2212020 Soil Colloids (Chapter 8) Notes - AGRI1050R50 Intro.docx

2/21/2020 Soil Colloids (Chapter 8) Notes - AGRI1050R50:
Introduction to Soil Science (2020S)
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Soil Colloids (Chapter 8) Notes
Soil Colloids (Chapter 8) Notes
Did you know ....
Did you know soil fertility or the ability for a soil to provide
nutrients is seated in the type of minerals it
contains? Chapter 8 will cover the various types of soil
colloids including all the layer and non-layer
silicates, cation exchange, anion exchange, and sorption.
Lecture content notes are accompanied by videos listed below
the notes in each submodule (e.g. Soil
Colloids (Chapter 8) Videos A though H). Print or download
lecture notes then view videos in
succession alongside lecture content and add additional notes
from each video. The start of each
video is noted in parenthesis (e.g. Content for Video A) within
each lecture note set and contains
lecture content through the note for the next video (e.g. Content
for Video B).
Figures and tables unless specifically referrenced are from the
course text, Nature and Property of
Soils, 14th Edition, Brady and Weil.

Content Video A
Soil Colloids
Smallest soil particles < 1 µm
Surface area - LARGE
Surface charge - CEC
Adsorb water
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Types of Colloids
Crystalline Silicate clays: ordered, crystalline, layers
Non-crystalline silicate clays: non-ordered, layers, volcanic
Iron/Aluminum Oxides – weathered soils, less CEC
Humus – OM, not mineral or crystalline, high CEC
Soil Colloids

Content Video B
Layer Silicates - Construction
Phyllosillicates
Tetrahedral Sheets
1 Si with 4 Oxygen
Share basal oxygen
Form sheets
Octahedral Sheets
6 Oxygen with Al3+ or Mg 2+
Di T i O t h d l b d # f di ti i
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Di or Tri Octahedral based on # of coordinating ions
http://web.utk.edu/~drtd0c/Soil%20Colloids.pdf
http://web.utk.edu/~drtd0c/Soil%20Colloids.pdf

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Size and Charge
Isomorphic substitution and surface charge
Content Video C
1:1 Silicate Clays
Kaolinite
Hydrogen bonding – Fixed Structure
Low Isomorphic Substitution
Relatively low CEC
Water Holding capacity lower most clays
Inert clay – lots of uses
2:1 Silicate Clays
Expanding Type Minerals
Smectites
Isomorphic substitution high – CEC
Oxygen bonding – weak
Shrink-swell clays
Montmorillonite
Vermiculites
Isomorphic substitution high again – Highest CEC of 2:1
Interlayer space smaller, ions/water held tighter
Less shrink/swell than smectites
Non-Expanding Type Minerals
Fine-grained Micas

Illites and Glauconites
Al 3+ for Si4+ - Strong Negative – K+ fits/satisfies charge
Non-Expansive
Chlorites
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2:1:1 with Mg2+ in the octahedral sheets
Hydrogen bonding – strong
CEC and physical properties similar to fine-grained micas
Review Silicate Layer Clays
Content Video D
Non-Silicate Clays
Iron/Aluminum Oxides
No silica
No tetrahedral sheets
Al3+ and Fe3+ main cations
Low Isomorphic Substitution, Low CEC, Sorb P
Non-Expansive – Low surface area
Gibbsite – Aluminum Hydroxide
Goethite – Iron Hydroxide

Humus - OM
Non-Crystalline
Carbon based
Difficult to characterize
HI CEC
Vital to soil fertility
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Soil Colloids
Where did you come from, where did you go?
Soil Orders - Major Colloids
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Content Video E

Sources of Charge
Isomorphic Substitution – Constant Charge
Net Negative Charge
2:1 layer clays
Mg2+ for Al 3+ - octahedral sheets
Al 3+ for Si4+ - tetrahedral sheets
Net Positive Charge
Less common
Al 3+ for Mg2+
pH dependent – Variable Charge
Mostly negative charges
Basic pH
OH groups
Broken Edges
Important in 1:1 (Kaolinite) and Iron/Aluminum Hydroxide
Some positive charge
Moderate/Extreme Acidity
Humus – Wide Range +/- sites
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Sources of Charge Review

d
Content Video F
Sources of Charge
Outer–sphere complex:
Waters bridges
Weak attraction
Inner-sphere complex
Direct bonding to colloid
CATION EXCHANGE
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Cation Exchange
Reversible
Stoichiometric Balance – cmolc “neutralized”
Mass Balance – Le Chatelier’s Principle
Flood the system, cation will replace on the exchange
Loss/precipitation of product will pull reaction in one direction,
loss of reversibility

Selectivity – Size and Charge
Higher Charge, Smaller Radii > Stronger adsorption
Lyotrophic Series
Al3+ > Sr2+ > Ca2+ > Mg2+ > Cs+ > K+ = NH4+ > Na+ > Li+
Cation Exchagne Capacity
CEC mass of exchangeable cation adsorbed per unit mass of soil
cmolc/kg soil
Charge for charge basis – NOT ion for ion
1 cmol Na+ = ½ cmol Ca2+ = 1/3 cmol Al3+
pH dependent
Lab Exercise for quantification of CEC
Content Video G
CEC - Soil Order
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Soil
Solution

- Plant Uptake
Vary with climate:
Humid/Wet/Warm/Acidic pH: Ca2+, Al3+, Al(OH)x, H+
Less Wet/Neutral pH: Ca2+, Mg2+, Na+
Soil Cations - Plant Nutrition
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Anion Exchange
CEC majority of ion exchange
Opposite of CEC
Negatively charged anions – satisfy positive charge
Sulfate, Nitrate, Phosphate
Inner-sphere complex
Less plant available, but less leaching loss
Weathering, Clays, CEC

Content Video H
Sorption
Sorption – adsorption + absorption
Soil – Bind pesticides – Slow down leaching
Kd = { (mg chemical sorbed/kg soil) / (mg chemical/L solution)
}
Koc = { (mg chemical sorbed/kg organic carbon) / (mg
chemical/L solution) }
Higher Kd or Koc – more tightly bound
Management strategy – hi or low
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2/21/2020 Soil and Hydrologic Cycle (Chapter 6) Notes -
AGRI1050R50: Introduction to Soil Science (2020S)
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Soil and Hydrologic Cycle (Chapter 6) Notes
Soil and Hydrologic Cycle (Chapter 6) Notes

Did you know ....
Did you know that the majority of the water you drink comes
from underground aquifers? And that soils play a
very important part keeping that water clean for consumption.
Chapter 6 continue our discussion on water and
soils and includes discussion of the global hydrologic cycle and
soils role in it, what happens to water when it
comes in contact with the soil surface, how water moves from
the soil up through the plants and out throgh the
leaves, controlling water loss, and managment of soil water.
the notes in each submodule (e.g. Soil and
Hydrologic Cycle (Chapter 6) Videos A though D). Print or
download lecture notes then view videos in
from each video. The start of each video is
noted in parenthesis (e.g. Content for Video A) within each
lecture note set and contains lecture content
through the note for the next video (e.g. Content for Video B).
course text, Nature and Property of Soils, 14th
Edition, Brady and Weil.

Content Video A
Hydrologic Cycle
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Fate and Transport
P = ET + SS + D
Interception
Infiltration
Runoff
Soil Storage Water
Vegetation
Cover
Stem Flow
Soil Texture
Soil Management
Maintain cover
Increase structure

Decrease compaction
Soils and Urban Development
Content Video B
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Soil-Plant-Atmosphere Continuum
Vapor Loss
Evapotranspiration – Evaporation + Transpiration
PET – Potential ET
Factors Effecting ET:

Soil Moisture
Plant Stress
Solar Radiation – LAI
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Water Efficiency
Tremendous amount of water to produce our food and fiber.
Efficiency peaks at ~1kg dry matter/1000 kg or m3 of water
Content Video C
Control ET Loss
Transpiration – Plant loss

Evaporation – Soil Surface Loss
Vegetative Cover
Crop Residue
Mulch
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Vegetables – Plastic
Irrigation - drip
http://photogallery.nrcs.usda.gov/res/sites/photogallery/

Liquid Losses
Memphis Sand Aquifer
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p q
http://pubs.usgs.gov/wri/wrir88-4182/pdf/wrir_88-4182_a.pdf
Content for Video D
Artificial Drains
http://pubs.usgs.gov/wri/wrir88-4182/pdf/wrir_88-4182_a.pdf

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Septic Tanks
Irrigation
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2/21/2020 Soil Aeration and Temperature (Chapter 7) Notes -
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Soil Aeration and Temperature (Chapter 7) Notes
Soil Aeration and Temperature (Chapter 7) Notes
Did you know ....
Did you know wetlands play a critical role in soil and water
interactions and have many ecological positives?

Chapter 7 will cover soil aeration and temperature including
how water and temperature effect soil properties
and functions as well as wetlands.
the notes in each submodule (e.g. Soil Aeration
and Temperature (Chapter 7) Videos A though D). Print or
download lecture notes then view videos in
from each video. The start of each video is
noted in parenthesis (e.g. Content for Video A) within each
lecture note set and contains lecture content
through the note for the next video (e.g. Content for Video B).
Edition, Brady and Weil. .
Content Video A
Soil Aeration and Temperature
'Land, then is not merely soil; it is the fountain of energy
flowing through a circuit of soils, plants, and
anumals.'
Aldo Leopod, A Sand County Almanac, 1949

Soil Air Composition and Exchange
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Soil Gases and Aeration
Respiration – Ventilation

Oxygen Availability
Macroporosity
Water content
Oxygen consumption
Oxygen limited: 0.1 L/L
80-90% Pore Space Water – 10-20% Air
Microbial Activity / Root Respiration limited
Water Saturation – Water Logged
http://www.tn.gov/tsla/exhibits/disasters/newmadrid.htm
Redox
Redox Potential – Eh – potential to transfer electrons
Ionic Species – Valence State – Availability
Oxygen – Oxidizing Agent – TEA

Important Notes:
Iron Looses Electron – 2+ to 3+ - Electrons are negatively
charged, so it increases valence state.
Lower the pH – Produced an H+
BALANCE – Oxidation – Reduction
http://www.tn.gov/tsla/exhibits/disasters/newmadrid.htm
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Table 7.1 Common Soil Inorganics Reduced/Oxidized Forms
Aerated/Oxidizing Conditions: Eh 0.4 to 0.7 Volts
Anaerobic/Reducing Conditions: Eh 0.32 to 0.38 Volts
Content Video B

Ecological Effects Soil Aeration
Microbial Community – Residue Breakdown
Inorganic Elements – Redox
Heavy Metals – Toxic
Soil Colors – Redox Status – Iron and Manganese
Greenhouse Gas Emission
Plant Roots – Oxygen needed
Content Video C
Wetlands
Wet/Saturated/Anaerobic Conditions
Hydric Soils
Periods of saturation – Diffusion of Oxygen into soil limited
Reducing conditions

Redoxomorphic features
Hydrophytic plants
Wetland Value
Species Habitat
Water Filtration
Flooding Reduction
Shoreline Protection
Recreation
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Recreation
Content Video D
Soil Temperature
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More Processes Affected by Temperature
Freeze/Thaw (f) – Frost Heaving (g)
Forrest Fire – Surface temperature increase, movement VOC
downward, decreased infiltration rates
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Content Video E
Soil Temperature
Temperature
Solar Radiation:
Small percentage used to heat soil
Lots of interception

Specific Heat
Energy for evaporation
Albedo – Reflection back off the surface
Aspect – Angle of the sun
Thermal Properties
Specific Heat amount of energy required to increase the
temperature of water by 1°C
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Specific Heat - amount of energy required to increase the
temperature of water by 1 C

Heat of Vaporization – energy for evaporation HI
Thermal Conductivity
Managing Soil Temperature
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2/21/2020 Soil Water (Chapter 5) Notes - AGRI1050R50:
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Soil Water (Chapter 5) Notes
Soil Water (Chapter 5) Notes
Did you know ....
Did you know that only 3% of the water on earth is fresh water?
And that soils play a very important part in the
movement and filtering of water before we find it in our taps.
Chapter 5 will start our discussion on water and
soils and includes discussion of water's properties that make it
so unique, energ of water movement in in soils,
measuring water content in soils, and how water becomes plant
available.

the notes in each submodule (e.g. Soil Water
(Chapter 5) Videos A though C). Print or download lecture
notes then view videos in succession alongside
lecture content and add additional notes from each video. The
start of each video is noted in parenthesis (e.g.
Content for Video A) within each lecture note set and contains
lecture content through the note for the next
video (e.g. Content for Video B).
Edition, Brady and Weil.
Content Video A
Soil Water
“When the well is dry, we will know the importance of water.”
Benjamin Franklin
Water Trivia
Only 3% of Earth’s water is fresh water. 97% of the water on
Earth is salt water.

Water covers 70.9% of the Earth’s surface.
There is more fresh water in the atmosphere than in all of the
rivers on the planet combined.
American residents use about 100 gallons of water per day
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American residents use about 100 gallons of water per day.
The first water pipes were made from wood.
More than 25% of bottled water comes from a municipal water
supply, the same place that tap water
comes from.
An inch of water covering one acre (27,154 gallons) weighs 113
tons.
Water makes up between 55-78% of a human’s body weight.
Data courtesy of USEPA
Structure and Properties
Polarity
Hydrogen Bonding
Cohesion – Stick together
Adhesion – Stick to other materials

Hydration
Surface Tension
Capillary Action
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Content Video B
It’s all about the ...... Energy
HI to LOW
Relative not absolute values – Difference determines direction
of movement

Potential – standard atmospheric pressure (bar) or Pascal
Gravitational
Osmotic
Matrix
Freedom of movement –Wet vs Dry Soils
Water Content and Potential
Measuring Soil Water
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Movement of Water in Soils
Saturated Flow
Heavy Rain - Fast
Hydraulic Gradient (Table 5.3)
Contamination and Loss - pesticides, nutrients , pathogens
Unsaturated Flow
Dominant movement of water
Texture
Driven by matric potential
Slow
Stratified Layer
Major textural change - Fragipan
Changes flow routes

Positive or Negative – Box 5.3
Vapor Flow
Wetting Front
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Content Video C
Qualitative Descriptions
Maximum Retentive Capacity – water saturated
Field Capacity
Plants with adequate water

Moisture is adequate but not too high for activities
Proper aeration for microbial community
Permanent Wilting Coefficient – Wilting Point
Hydroscopic Coefficient
Plant-Available Soil Water
Texture
Organic Matter
Compaction
Roots
Capillary Action
Root Extension
Activity Details

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Intro Soils - Lab 3
Soil Colloids – Cation Exchange Capacity
o Lecture and Text Materials: Soil Colloids (Chapter 8)

o Labs submitted without advised instructions will result in a 3
point deduction:
answers
o Labs submitted early will receive feedback to aid in exam
preparation with the opportunity to
resubmit the lab. Do not miss out on a great opportunity to be
ensure understanding of the
materials and increase your lab grade.
Lab 3 –Soil Colloids and Cation Exchange Capacity
Soil colloids are the smallest size fraction of the soil particles
and are the most chemically active portions

of the soil; soil colloids include clays and humus. These
particles are generally <0.1µm in size and are
collectively called the soil colloid fraction. The soil colloids
have very large per unit volume surface areas
and thus are critical in attracting and holding water and
nutrients in the soil profile. There are four types
of soil colloids including crystalline silicate clays, non-
crystalline silicate clays, iron and aluminum oxides,
and humus (organic matter). The clay minerals are a result of
the weathering or decomposition and
recrystallization of primary minerals into secondary minerals.
The composition of these clay minerals is
contingent on the weathering conditions, parent materials, and
climate under which they are formed.
The surfaces of soil colloids carry electrostatic charges, most of
which are net negative. Colloid charge
can either originate from two main sources. Charge can be
constant from isomorphic substitution of a
higher charged ion for a lower charged one in the tetrahedral or
octahedral sheets in the layer silicates.
Charge can also be pH dependent originating from humus or
protonation on broken edges of the clay
crystals in layer silicates and the iron and aluminum
hydroxides. As the pH in soil increases, to do these

pH dependent charges.
Net negative charge serves as the seat of soil chemistry and
fertility. The negative charges are
neutralized by positive cations in the soil solution and include
calcium, magnesium, potassium, sodium,
ammonium, and hydrogen. These cations are retained in the soil
solution and used for plant and
microbial nutrition. The mass of exchangeable cations sorbed
per unit mass of soil is the cation
exchange capacity (CEC). The CEC of soils is a good indicator
of soil fertility, and the capacity of a soil to
sorb and make available existing and applied plant nutrients.
The exchange of cations is determined by several principles:
(1.) Exchange reactions are reversible and rapid. The cations in
soil are exchangeable and will
move in the direction of the most available product or reactant.
(2.) The reactions are charge equivalent. Ultimately, the
negative charges created on colloid
surfaces will be neutralized by cations in soil solution, but they
are neutralized on a

stoichiometric basis not on an ion to ion basis. The soil ions
have varying levels of charge
per mole, discussed below and will be satisfied on a charge to
charge basis.
(3.) The law of mass action will be obeyed. If the system is
flooded with a particular cation, it
will move onto the exchange sites. The law of mass action is
utilized in determining
exchangeable cations to calculate CEC.
(4.) Size and charge dictate which ions if available will move
onto the exchange site. The higher
the charge and smaller the radii of the ion the stronger it will be
held. The lyotrophic series
lists the order in which cations will be exchanged on the soil
colloid surface based on
complementary ions in the soil solution. Waters of hydration
around ions give rise to the
formation of outer-sphere complexes where the ions are more
loosely held and are easily
exchangeable. Ions that form inner-sphere complexes bond
directly with the colloid surface

forming a stronger bond with less exchangeability.
Cation exchange capacity is quantified by measuring the amount
of exchangeable ions that can be
replaced on the soil colloid surface. Simply, the soil sample is
flooded with a high concentration of a
cation which through mass flow displaces all of the soluble
cations (most common in soils are sodium,
potassium, magnesium, calcium, and in acidic soils hydrogen
and aluminum) off the soil colloids and into
solution. A benchtop method first uses ammonium to replace
cations on the soil exchange sites,
followed by a second exchange which moves another ion like
sodium or potassium onto the exchange
sites. The amount of ammonium can be quantified to calculate
the chemical equivalent CEC (cmolc/kg)
(Text Figure 8.22).
Soil testing laboratories do not generally directly measure CEC,
instead CEC is estimated using the
quantity of soil cations tested in a standard soil test. Soil
testing facilities use standardized extractants
(Meilich I or III, Bray-I) to displace the all of the exchangeable

ions in soil to determine how much of
those particular elements will be available for plant uptake
during a crop season. Those determinations
are then used to recommend a range of nutrient additions,
fertilizer and lime, required to meet crop
needs for an expected crop yield. Soil testing facilities
routinely utilize inductively coupled plasma
spectrometry (ICP) coupled with atomic adsorption (AA)
spectroscopy to determine a wide range of
elemental concentrations. Inductively couple plasma
technologies heat the samples to a very high
degree to create ionization; individual ions emit specific
wavelengths of light which are quantified
downstream by various detection methods including atomic
adsorption, mass spectrometry and others.
These tools can analyze for multiple elements simultaneously
and can also be used for several matrices
including plants, soils, manures, and water. CEC can then be
estimated using the by summing up the
contributions from the major soil cations in the extracted
solution. More traditional benchtop methods
analyze the elements individually using colorimetric assays for
end point quantification. Again, the mass
of these soluble, exchangeable cations per unit of soil and
represent the capacity of that soil to

exchange cations, CEC.
At pH 7, neutral conditions, some soils do not have
exchangeable hydrogen ions and aluminum ions, and
some soils to not exhibit exchangeable sodium ions, so caution
is taken to know what exchangeable ions
are in the soils which are tested, reported, and utilized for CEC
calculations. CEC estimation using soil
test data is easy to generate using already measured soil test
nutrients, but just as the name implies, it is
an estimate, and should be interpreted as such. There are
various means of determining CEC beyond
the scope of this exercise, but again, it is important to note the
method used to determine CEC and
potential pitfalls and the agronomic ramifications of over or
underestimation of plant nutrients, and
thus CEC. The importance and value of CEC cannot be
understated. CEC is the ability of a soil to sorb
ions and molecules, making them available or not to the plant
and microbial community or ultimately to
leaching or runoff, and is key to managing soil fertility.

Estimating CEC using Soil Test Values (ppm)
To calculate the CEC using soil test values, chemistry concepts,
the charge for charge neutralization rule,
and the units for CEC should be reviewed. The end goal is to
convert a parts per million (ppm or mg/kg)
quantity from soil test into CEC which is conveyed in cmolc/kg
of soil. Recall from chemistry, each ion
(element, metal) has a specific atomic weight found on the
periodic table in units of grams per mole
(reference pg. 923 in text). We can utilize that information as
well as the equivalent charge per ion to
make this conversion. It is important to be aware of the units
used and understand the end point unit.
Ultimately, the cmolc from each cation are summed together to
determine the estimated CEC. Soil labs
also utilize the units of meq/100 grams of soil but cmolc/kg is
the standard international unit.
Table 1: Cations, Atomic Wt, Charge Equivalence
Cation Atomic Wt (g/mol)
Equivalent

Charge/Valence
Calcium (Ca2+) 40 2
Magnesium (Mg2+) 24 2
Potassium (K+) 39 1
Sodium (Na+) 23 1
Hydrogen (H+) 1 1
Example calculations:
Equation 1: Determining cmolc from the calcium ion
contribution from the soil test calcium values. Sum
the values from the ‘top’ (above the dividing lines) then divide
by the sum of the ‘bottom’ (below the
dividing line) to produce cmolc for each particular ion/kg soil.
Equation 2: Review of unit cancellations. Each member of the

equation is utilized to convert one unit to
another to ultimately end with cmolc/kg soil. Mark-thru lines
are unit cancellations; in order for a unit to
‘cancel’ it must occur in the top and bottom of the overall
equation.
Equation 3: Procedure for calculating the CEC contribution
from the additional ions (calcium is shown in
Equation 1). You will simply use the exact same equation
(Equation 1) replacing each time the ppm
(mg/kg) from the soil test for each ion, the molecular weight of
the particular ion, and the equivalent
charge of the particular ion (provided in the table above).
Equation 4: CEC is the sum of the contribution from each

individual major soil cation. Again, calcium,
magnesium, and phosphorus are always used and include
sodium, hydrogen, or aluminum in soils with
those exchangeable ions.
Calculating CEC using soil test values (lbs/acre)
Many soil labs also report the various elemental analysis in
terms of lbs/acre since fertilizer
recommendations are still calculated in that manner. Here, the
calculations for estimated CEC is still the
summation of the contribution from each individual ion but
using the equivalent weight in pounds per
acre equal to 1 meq/100 g (older unit estimation, same as
cmolc/kg soil) in one acre soil to a depth of 6
inches (Table 2). To obtain this value, divide the molecular
weight by the valence (equivalent weight)
and multiply by 20. To calculate the estimated CEC
contribution from each ion, simply divide the
lbs/acre of each ion by its meq weight in lbs/acre (far right
value) from Table 2.

For instance, if a soil test result is 1500 lbs/acre of calcium, its
contribution to CEC would be calculated
as (1500 lbs/acre / 400 meq) or 3.75 meq/100g of soil. Each of
the ions would be calculated individually
and summed to compute the estimated CEC using lbs/acre.
Table 2: CEC Calculations using lbs/acre
Cation
Atomic Wt
(g/mol)
Equivalent
Charge/Valence
Equivalent
Weight
Amount in 1 acre soil 6-inch
deep @ 1 meq cation/100g
Lbs/acre

Calcium (Ca2+) 40 2 20 400
Magnesium (Mg2+) 24 2 12 240
Potassium (K+) 39 1 39 780
Sodium (Na+) 23 1 23 460
Hydrogen (H+) 1 1 1 20
Estimating CEC using Soil Texture
Cation exchange is based in the soil colloids, clays and humus,
so CEC can actually be estimated using
soil texture. Ranges of common estimates of cation exchange
capacity of some of the major soil textural
classes are included below. It should be apparent that
increasing clays also increase CEC and thus the
ability of a soil to maintain and provide soil nutrients for plants
and the microbial community.

1.) Sands 1-5 cmolc/kg
2.) Sandy Loams 5-10 cmolc/kg
3.) Loams/Silt Loams 5-15 cmolc/kg
4.) Clay loams 15-30 cmolc/kg
5.) Clays > 30 cmolc/kg
Using knowledge of the clay percentage, organic matter
percentage, as well as information of the parent
material of the local soil type one can estimate CEC. For
instance, if you have a Tennessee Alfisol known
to contain 15% clay and 3% organic matter. You also happen to
know the dominant clay in this area are
kaolinites. At neutral pH, the CEC of kaolinite is
approximately 8 cmolc/kg and OM approximately 200
cmolc/kg.
Kaolinite: 15% or 0.15 kg x 8 cmolc/kg = 1.2 cmolc
OM: 3% of 0.03 kg x 200 cmolc/kg = 6 cmolc
Total Estimated CEC: 1.2 + 6 = 7.2 cmolc/kg

Intro Soils - Lab 3 Assignment Questions
Soil Colloids – Cation Exchange Capacity
o Utilize Lecture and Text Materials: Soil Colloids (Chapter 8)
o Note: Again, if I cannot recreate how/where you came up
with any calculated number in this
exercise you will not get credit for that answer. If you utilize
reference values for any of your
calculations, please include the reference, i.e., table/figure
number from the text.
1.) Farmer Brown has purchased a new area of land to add to his
row crop operation. He has
collected soil samples to get a baseline assessment of the land
to obtain soil test values and to
determine how much lime and fertilizer will be needed for his
corn crop. His soil test arrived
back from Lab XX and included the amount of several soil
cations in the soil, but did not

estimate CEC of his new property. Below are the values
reported of the soil major cations:
Calcium: 1800 ppm
Magnesium: 450 ppm
Potassium: 380 ppm
Sodium: 25 ppm
Calculate the estimated CEC using the soil test ppm values
using information from Table 1 and
Equations 1 thru 4. Reminder to show your work!
2.) Farmer Brown decided his pasture was not performing very
well either, so he sent this sample
to another soil lab for similar assessment. This time, his
pasture soil test values arrived and this
lab too failed to estimate CEC, but this time, his cations were
reported in lbs/acre. Below is a
list of the cations and their test values:

Calcium: 2700
Magnesium: 344
Potassium: 218
Sodium: 14
Calculate the estimated CEC on this pasture soil using the
above soil test values. Utilize the
information from Table 2 for these calculations.
Review Questions
3.) Define what constitutes a soil colloid and list 4 main
characteristics.
4.) Discuss isomorphic substitution: Include a definition,
where it occurs, discuss what ions might
be included in isomorphic substitution, and name three clays in
which their charge is dependent
on isomorphic substitution.

5.) List at least one major colloid from each of the four types of
colloids, include their colloid type,
and CEC; rank them in order of decreasing CEC, and include
their major source of charge
(constant or pH dependent).
6.) Rank the following soil orders highest to lowest based on
expected CEC: Mollisols, Alfisols,
Ultisols, Histosols, and Vertisols.
7.) Discuss the four main principles that govern cation
exchange?
8.) Why are cations not exchanged ‘ion for ion’ but rather on
charge equivalence?
9.) Clay type and amount in soils are the result of weathering of
parent materials. In general,
discuss how the weathering process shapes clay formation
(Utilize Figures 8.16 and 8.28).

10.) When using a new herbicide, why might a famer or crop
consultant want to understand the
combination of the Kd or Koc and major soil characteristics
(texture and CEC) prior to using this
product? What information do the Kd or Koc provide?
11.) BONUS! Estimate the CEC of a Soil in Texas known for
its shrinking and swelling smectititic clay.
The soil contains 25% clay and 2% organic matter.

2212020 Soil Colloids (Chapter 8) Notes - AGRI1050R50 Intro.docx

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