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3/19/2020 Soil Acidity (Chapter 9) Notes - AGRI1050R50: Introduction to Soil Science (2020S)

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Soil Acidity (Chapter 9) Notes

Soil Acidity (Chapter 9) Notes

Did you know .... Did you know that wood ashes can actually help change the pH of your soil? Chapter 9 highlights soil acidity, its sources, how it occurs in soil naturally as well as man-induced, why pH is important for nutrient availabilty, and finally how to manage soil acidity.

Lecture content notes are accompanied by videos listed below the notes in each submodule (e.g. Soil Acidity (Chapter 9) Videos A though E). 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 referenced are from the course text, Nature and Property of Soils, 14th Edition, Brady and Weil.

Content Video A

Soil Acidity

http://kimscountyline.blogspot.com/

Chemistry Review

Reversible Reaction: double arrows going left and right

LEFT of Arrow: Reactant RIGHT of Arrow: Product

AGRI1050R50: Introduction to Soil Science (2020S) LH

3/19/2020 Soil Acidity (Chapter 9) Notes - AGRI1050R50: Introduction to Soil Science (2020S)

https://gotoclass.tnecampus.org/d2l/le/content/8094442/viewContent/60403398/View 2/8

Law Mass action:

Add Reactant (to the left side) push reaction to the RIGHT–Make more product (right side)

Add Product (to the right side) push reaction to the LEFT –Make more reactant (left side)

Dissociation – Break apart into constituents, generally adds to acidity (H+)

See carbonation above

pH, Acidity, and Alkalinity

Kw = [H+] + [OH-] = 10-14 pH = - log10 [H+]

Example:

[H+]= 0.0001 M (10-4)

10-4 M = pH = 4

Log Scale – Every step, pH change is 10X

Chemistry Review: Moles (M) = g/L

pH and pKa Acids – donate protons – H+

Acid dissociative constant – pKa

Dissociate – Break apart – Produce more H+

½ the acid dissociate and ½ stays in solution

pH > pKa – More likely to dissociate

pH < pKa – More likely to stay in tact (undissociated)

pH = pKa – in equal concentrations

Weak Acids –Reaction pKa between 0 and 14

Lots of weak acids in soils – Offer BUFFERING CAPACITY

Carbonate, Nitrate, Phosphate, Sulfate

Strong Acids – Reaction pKa < 0

Content Video B

3/19/2020 Soil Acidity (Chapter 9) Notes - AGRI1050R50: Introduction to Soil Science (2020S)

https://gotoclass.tnecampus.org/d2l/le/content/8094442/viewContent/60403398/View 3/8

pH Scale

Sources and Consumers of H+

Sources of Acidity Microbial/Plant Activity

Produces CO2 gas – Carbonic Acid

Uptake nutrients – Release H+

Nutrient Cycling – Reduction of N, S, Fe

Humus/pH dependent charge

Atmospheric Deposition

Weathering

3/19/2020 Soil Acidity (Chapter 9) Notes - AGRI1050R50: Introduction to Soil Science (2020S)

https://gotoclass.tnecampus.org/d2l/le/content/8094442/viewContent/60403398/View 4/8

Weathering

Rain Water – pH 5.6

Based on # of exchangeable cations (CEC)

Leach ‘base cations’: Ca, Mg, Na, K, Al

Leaves Al on the exchange site

Aluminum and Soil Acidity Aluminum – Major Constituent Soil Acidity

Lots of Al3+ in soils – minerals, clays, oxides

Weathering – displaces Al3+ with excess H+ in solution

Now both exchangeable AND in soil solution

Hydrolyses Water – More H+

Breaks water into H+ and OH-

Produces Al(OH)x

pKa in typical soil ranges – perpetuates issue

Aluminum Hydrolysis Al(OH)x – dependent on soil pH

Looking to ppt soluble Al3+ into Gibbsite

Problem: Aluminum is highly toxic to plants!

Weathering – Loss of Cations Excess H+ ions in system

Move ‘base cations’ off the exchange and into solution

Base Cations lost to leaching

Leaves Al3+and its hydroxides on exchange

More Acidity

3/19/2020 Soil Acidity (Chapter 9) Notes - AGRI1050R50: Introduction to Soil Science (2020S)

https://gotoclass.tnecampus.org/d2l/le/content/8094442/viewContent/60403398/View 5/8

Content Video C

Concept of Base Saturation CEC – Saturation Percentage

Percentage of the CEC taken up by particular ions

Acid-Saturation – Al3+ and H+

Non-Acid Saturation (Base saturation): Ca2+, Mg2+, K+, Na+

Do not Hydrolyze like Al (and Fe) to produce more H+

Not really ‘bases’ just not acidifiers

Utilized in Soil Taxonomy – Ultisols vs Alfisols

Relative measure of level of weathering as well as soil fertility

Saturation and pH Acid Saturation vs Base Saturation (CEC)

Wide pH ranges – ph Dependent charge – variable CEC

Aluminum toxicity >20% acid saturation

Pools of Acidity – Resist Change Active – [H+]

Concentration of H+ ions in solution

Smallest portion

Exchangeable Al3+ and H+

Salt Exchangeable

Important in acidic soils

Residual

Non-exchangeable Al3+ and H+

Tightly bound

Largest Pool

Soil Buffering Capacity

Active and Exchangeable – Easily neutralized

Buffered by addition of Al and H from the residual

Resist major changes in soil pH

Generally: The higher CEC higher buffering capacity

3/19/2020 Soil Acidity (Chapter 9) Notes - AGRI1050R50: Introduction to Soil Science (2020S)

https://gotoclass.tnecampus.org/d2l/le/content/8094442/viewContent/60403398/View 6/8

Generally: The higher CEC, higher buffering capacity

Weak Acids – Buffering Capacity Reversibility – Mass Flow = Buffering Capacity

Move either direction to keep things the same – Buffer the system

Aluminum Hydroxide Formation

Humus

Weathering – CEC

Buffering Capacity

Content Video D

Man-Induced Soil Acidification Commercial N Fertilizers:

Acid Deposition

Industrial Activity

Fossil Fuel Burning

SOX –NOX Gases – Come back as Sulfuric and Nitric Acid

Mining activities – Acid Mine Drainage

Significant environmental issue

Metal Oxide Oxidation – LOTS of Acidity produced

Runoff/Rainwater destroy ecosystems

Importance of pH Nutrient availability Figure 9 22

3/19/2020 Soil Acidity (Chapter 9) Notes - AGRI1050R50: Introduction to Soil Science (2020S)

https://gotoclass.tnecampus.org/d2l/le/content/8094442/viewContent/60403398/View 7/8

Nutrient availability – Figure 9.22

Maronutrients: Ca2+, Mg2+, K+ – LESS available at extreme pH

Micronutrients: Mn, Zn, Cu, Co, Fe – MORE available at extreme pH – even toxic

Nutrient Interactions

Plants and Microbes: Al (and Mn) toxicity

Generally: pH <5.2 major issues

Accumulates in plant roots – Test roots

Wide range of tolerance

Fungi: Acid Lovers

Bacteria: Neutral pH (adapted wide range)

Content Video E

Reducing Soil Acidity Increase the pH

Add soil amendments – LIME

Change chemical nature of the root zone

Overcome soil buffering capacity

Tons per acre

Liming Materials

3/19/2020 Soil Acidity (Chapter 9) Notes - AGRI1050R50: Introduction to Soil Science (2020S)

https://gotoclass.tnecampus.org/d2l/le/content/8094442/viewContent/60403398/View 8/8

Reflect in ePortfolio Download Print

Reducing Soil Acidity Add Lime – CaCO3

Active Acidity: Increase

Exchangeable Acidity: {Equations 9.25, 9.26, and 9.27}

Increase pH – Promote movement Ca2+ onto exchange – precipitate Al(OH)3

How much lime do I need? Soil pH in water – active acidity

Soil ph in buffer – exchangeable acidity

Adams-Evans for TN – buffered at pH 8

Change from pH 8 – amount of exchangeable acidity

Big drop – more lime

Small drop – less lime

Lime requirements – crop specific and target pH specific

Regression combination of soil water pH and buffer pH

Vary by state – parent material (aka clays)

Review What is pH?

Define soil acidity? 2 main acidifying ions

Sources of soil acidity– natural and human induced

Acid and base saturation – importance?

How do soils buffer pH?

Problems that arise from soil acidity – at what pH should I be concerned – how might I diagnose the problem?

How do we reduce soil acidity?

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