Soil LaB

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

AGRI1050R50: Introduction to Soil Science (2020S) LH

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

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.pdf

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

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

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

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

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

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

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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