Soil Acidity and pH The Concept: • Definition: negative logarithm (base 10) of the H+ concentration in solution: – log [H+], where [H+] = mol L–1 = M • If [H+] = 0.0001 M, then pH = – log (0.0001) and pH = 4 {note that 0.0001 M = 10–4 M}

Soil Reaction: Acidity Chapter 9 p. 363-411

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Lowry–Brønsted Acids and Bases

The pH Scale

• Acids are defined as substances that donate (lose) protons • Acids that can donate more than one proton are termed polyprotic • Acidity is described by the acid dissociation constant, pKa (–log Ka) • For example, pKa = 7.20 for the reaction: H2PO4– = HPO42– + H+

• pH ranges from 0 to 14 – pH < 7 = acidic – pH > 7 = basic or alkaline – pH 7 is neutral [OH–] = [H+]

• Insensitive: pH 4 is 10 times more acidic than pH 5 • Soils generally range from 4.5 to 9.5 3

Lowry–Brønsted Acids and Bases Ka =

[H+ ][HCO3− ] = 10 − 6.35 [H2CO3 ]

log K a = −6.35; pK a = 6.35 Ka =

Lowry–Brønsted Acids and Bases • pKa – pH = log [H2CO3] – log [HCO3–] • When the pH of a soil solution is less than the pKa, log [H2CO3] – log [HCO3–] > 0 and the acid is predominantly undissociated • When the pH of a soil solution is greater than the pKa, log [H2CO3] – log [HCO3–] < 0 and the acid is predominantly dissociated

H2CO3 → H+ + HCO3−

What is pKa?

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[H+ ][HCO3− ] = 10 − 6.35 [H2CO3 ]

log K a = log [H+ ] + log[HCO3− ] − log[H2CO3 ] or pK a − pH = log[H2CO3 ] − log[HCO3− ] 5

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Lowry–Brønsted Acids and Bases

Lowry–Brønsted Acids and Bases

When pKa = pH, [H2CO3] = [HCO3–]

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Lowry–Brønsted Acids and Bases

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Lowry–Brønsted Acids and Bases

• Acids that have pKa values that are between 0 and 14 are termed weak acids (inorganic & organic) • Acids that have pKa values that are less than 0 are termed strong acids (inorganic only) • Acids that have pKa values that are greater than 14 are termed strong bases (inorganic and organic)

Strong Acids Acid reaction HClO4 = 0

ClO4–

pKa +

H+

–7

HCl0 = Cl– + H+

~–3

H2SO40 = HSO4– + H+

~–3

H2SeO40 = HSeO4– + H+

~–3

HNO30 = NO3– + H+

–1

H2CrO4 = HCrO4 + 0

–

H+

–0.2

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Soil is a Weak Acid System

Lowry–Brønsted Acids and Bases Weak Acids

• Important weak acids in soils:

Acid reaction

pKa

HSO4– = SO42– + H+

1.99

H3PO40 = H2PO4– + H+

2.15

HF0 = F– + H+

3.17

H2CO30 = HCO3– + H+

6.35

H2PO4– = HPO42– + H+

7.20

B(OH)30 = B(OH)4– + H+

9.23

H4SiO40 = H3SiO4– + H+

9.86

– Carbonic acid – All aqueous organic functional groups – pH-dependent mineral surface sites – All solid organic functional groups

• Weak acid systems are buffer systems • Buffer ≡ resists change

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Soil is a Weak Acid System

Acid Neutralizing (Buffering) Capacity

When protons are removed, or when base is added, the weak acid dissociates to buffer solution pH

CH3COOH

• Dissociation and protonation of weak acids – Inorganic & organic – In solution & on solid surfaces

CH3COO– + H+

• Mineral weathering • Base saturation

When protons are added, or when base is removed, the weak acid protonates to buffer solution pH

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How does Soil become Acidic?

Types of Soil Acidity

• Weathering (leaching)

• Active acidity – [H+] in soil solution, measured by standard pH measurement – Very small amount, may take only a few kg of CaCO3 per hectare to neutralize – Acidity that the plant “sees”

• Exchangeable acidity (≡ total acidity) – Exchangeable Al3+, AlOH2+ , and Al(OH)2+ (and H+) – 1,000 to 100,000 times greater than active acidity

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How does Soil become Acidic?

– Rain ≈ pH 5.6 – Mineral dissolution consumes H+ and releases Ca2+, Mg2+, K+, Na+, and Al3+ – Basic cations leach more readily, leaves Al and its hydrolysis products behind on exchange complex – Why is Al3+ an acidic cation? • • • •

Al3+ + H2O → AlOH2+ + H+ AlOH2+ + H2O → Al(OH)2+ + H+ Al(OH)2+ + H2O → Al(OH)30 + H+ Al(OH)30 + H2O → Al(OH)4– + H+

(pKa = 5.0) (pKa = 5.1) (pKa = 6.7) (pKa = 5.9)

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How does Soil become Acidic?

• Activities of organisms – Release of H+ by plants and microbes during nutrient uptake – Release of CO2 → carbonic acid

• Ca2Al2Si2O8 + 8H+ → 2Al3+ + 2Ca2+↓ + 2H4SiO4↓ – Al3+ + H2O → AlOH2+ + H+ – AlOH2+ + H2O → Al(OH)2+ + H+ – Al(OH)2+ + H2O → Al(OH)30 + H+ – Al(OH)2+ + H2O → Al(OH)3(gibbsite) + H+ – Al(OH)30 + H2O → Al(OH)4– + H+

• Additions of ammonium fertilizers – NH4+ + 2O2 → NO3– + H2O + 2H+

• Oxidation produces acidity (occurs with reduced S compounds) • Additions of Al and Fe compounds.

• 3{Ca–Soil2} + 2Al3+ → 2{Al–(Soil)3} + 3Ca2+↓

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Importance of Soil pH

Importance of Soil pH

• Nutrient and metal availability – P is limited in both acid and alkaline conditions – Ca, Mg, & K deficient in acid soils – Metal micronutrients (Cu, Fe, Mn, Zn, etc.) deficient in alkaline soils – Anionic micronutrients (e.g., Mo, S) deficient in acid soils – Boron deficient in both acid and alkaline soils – Metal toxicities in acid soils (e.g., Al and Mn)

• Plant growth – Al and Mn are toxic to plants – Some plants require acidic conditions to enhance Fe availability, e.g., azalea, blueberries

• Microbial activities – Optimum activities pH 5.5 to 7.5 – Fungi tolerate acidity best – Bacteria tolerate alkalinity best

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pH and % Base Saturation

Base Cation Saturation • As pH increases, soluble and exchangeable concentrations of Al decrease relative to those of Ca, Mg, K, Na (the base cations) • Relative measure of weathering and soil fertility • % Base saturation: %BS =

cmolc (Ca2 + + Mg2 + + K + + Na + ) kg−1 × 100 CEC (cmolc kg−1)

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Reducing Soil Acidity Alteration of Soil pH

• Liming materials: carbonates, oxides, or hydroxides of calcium and magnesium

• Reduction of acidity

– Calcite – CaCO3 – Dolomite – CaMg(CO3)2 – Calcium oxide or quicklime - CaO – Calcium hydroxide or hydrated lime - Ca(OH)2

– Application of liming materials

• Increasing acidity or lowering alkalinity – Application of acid-forming materials

• All these neutralize 2 H+ per mole of liming compound (base cation) 23

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How Liming Works: Reactions

Determining pH and Lime Requirement

• • • • •

Reverses the soil acidification process 3CaCO3 + 6H+ → 3Ca2+ + 3CO2 ↑+ 3H2O 2{Al–(Soil)3} + 3Ca2+ → 3{Ca–Soil2} + 2Al3+ 2Al3+ + 6H2O → 2Al(OH)3 + 6H+ Calcite dissolution provides the Ca2+ required to displace Al3+ from the exchange complex and alkalinity to facilitate Al3+ precipitation (a requirement) • Note that overall, liming is pH neutral!

• pH – Measured in soil-water suspension with pH meter – Usually 1 or 2 parts water to 1 part soil (mass) – Sometimes use CaCl2 solution

• Lime Requirement: – Add pH 8 buffer to soil-water suspension (AdamsEvans buffer in Tennessee, Shoemaker-McLean-Pratt in other states) – Measure pH in water – Buffer pH decrease from pH 8 is from exchangeable acidity – Calculate H+ released and lime required

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TN lime recommendations (t A–1) raise pH to a target pH of 6.5 • • • •

Soil pH in Adams-Evans Buffer Soil pH in water

7.8

7.6

7.4

7.2

7.0

6.0

1.0

2.0

2.0

2.0

2.5

5.8

1.5

2.0

2.0

2.5

3.0

5.6

1.5

2.0

2.5

3.0

3.5

5.4

1.5

2.0

2.5

3.0

4.0

5.2

2.0

2.0

2.5

3.5

4.0

Practical Aspects

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Usually apply no more than 2 to 3 tons Try to use Mg containing materials Apply in advance of crop needs Test every 2 to 3 years

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Soil Acidification • Application of elemental sulfur - S0 (microbial mediated process) S + 1.5 O2 + H2O → 2 H+ + SO42–

• Ferrous sulfate (FeSO4) applications for iron-loving plants (chemical/microbial) FeSO4 → Fe2+ + SO42– + Fe2+ + 0.25 O2 + H+ → Fe3+ + 0.5 H2O + Fe3+ + 2 H2O → FeOOH + 3 H+ = FeSO4(s) + 0.25 O2 + 2 H2O → FeOOH(s) + 2 H+ + SO42– 29

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