPa)
what is saturation?
(Ψm = 0 kPa) occurs when all pores are filled with water (total porosity) (limited O2 causes slow decomposition + gleying)
what is field capacity?
(Ψm ~ -33 to -30 kPa for medium-fine soils and -10 for sandy soils) occurs when there has been about 3 days of free drainage after saturation. “gravity drained water content”
what is the permanent wilting point?
(Ψm = -1500 kPa) occurs when the matric potential is below the point at which plants can take up water. plants exposed to this matric potential usually lose turgor pressure in their leaves and die rapidly.
what is plant available water (PAW)?
difference in θ between field capacity (FC) and permanent wilting point (PWP).
(PAW = FC - PWP)
PAW = the average θ available to most plants, based on average plant physiology (can be volumetric or gravimetric)
what soil moisture potential is soil water held the most tightly?
-3100 kPa
(all other factors being equal) as pore size increases, matric potential should …
increase (become less negative)
as you decrease soil texture (diameter), the __________ increases, which allows for more water to be held between the soil particles
surface area
list these soil textures: loam, sandy loam, clay loam, silt loam, clay, and sand from greatest plant available water (PAW) to least
silt loam –> clay loam –> loam –> clay –> sandy loam –> sand
with an _______ in soil organic matter, there is an increase in the field capacity and a decrease in the permanent wilting point of a soil, therefore this results in a higher _________ ___________ ___________
increase
plant available water percentage
sand particles have larger pore sizes, _______ total porosity which results in a ________ PAW percentage and _______ flow when saturated
lower
lower
faster
with increased aggregation, there is an increase in the amount of _______________ which in turn ___________ the field capacity and the PAW percentage
macropores
increases
what is the definition of flow (flux)?
movement of material (a given volume of water) from one location to another per unit of time (always moves from high to low total water potential)
what are the three different types of water flow in soil?
- saturated flow
- unsaturated flow
- vapor flow
_________ ______ is the movement of water through completely full soil pores, usually directly after heavy rain or irrigation
saturated flow
what 3 factors affect saturated flow rate?
- texture
- aggregation
- pore connectivity
larger particles of soil will have _______ saturated flow because the water has a more direct flow path and total porosity is lower, versus water held in smaller particles has to flow through very tight, small particles with a less direct path and there is a higher total porosity, causing flow rate to be much __________
faster
slower
more aggregation = _____ macropores which leads to ________ flow
more
faster
more pore connectivity = _________ flow path = ______ flow
shorter
faster
____________ ________ is the movement of water through soil that is not saturated
unsaturated flow
lower matric potential causes ________ connectivity = ________ water flow versus higher matric potential causes _______ connectivity = __________ water flow
poorer
slower
better
faster
what is tortuous?
curving and winding path of water through soil particles
what type of soil texture (silt/clay or sand) will have a faster flow rate in unsaturated flow (very dry)?
silt/clay because there is greater connectivity of water films because particles are closer together –> decreased tortuosity
which way is water going to flow:
horizon A: Ψt = -1.1 MPA
horizon B: Ψt = -0.8 MPa
B horizon to A horizon (more positive to more negative)
which way is water going to flow:
soil A: Ψt = -0.5 MPa
soil B: Ψt = -0.7 MPa
from soil A to soil B (more positive to more negative)
what is stratified soil?
soil with horizons of contrasting textures
explain how stratification can be engineered better for golf courses and nuclear waste sights?
there is a sandy soil layer closest to the surface and beneath that there is gravel surrounding the nuclear waste canisters so that water is drawn (due to capillary action) towards the smaller, sandy soil particles and not the gravel layers deeper underneath
what is the effect of contrasting layers on water flow in stratified soils?
contrasting layers slows down water flow
_______ ______ is the movement of water vapor in soil
vapor flow
what is the hydroscopic coefficient?
moisture at -3100 kPa
when matric potential (Ψm) is at or below-3100 kPa: water films are ~4-5 molecules thick so the water is held very rigidly, hence the water can only move in _________ _______
vapor phase
water vapor moves from areas of ______ to _______ vapor pressure
high to low
water vapor movement is very important in ______ areas and to xerophytes (_______________) and seedlings
arid areas (desert soils)
xerophytes = desert adapted plants
what affects how much water plants can actually get?
-vertical depth of the roots
-volume (horizonal) of roots
how do plants get water from soils?
-capillary action (cohesion and adhesion)
-root extension (how deep are the roots)
-root distribution (adjacent plants affect this)
-root soil contact
when soil dries, roots ______ which causes ______ connectivity between roots and water-filled pores, so plants get a _______ percentage of remaining water
shrink
less
smaller
list the types of water flow from the fastest to slowest (volume per unit time)
(fastest) saturated slow, unsaturated flow, vapor flow (slowest)
(vapor flow would be fastest if you’re tracking the maximum speed of a single water molecule)
what is TPAW?
total volume of water available to plants, in cm^3, mL or L
available water depth = TPAW (total plant available water) = ?? (equation)
TPAW = θv PAW x (soil depth)
(θv PAW (unitless) should be in decimal form, not percentages)
(PAW = FC - PWP)
-units of TPAW are in soil depth (usually inches)
how do you calculate TPAW using bulk density and gravimetric % PAW??
TPAW = θm PAW * Db * Dw * L
(gravimetric %PAW * bulk density * density of water (1) * depth (L))
how can you convert θv to total water volume?
total water volume (Vw) = %θv / 100 * Vs (volume of a soil)
how can you convert θv to total water depth?
total water depth (Dw) = θv (volumetric water content) * Ls (depth of soil)
what are the 5 factors affecting TPAW (total plant available water)?
- soil texture
- soil organic matter
- bulk density + aggregation
- osmotic potential
- soil depth and layering
sandy soils have ______ TPAW
clayey soils have ______ TPAW
silty soils have _______ TPAW
low (due to having mostly macropores and low total porosity)
medium (due to having mostly small micropores)
high (due to having mostly medium micropores and medium pore volume)
with an increase in soil organic matter, TPAW _________ because …
increases in organic matter increase porosity, along with the field capacity and the permanent wilting point of a soil, resulting in a higher PAW and TPAW
with increased aggregation, you have a ________ porosity and field capacity, resulting in a _________ TPAW
higher
higher
with greater compaction, there is a ______ bulk density, resulting in a ________ porosity and __________ root penetration resistance, hence causing a ___________ TPAW
higher
lower
greater
lower
in very dry soils, with greater bulk density what occurs?
greater root penetration resistance
in very wet soils, with greater bulk density, what occurs?
a decrease in oxygen
with increased salt concentrations, osmotic potential ________ and there is a __________ in TPAW
decreases
decrease
deeper rooting depths = _____ TPAW
higher.
stratified layers keep water closer to roots
what is soil aeration?
process by which air in the soil is replaced by air from the atmosphere
more mixing = ______ __________
more aeration
aeration supplies O2, which is needed for _________ ____________
redox reactions (specifically oxidation)
plant roots and microbes need O2 for ___________ ____________. when there is low O2 levels, microbes switch to ____________ _______________
aerobic respiration
anaerobic respiration (slower process)
anaerobic respiration causes dramatic shifts in soil _________ ___________ and soil _________ __________
microbial populations
solution chemistry
how can we measure soil aeration?
- % O2 and CO2
- air-filled porosity
- redox potential (Eh)
why do we not use nitrogen as an index of aeration?
because there is the same amount of nitrogen in the soil as the atmosphere (78%)
higher %O2 = ______ aerated
higher %CO2 = _______ aerated
more
less
more air filled pores = ________ aerated
more water filled pores = ________ aerated
more
less
higher redox potential = more O2 and other oxidizing agents = _______ aerated
more
there is more ____ in the atmosphere than in soils and there is more ______ in soils than in the atmosphere
O2
CO2
what 2 processes allow gas to move throughout soils?
- mass flow = movement along a pressure gradient (from high to low pressure)
- diffusion = movement along a concentration gradient (high to low concentrations)
mass flow is the _______, diffusion in air is ________, and diffusion in water is __________
fastest
middle
slowest
water effectively _______ the flow of O2, therefore moist soils have ____ O2
blocks (O2 diffuses 10,000x slower through water than air)
low
how can you calculate the % of air-filled porosity?
% air filled porosity (%AFP) = [% total porosity] - θv (volumetric water content)
more macropores = ______ air-filled pore spaces = _______ aeration
more
better
fewer macropores = _______ water-filled pore spaces = ________ respiration = ______ aerated
more
more
less
why is a moist soil with low OM better than a moist soil with high OM for aeration?
because higher OM encourages more microbes in the soil body which release more CO2 during decomposition.
which would lead to better aeration: a histosol or an oxisol?
oxisol because histosols have high OM and poor drainage compared to oxisols which are much more weathered and have low OM
________ is the gain of electrons (more negative charge)
reduction
_______ is the loss of electrons (more positive charge)
oxidation
the ________ _________ is associated with reduction and the _________ ________ is associated with oxidation.
oxidation agent
reduction agent
Hydrogen always has an oxidation state of ___, and oxygen always has an oxidation state of___
+1
-2
what is denitrification?
anaerobic conversion of NO3- to gaseous nitrogen, causing loss from soil
in the soil redox ladder, what oxidizing agent provides the most energy for microbes in the soil? what oxidizing agent provides the second most energy for soil microbes?
aerobes (oxygen)
denitrifies (NO3- (nitrate))
oxygen is ________ and nitrate is __________
aerobic
anaerobic
what molecules would you expect to find in poorly aerated soils?
-Fe2+
-N20
-Mn2+
-H2S
(reduced form)
what molecules would you expect to find in well aerated soils?
-Fe3+
-NO3-
-Mn4+
-SO4 2-
(oxidized form)
what 6 factors do aeration and redox reactions effect on the soil?
- soil color
- nutrients
- toxins + contamination
- OM degradation
- plant growth + function
- greenhouse gas emissions
what 5 factors affect aeration and redox reactions?
- soil structure and drainage
- respiration rates
- location in soil profile + landscape
- climate (temp + moisture)
- vegetation
-soil texture and drainage-
smaller particles = smaller pores = ________ aeration.
greater aggregation = _______ macroporosity = _______ aeration.
more drainage = _______ aeration.
less
greater, more
greater
-respiration and roots-
closer to plant root = ______ CO2 and _____ O2 = ______ aeration.
more, less
decreased
-respiration and roots-
closer to Carbon source = ______ microbial respiration, = _____ CO2 and ________ O2 = ______ aeration
increased
more, less
decreased
-location in soil profile-
lower in the soil profile = less O2 = ______ aeration and _______ redox potential
less
lower
-landscape position and drainage-
lower in the landscape = ________ drainage = ________ aeration
poorer
less
-climate and seasonality-
dry season = ____ water = _______ aeration.
moist season = ________ respiration = _______ aeration
less, more
more, less
-vegetation-
more plants or “thirstier plants” = _______ transpiration = ______ soil moisture = ______ aeration
higher
lower
greater
-vegetation-
higher root density = ______ respiration = _____ CO2 and _____ O2 in root zone = ______ aeration
more
more, less
decreases
poor soil aeration leads to insufficient ____ levels which _______ plant growth. poor soil aeration leads to nutrient loss because metals experience some _______ and the process of denitrification __________ ________
O2, decreases
leaching
decreases NO3- (nitrates)
poor soil aeration causes metals to be more _________ and more _______ _________ compounds are produced
soluble (metals are more soluble in their reduced form)
toxic organic
poor soil aeration _________ microbial growth because anaerobic metabolism is _______ than aerobic which in turn _________ OM decomposition
decreases
slower
decreases
poor soil aeration also results in external impacts as it _______ greenhouse gas emissions because denitrification releases __________ ___________ and methanogenesis releases ________ which are both GHG’s.
increases
nitrous oxide (N20)
methane (CH4)
poor soil aeration also results in external impacts as leached _______ pollute ____________
metals
groundwater (metals are more soluble in their reduced form)
what are wetlands? why are they important?
hydric soils that have frequent saturation and poor drainage. plants found here are called hydrophytes (water-loving plants). wetlands are important because this habitat is home to many threatened and endangered species and is shrinking
what are the 4 main ecosystem services of wetlands?
-water supply and filtration (mitigate floods)
-greenhouse gas regulation (stores carbon)
-nutrient recycling
-habitat for culturally important organisms
explain why wetlands are an important habitat for organisms
40% of species live or breed in wetland habitats.
60% of all threatened species + 40% of all endangered species live in wetlands
hydric soils (found in wetlands) form due to regular _________ and typically have __________ conditions
flooding (or ponding) (saturated conditions)
anaerobic
what are 4 properties of hydric soils?
-gleyed subsoil (far from plant roots) = greying
-mottling = patchy greying
-organic matter accumulation (dark color)
-red or orange colors near hydrophyte plant roots
the presence of mottling indicates alternating _________ and _________ conditions during soil formation
anaerobic and aerobic
what is subsidence?
sinking of sand due to loss of material (sediment, soil, organic matter, or water). if wetlands are drained (pipes are put underground), drying and aerobic respiration will result in organic matter loss
what are 4 impacts of drained wetlands?
-subsidence
-loss of ecosystem services and biodiversity
-increased risk of flooding
-increased fire hazard (many wetland soils are flammable when dry)
what is the importance of soil temperature?
-plants
-soil weathering
-microbial processes
soil temperature affects plants _____ than air temperature.
too cold temps = ______ growth
too hot temps = _______ and _________ stress (transpiration and respiration are too high)
more
slow
heat and water
(most plants can only grow between ~8-35 degrees C)
an ________ in temperature = ________ increase in microbial activity
increase
exponential
what are four effects of cold soil temperature?
-freeze-thaw (water molecules expand and contract –> increased physical weathering)
-cryoturbation (mixing due to freeze-thaw)
-ice lenses (frozen solid layer of ice crystals)
-frost heaving (frost heave pushes objects (plant roots) upwards during freeze-thaw and tends to break roots)
increased temperatures = ___________ weathering
faster
after surface fires, ____________ _____________ burns which decreases the ___ horizon. (oils and char are combustion byproducts)
organic matter
O horizon
what 4 factors influence soil temperature?
-soil radiation absorbed
-soil’s specific heat
-water content
-depth in soil
only about _____% of solar radiation actually warms up soils
(solar radiation mostly evaporates water and is reflected back to the sky)
10%
what is albedo?
measure of a surface’s ability to reflect solar energy
darker colors = _______ albedo = _________ temperatures.
lighter colors = _________ albedo = _________ temperatures
lower, higher
higher, lower
slope angle and aspect also affect solar radiation absorption:
north facing slope = _____ direct sunlight = _______
south facing slope = ______ direct sunlight = ________
less, cooler temps
more, warmer temps
(can manipulate the aspect by placing crops on little ridges that force it to face either north or south)
what is specific heat (C) or specific heat capacity?
the amount of energy required to increase the temperature of a material by 1 degree C per unit mass.
units = cal/g or J/g
the specific heat (C) of moist soil depends strongly on soil moisture because _______>________
C water>C dry soil
(C water= 1 cal/g)
(C dry soil = 0.2 cal/g)
what is heat capacity?
the energy required per degree C to increase temperature of an object
as soil moisture increases, the specific heat capacity of a soil _________, therefore the energy needed to warm soil ________
increases
increases
what is heat of vaporization?
the energy needed to evaporate a liquid per unit mass.
(water = 540 cal/g)
water takes _________ from _____ to evaporate water which in turn cools the soil (prevents warming)
energy, soil
(latent heat = energy used to evaporate water in a given context)
how do you calculate the specific heat capacity of moist soils if you know the mass of the soil and water?
C moist soil = [CdryMdry + CwaterMwater] / [Mdry + Mwater]
how do you calculate the specific heat capacity of a soil if you know the θm of a soil?
C moist soil = FdryCdry + FwaterCwater
(Fdry = dry soil mass/[dry soil + water mass]
(Fwater = water mass/[dry soil + water mass]
on a hot day/season, as soil depth increases, temperature will __________
decrease (insulation effect)
on a cool day/season, as soil depth increases, temperature will ___________
increase (insulation effect)
heat flow rate is the fastest for __________, the slowest for _______ and in between for __________
minerals
air
water
moister and/or denser soil = _______ transfer of heat to deeper soil depths –> less __________ ____ at depth
faster
temperature lag
what are daily temperature fluctuations due to the suns angle called?
diurnal changes
as depth increases in the soil body, there is ______ seasonal and daily temperature fluctuations, the temperature is closer to ________ _______ and there is a _______ lag time.
several feet underground has a near ________ temperature.
less
annual average
longer
constant
how can you manage soil temperature (5 things)?
-plant residues + mulches
-plastic mulches
-cover crop or canopy
-tillage
-moisture control
how does mulch and plant residue affect soil temperature?
mulch decreases temperature and causes the temperature to be closer to the average (buffering). there is also a longer lag time
how does clear vs. white vs. black plastic mulch affect soil temperature?
clear plastic mulch: increases temperature due to greenhouse effect
black plastic mulch: smaller increase in temps due to low albedo
white plastic mulch: no increase in temps due to high albedo
how does crop cover or a canopy affect soil temperature?
they block sunlight rays and lower the average temperature
how does tillage affect soil temperature?
more tillage = reduced plant residue cover which means that the soil temperature is closer to the air temperature
how does moisture control affect soil temperature?
drier soil in the spring = improved warming and germination
what are the structural elements/macronutrients in soil?
air: Carbon
water: Hydrogen and Oxygen
what are 3 important measurable chemical properties of the soil solution?
-acidity and pH
-alkalinity
-electrical conductivity (EC)
the ______ ________ is where solution and solids meet. these important reactions are:
-dissolution and precipitation
-absorption and desorption
-redox reactions
-acid-base reactions
soil surface
what are colloids?
particle with size <1micrometer (μm), they are so small that they will not settle out of suspension (w/o help)
colloids have very high _________ ______ per unit mass due to small size and have _________ surfaces. in soils, colloids can be _________ or __________
surface area
charged (positive and negative)
organic or inorganic (mineral)
(solid or liquid)
where can soil colloids be found?
the soil, air and water
(also includes sols, aerosols, and emulsions)
why are colloids important?
-soil clays “store” soil nutrients and other cations.
-organic colloids in soils (humus) absorb non-polar molecules such as pesticides.
-colloids control soil pH.
-compacted clay liners are used under landfills and toxic waste dumps
primary minerals that are colloid sized are layer __________.
secondary minerals that are colloid sized are layer __________ and _________/___________.
silicates
silicates
oxides/hydroxides
soil colloids are responsible for most of the _________ occurring in soils
sorption (adsorption and absorption)
what is adsorption?
the loose adherence of solutes onto soil surfaces through electrostatic attraction or other intermolecular forces
what is absorption?
physical movement/flow of soil solution (including solutes) into soil pores
what are the different types of soil colloids?
- crystalline silicate clays
- non crystalline silicate clays
- Fe and Al oxides
- organic (humus)
_____________ means having constituent atoms, molecules or ions arranged in an orderly, repeating pattern extending in all 3 spatial dimensions.
crystalline
what is a phyllosilicate?
crystalline mineral containing SiO4 2- that is organized in repeating layers
(phyllo = layer, thin sheet + silicate = SiO4 mineral)
what is a sheet, layer and interlayer?
sheet = fundamental unit of a layer repeating in 2 dimensions
layer = group of 2 or more sheets
interlayer = space between layers (water and solutes may enter)
what is a tetrahedron shape (T)?
what is an octahedron shape (O)?
pyramid with a triangular base
2 pyramids sharing a rectangular base
what are the two different types of phyllosilicate layer patterns?
1:1 phyllosilicate (TO,TO)
2:1 phyllosilicate (TOT-TOT)
only ___ _____________ clays have interlayers
2:1 phyllosilicate
what are the 2 types of electrical charge on soil surfaces?
-permanent charge
-pH-dependent charge
what is permanent charge?
-always negative, independent of pH
-due to isomorphous substitution
-occurs inside mineral structure
what is pH-dependent charge?
positive and negative charge and the magnitude of charge vary with pH. the change in charge is due to interaction with H+ and OH-.
-lower pH = more positive charge
-higher pH = more negative charge
more charge = more ____________ absorption, including _____ exchange
electrostatic
ion
____________ ___________ is the replacement of atoms or ions with others of similar size (ionic radius) within a crystal structure
isomorphous substitution
usually the new atom/ion has a less ________ ________ –> charge of the clay layer becomes more _________
positive charge
negative
in a ____________ sheet, Al3+ replaces Si4+
in a __________ sheet, Mg2+ (or Fe2+) replaces Al3+
tetrahedral
octahedral
what is pH?
the negative log of the hydrogen ion concentration
[pH = -log(H+)]
as pH increases, the [H+] __________.
low pH conditions = _________
decreases
acidic
non-crystalline silicate clays are not as order or layered, with a ______ or _____ shape. they are ________ but not very sticky and form from ____________ _____
sphere or tube
plastic
volcanic ash
Fe and Al oxides are usually ___ with O2- and/or OH- and have varying degrees of _______
3+
order
organic (humus) colloids are________ ________ that is less than _____ in size and are partially decomposed plant or microbial tissues. they are non __________ and non ___________
organic matter
1μm
non-organic and non-mineral
what is ion exchange?
rapid swapping between difuse hydrated ions in solution and ions adsorbed on soil surfaces.
what is cation exchange?
rapid swapping between cations in the soil solution and cations adsorbed to negatively charged surfaces
what is anion exchange?
rapid swapping between anions in the soil solution and anions adsorbed to positively charged surfaces
what are the 4 major phyllosilicate types?
- Mica
- Vermiculite
- Smectite
- Kaolinite
describe Mica
-2:1 layering, interlayer contains K+ (no water) and is non-expanding.
-highest permanent charge, S.A. = medium-low
-uses: drywall, glitter, makeup, insulation, nail polish
describe Vermiculite
-2:1 layering, interlayer contains mostly Mg 2+ with <2 layers of water, limiting expansion, mostly permanent charge
-medium-high layer charge and medium-high S.A.
-uses: potting media, soil conditioner, and insulation
describe Smectite
-2:1 layering expanding clay, interlayer expands to accommodate many H2O layers (greatest expansion)
-medium permanent charge, highest S.A.
-highest ultra microporosity
-uses: cat litter
describe Kaolinite
-1:1, no expansion (no interlayer) adjacent layers bound by hydrogen bonding
-no isomorphous substitution, 100% pH-dependent charge
-low S.A., low plasticity, stickiness and cohesion
-uses: infrastructure, bricks, pottery, and pest/pathogen protection
which major phyllosilicate swells the most? the middle? the least?
smectite swells the most
vermiculite swells the second most
mica and kaolinite swell the least
where does negative ion exchange occur?
-external surfaces: mineral edges
-internal surfaces: phyllosilicate interlayers
what are the 3 important principles of cation exchange?
-reversibility
-charge equivalence
-cation selectivity
what is reversibility?
at equilibrium (steady state), cation exchange reactions process just as easily forwards as they do backwards
what is charge equivalence?
exchange takes place on a charge-for-charge basis (adsorbing cation charge = desorbing cation charge)
what is cation selectivity*?
strength of adsorption increases as
-cation valence increases
-hydrated radius decreases
what is a hydrated radius?
distance from the center of the ion to outermost associated water molecule.
the greater the hydrated radius, the less adsorption strength
the smaller the atomic radius, the __________ the hydrated radius
larger
what is Cation Exchange Capacity (CEC)?
the amount of cations soil, soil component or other material can absorb via exchange. (from negative surface charges)
units = charge per unit mass: cmol/kg = meq/100g
meq = millimoles of equivalent charge = .001 mol of charge
list the 4 major phyllosilicates from greatest to least CEC?
vermiculite –> smectite –> illite –>kaolinite
for pH-dependent charge, with an increase in pH, CEC ___________ and AEC __________
increases
decreases
what ways allow you to determine soil CEC?
- estimate based on negative charges of soil components (clay, SOM, Al+Fe Oxides)
- quantify all of the exchangeable cations in soil, then add up their charges
- direct measurement: cation replacement methods
what is the equation to determine soil CEC (assuming clay and organic matter are the dominant CEC sources)?
CEC soil = [F clayCEC clay] + [F om CEC om]
soil pH is a _______ ____________ that affects: (biological properties)
-soil ___________ activity
-availability of both ___________ and ________ to plants
master variable
microbial
nutrients and toxins
soil pH affects: (chemical properties)
-________ and leaching of nutrients and toxins
(______ pH = high CEC, ____ pH = high AEC)
-soil ____________
mobility
high, low
weathering
describe acid vs. base
acid: H+ donor or OH- acceptor (pH < 7)
base: H+ acceptor or OH- donor (pH > 7)
(plants usually grow best in pH 5.5-6.5)
where would you except to find very acidic soils?
old soil (highly weathered conditions) in warm/hot, moist environments (tropics, boreal forests, coniferous forests, deciduous forests) (high rainfall climates)
what is an acid soil?
general: soil with pH < 7
USDA: soil with pH < 6.5
what is a naturally occurring acidic parent material?
peat
what is acidification in soil?
the buildup of H+ and Al3+ (OH- acceptor) in soil
what are the natural causes of acidification?
biological activity
-plants and microbes produce organic acids (carbonic acid)
-aerobic respiration (CO2 + H20 –> H2CO3)
-chemical weathering of soils
-oxidation reactions
-hydrolysis of metals
-organic acids and organic matter (carboxylic acid R-COOH)
what are the 4 main human causes of acidification?
-acid rain
-extra CO2 from fossil fuels
-mining/construction with Sulfur-rich sediments
-fertilizers (primarily NH4+)
more root or microbial respiration = more __________ _______
carbonic acid (H2CO3) (carbonation: H20 + CO2 –> H2CO3 –> HCO3- + H+)
explain how exposure of sulfur causes acidification in soils?
reduced sulfur (S^0 and S^2-) (which comes from wetland sediments and mining waste materials that are high in sulfur) is converted via sulfur-oxidizing bacteria into sulfuric acid (H2SO4, which is a strong acid that reacts with water and break into 2H+ + SO4^2-)
which soil type is most likely to have the highest AEC?
Oxisols
which soil type is most likely to have the highest CEC?
Mollisols
as weathering increases, pH _________, AEC ___________ and CEC ___________
decreases
increases
decreases (may increase a bit at first)
this is due the difference in permanent charge to pH-dependent charge
explain how acid rain causes acidification in soils?
during combustion processes, sulfide oxides and nitrous oxides (SOx and NOx (gasses)) are produced and then they react with water vapor in this atmosphere via oxidation reactions and create sulfuric acid (H2SO4) and nitric acid (2HNO3). they then dissociate and produce loose H+ ions that lower the pH in soils.
aluminum is an ______ _______ abundant in soils (~5% by weight). with increased weathering, there is _____ dissolved and exchangeable Al3+
acid cation
more
why is aluminum important for calculating soil acidity and considered how it impacts plants in the soil?
-soluble aluminum is toxic to plants.
-Al3+ binds to negative charge sites which decreases nutrient sorption
-generally contributes to both AEC and CEC
what type of chemical reaction is associated with Al3+ and H+ which lowers the pH?
hydrolysis reactions
what type of chemical reactions will result in a lower soil pH?
oxidation reactions
what is AEC?
anion exchange capacity, the amount of anions in a soil can absorb via exchange (due to positively charged surfaces).
AEC __________ with decreasing pH
increases (opposite from CEC)
when soil pH increases, the amount of H+ on the surface ___________, there is an increase in the ____________ charged surfaces, causing a _________ CEC and a ____________ AEC
decreases
negatively
higher
lower
when soil pH decreases, the amount of H+ on the surface ___________, there is an increase in the ____________ charged surfaces, causing a _________ CEC and a __________ AEC
increases
positively
lower
higher
in most soils, AEC is _______ than CEC because the concentration of permanent charges tends to be greater than the concentration of pH-dependent charges
less
where are acid cations found in soils? what is their amounts comparatively?
-soil solution (size = smallest)
-exchangeable on surfaces/salt-replaceable (size = medium)
-held very tightly in/on inner particles (size = largest)
list the 4 main acid cations that contribute to soil acidity
-H+
-Al3+
-Fe3+
-Fe2+
list the main non-acid (base) cations that do not contribute to soil acidity (do not accept OH-)
-Na+
-K+
-Ca2+
-Mg2+
what is cation saturation percentage?
% of CEC occupied by a given cation
what is nonacid cation saturation?
“base” cation saturation = % of CEC occupied by non-acid cations
%BS = [Ca2+ + Mg2+ + K+ + Na+] / CEC x100 (%)
(units are in cmolc/kg / cmolc/kg)
what is acid saturation?
% of CEC occupied by acid cations
%AS = [H+ + Al3+] / CEC x 100 (%)
how can you find acid saturation when given the base saturation?
%AS = 100 - %BS
%BS = 100 - %AS
(%AS + %BS = 100)
what is buffering? what are buffers?
resistance to change in pH
buffers in soil:
-any soil colloids (especially organic)
-acid soil: Al3+ (aluminum)
-alkaline soil: CO3 2- (calcium carbonate)
what impacts does soil pH have on biota?
-aluminum toxicity
-transition metal toxicity
-toxic compound mobility
-nutrient availability
-microbial effects
how does pH affect nutrient availability in soils?
too acidic (pH < 5.5) = Al and toxic metals become more soluble at low pH’s –> causes toxic concentrations which isn’t good for plant growth
too alkaline (pH > 7) = phosphorus deficiency due to P precipitating with Fe, Al, Ca, and Mg. transition metals insoluble (Fe, Mn, Cu, and Zn)
how can we manage acidic soils?
liming: addition of alkaline materials to raise the pH
what are liming materials?
most common: carbonates from limestone (calcite and dolomite)
others: oxides, hydroxides, silicates
to raise the soil pH, where must you neutralize the pH?
- active acidity (soil solution)
- exchangeable acidity (soil surface exchange sites)
plants evolve to grow in their ________ soil pH but most crops grow best at pH ____-____
native
6-6.5
what are the benefits of liming acidic soils?
- increased availability of many plant nutrients
- directly supplies Ca and/or Mg (dolomite or calcite)
- decreases metal toxicities (Fe, Al, Mn)
- increases beneficial soil microbes
__________ is a base containing an alkali metal or alkaline earth metal (a basic anion with a nonacid cation)
alkali
(common basic anions: CO3 2-, OH-, O2-)
what is alkalinity?
the measure of bases in a soil (ability of a soil to neutralize acid) pH >7
alkaline soils are found mostly in _________ regions
dry/desert
__________ are the main alkalis in soils because there is less rain so the __________ don’t wash away and they accumulate, causing thicker __________ horizons
carbonates (CO3 2-)
carbonates
calcic
_______________ soil = soil with copious CaCO3 (moderately alkaline)
calcareous
what are alkalizing reactions?
-protonation of conjugate bases of weak acids
-weathering of hydroxide minerals
-carbonate reactions
what are highly alkaline soils problematic for plant growth?
-transition metals (Fe, Mn, Cu, Zn) become insoluble (unavailable) because they precipitate with OH-
-phosphorus deficiencies because it is precipitating with Ca and Mg and becomes unavailable to plants
alkalinity remediation is usually …
too expensive to be feasible
______ is a non-acid cation + anion (ionically bonded)
salt
example: NaCl, KCl, KNO3
what are salt-affected soils?
soils that contain enough salts to inhibit plant growth
____________ is the total accumulation of all salts
salinity
_____________ is the ration of sodium (Na+) to the sum of nonacid cations
sodicity
what are the sources of salt?
-parent material weathering
-groundwater
-irrigation water
-overland flow of runoff water
-sea water
-aeolean dust
when do salts accumulate?
when salt inputs are greater than outputs
inputs: low precipitation and hot causes high evaporation which decreases leaching
outputs: impermeable layers and high water table = poor drainage
how do we quantify salinity?
by measuring the EC: electrical conductivity
what is EC?
the ease with which electrons flow through a substance
units: dS/m (desi Semens per meter)
increased salts = ____________ EC
increased (salty water conducts electricity better than pure water)
how do we quantify sodicity?
by measuring the ESP: exchangeable sodium percentage
or by measuring SAR: sodium absorption ratio
what is ESP? what does ESP equal?
ESP is the degree to which soil exchange sites are Na+ saturated.
ESP = [exchangeable Na+] / CEC soil
(units = cmolc/kg / cmolc/kg (unitless))
what is SAR?
relative concentration of Na+ compared to Ca2+ and Mg2+
(total concentrations, not just exchangeable, in mmol/L)
the higher the sodicity, the __________ the ESP and SAR
higher
list the four classifications of salt affected soils in order from best to worst soil quality
-normal (best quality)
-saline
-saline-sodic
-sodic (worst quality)
what are the EC, ESP, and SAR measurements of a normal soil?
EC: less than 4
ESP: less than 15
SAR: less than 13
what are the EC, ESP, and SAR measurements of a saline soil?
EC: greater than 4
ESP: less than 15
SAR: less than 13
what are the EC, ESP, and SAR measurements of a saline-sodic soil?
EC: greater than 4
ESP: greater than 15
SAR: greater than 13
what are the EC, ESP, and SAR measurements of a sodic soil?
EC: less than 4
ESP: greater than 15
SAR: greater than 13
salinity: high EC causes __________ stress
sodicity: high ESP or SAR increases ______ and __________ soil structure
osmotic
pH and degrades
describe why a high EC causes osmotic stress
plant roots are not very salty and have a high osmotic potential versus if there is a high salt concentration in the soil solution there is low osmotic potential. water moves from areas of high osmotic potential to areas of low osmotic potential, so water moves away from the plant roots and towards the salt particles in the soil solution
why is the pH very high (>8.5) in sodic soils?
sodic soils have high concentrations of Na+ in their solution which react with carbonates to form sodium carbonate which is highly soluble and reacts with water easily to form 2Na+ + OH- + HCO3 which increases the pH as there is too much OH- and Na+
Na+ is a _____ flocculator and causes _______ dispersion which ___________ soil aggregates and __________ soil quality
poor
more
decreases
degrades
why is Na+ a poor flocculator and why is Ca2+ a good flocculator?
Na+ has a less positive charge and a greater hydrated radius, which causes a thicker ion swarm which causes dispersion instead of flocculation
Ca2+ has a more positive charge, a smaller hydrated radius which leads to a thinner ion swarm which causes flocculation instead of dispersion
with more Ca and Mg, there is ______ flocculation and ________ aggregates which creates _______ pores which results in _________ infiltration and drainage
more
more
larger
faster
with more Na+, there is ________ dispersion which causes ______ to move through the soil, clogging pores and forms pans or crusts which leads to ________ drainage
more
clays
slower
high EC and/or low ESP causes _____________
low EC and/or high ESP causes _____________
flocculation
dispersion
dispersion disrupts __________ –> dispersed clays near the surface are disrupted even more by rainfall and ponding –> ___________
aggregates
crusting
explain one practice farmers can use to recycle irrigation drainage water
farmers can put the least salt tolerant crop at the top of a slope, with the next least salt tolerant crop in the middle, and the most salt tolerant crop at the bottom of a slop. as the water drains downhill, it will have higher salt concentrations in it because only water, not salt will evaporate from the soil body
what can farmers do to reclaim saline soils?
-add lots of high quality (low salt) water that has a SAR of less than 13 and an EC less than 4.
-promote good drainage: install artificial drainage and/or use deep-rooted high-transpiring plants.
how can farmers reclaim sodic and saline-sodic soils?
-add gypsum (CaSO4), sulfuric acid (H2SO4) or sulfur (So) which replaces the Na+ adsorbed to the surface with Ca2+ (do not add calcite)
-irrigate with excess, high quality water
why add gypsum and not calcite?
gypsum is not alkaline and provides a source of soluble Ca2+
calcite will make soils more alkaline
