Chapter 8 The Colliodal Fraction: Chapter 9: Soil Acidity

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Chapter 8 The Colliodal FractionChapter 9
Soil Acidity
Kaolinite
Mica
Montmorillonite
Fulvic acid
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Figure 8.1
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Basic Phyllosilicate
Figure 8.4
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Important Clays Minerals

• Kaolinite Most common in soils
• Smectite Shrink-Well Clays
• Montmorillonite
• Vermiculite
• Micas Fine-grained micas
• Illite
• Chlorite
• Humus Humic acid and organic matter

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Figure 8.27
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Structure of Humic Acid
Figure 8.12
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Figure 8.10
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Genesis of Soil Colloids

• Alteration An example would be the change from
  muscovite mica to fine-grained mica through
  physical breakdown, loss of some elements and
  addition of others.
• Recrystallization Results from the complete
  breakdown of the original crystal structure and
  regrowth of clay minerals.
• Relative Stages of Weathering Fine-grained
  micas are an early stage while kaolinite and iron
  oxides are late stages

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Figure 8.14
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Principles Governing Cation Exchange Reactions

• Reversibility A cation is usually replaced by a
  hydrogen ion and this reaction is reversible
• MicelleNa H MicelleH Na
• Charge Equivalence Exchanges take place on a
  charge-by-charge basis
• MicelleCa2 2H Micelle2H Ca2
• Ratio Law The amount of exchangeable cation in
  the soil solution will affect the ions
  equilibrium

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Principles Governing Cation Exchange Reactions

• Mass Action If a product precipitates,
  volatilizes, or strongly associates with an
  anion, the reaction is not reversible.
• Cation Selectivity Higher charge and hydrated
  radius affect the exchangeability of cations
• Al3 gt Ca2 gt Mg2 gt K NH4 gt Na

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Figure 8.19
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Figure 8.22
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Figure 8.20
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CEC and pH in Soil Orders
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Relation of CEC to pH
Figure 8.23
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Acidifying a Soil
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Pools of Soil Acidity

• Active Acidity
• H ion activity in the soil solution.
• Exchangeable (Salt Replaceable) Acidity
• Associated with exchangeable aluminum and
  hydrogen ions in the parent material.
• The take the place of beneficial cations.
• Residual Acidity
• Hydrogen and Aluminum ions bound to organic
  matter and clays. May free-up Cation exchange
  locations with liming
• May be 100,000 times greater acid input than the
  other pools

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Pools of Acid
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Acid Rain
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Geographic Distribution of Acid Rain
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Effect of Soil pH on Root Growth
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Plant Macronutrients

• Calcium and Magnesium are essential to plants and
  are tied to soil acidity
• Calcium
• Calcium is second only to N and K as a plant
  macronutrient
• Used in cell walls, cell elongation, membrane
  permeability, and enzymes
• Taken up exclusively by young root tips and
  redistributed in the plant with transpiration
  water
• Symptoms Young leaves not unfolding, lack of
  root development, blossom-end rot, empty peanut
  shells

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Symptoms of Low Calcium
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Plant Macronutrients

• Magnesium
• Taken up in less quantity than Calcium
• Used in the Chlorophyll molecule
• Used in the synthesis of oils, proteins, and
  enzymes
• Symptoms Mottled green and yellow coloring in
  the older leaves.
• Both Calcium and Magnesium deficiencies can be
  remedied by applying lime (CaO) or Gypsum
  (CaSo42H2O)

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Principle Pathways of Calcium and Magnesium
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Calcium and Magnesium Cycle
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