Due in 12 hours. Need best work.  Check the Soil_Lab 3 for the lab. I attached the notes for all the chapters.  

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:

 Proper document name (LastName_SoilsLab3)

 Name included in document

 Legible numbering and spacing including questions with answers

 Use of spell and grammar check

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.

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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.

Lecture content notes are accompanied by videos listed below 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).

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 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

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2/21/2020 Soil Aeration and Temperature (Chapter 7) Notes – AGRI1050R50: Introduction to Soil Science (2020S)

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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.

Lecture content notes are accompanied by videos listed below 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
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 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

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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

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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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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.

Lecture content notes are accompanied by videos listed below 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
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

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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2/21/2020 Soil and Hydrologic Cycle (Chapter 6) Notes – AGRI1050R50: Introduction to Soil Science (2020S)

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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

2/21/2020 Soil and Hydrologic Cycle (Chapter 6) Notes – AGRI1050R50: Introduction to Soil Science (2020S)

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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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2/21/2020 Soil and Hydrologic Cycle (Chapter 6) Notes – AGRI1050R50: Introduction to Soil Science (2020S)

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Vegetables – Plastic

Irrigation

– drip

http://photogallery.nrcs.usda.gov/res/sites/photogallery/

Liquid Losses

Memphis Sand Aquifer

http://photogallery.nrcs.usda.gov/res/sites/photogallery/

2/21/2020 Soil and Hydrologic Cycle (Chapter 6) Notes – AGRI1050R50: Introduction to Soil Science (2020S)

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p q

http://pubs.usgs.gov/wri/wrir88-4182/pdf/wrir_88-4182_a

Content for Video D

Artificial Drains

http://pubs.usgs.gov/wri/wrir88-4182/pdf/wrir_88-4182_a

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2/21/2020 Soil and Hydrologic Cycle (Chapter 6) Notes – AGRI1050R50: Introduction to Soil Science (2020S)

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Septic Tanks

Irrigation

2/21/2020 Soil and Hydrologic Cycle (Chapter 6) Notes – AGRI1050R50: Introduction to Soil Science (2020S)

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2/21/2020Soil 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

AGRI1050R50: Introduction to Soil Science (2020S) LH

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2/21/2020 Soil Colloids (Chapter 8) Notes – AGRI1050R50: Introduction to Soil Science (2020S)

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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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2/21/2020 Soil Colloids (Chapter 8) Notes – AGRI1050R50: Introduction to Soil Science (2020S)

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Di or Tri Octahedral based on # of coordinating ions

http://web.utk.edu/~drtd0c/Soil%20Colloids

http://web.utk.edu/~drtd0c/Soil%20Colloids

2/21/2020 Soil Colloids (Chapter 8) Notes – AGRI1050R50: Introduction to Soil Science (2020S)

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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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2/21/2020 Soil Colloids (Chapter 8) Notes – AGRI1050R50: Introduction to Soil Science (2020S)

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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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2/21/2020 Soil Colloids (Chapter 8) Notes – AGRI1050R50: Introduction to Soil Science (2020S)

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Soil Colloids

Where did you come from, where did you go?

Soil Orders – Major Colloids

2/21/2020 Soil Colloids (Chapter 8) Notes – AGRI1050R50: Introduction to Soil Science (2020S)

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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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2/21/2020 Soil Colloids (Chapter 8) Notes – AGRI1050R50: Introduction to Soil Science (2020S)

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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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2/21/2020 Soil Colloids (Chapter 8) Notes – AGRI1050R50: Introduction to Soil Science (2020S)

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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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2/21/2020 Soil Colloids (Chapter 8) Notes – AGRI1050R50: Introduction to Soil Science (2020S)

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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

2/21/2020 Soil Colloids (Chapter 8) Notes – AGRI1050R50: Introduction to Soil Science (2020S)

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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 Colloids (Chapter 8) Notes – AGRI1050R50: Introduction to Soil Science (2020S)

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Biomolecules and Soil Colloids

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