Lecture 12a Soil Chemistry Soil pH

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Lecture 12aSoil Chemistry / Soil pH

• Soil pH is the single most important chemical
  property of the soil (like soil texture is to the
  physical properties)
• Knowing the pH of the soil will quickly allow you
  to determine if the soil is suitable for plant
  growth and what nutrients will be most limiting.
• "Life is a struggle, not against sin, not
  against the Money Power, not against malicious
  animal magnetism, but against hydrogen
  ions.--H.L. MENCKEN
• Strictly speaking, hydrogen ions are protons and
  do not exist in the naked state in fluids
  instead they react with water (H20) to form
  hydronium ions, such as H3O
• For most purposes H can be used to represent
  these hydrated protons.

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Acid Base Chemistry

• Acid-base chemistry is an important part of
  everyday life. The excess hydronium (H3O) ions
  in acids give them interesting properties.
• Acids can react with metals and other materials.
  The strong acid HCl is produced in your stomach
  to help digest food. In dilute concentrations,
  acids are responsible for the sour taste of
  lemons, limes, vinegar and other substances.
• Bases are also very reactive. The strong base
  NaOH is used in many household cleaning agents
  such as oven cleaner and drain clog-remover.
• How do we measure the concentration of an acid or
  base?

H3O depends on the Strength of acid and Initial
concentration Of acid
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Measuring Acidity

• The acidity (or basicity) of a solution is
  measured using the pH scale. (this scale is used
  because of the very small concentrations that are
  being measured)
• The pH scale corresponds to the concentration of
  hydronium ions in a solution.
• If you take the exponent of the H3O
  concentration and remove the negative sign, you
  have the pH of a solution.
• For example, in pure water the concentration of
  hydronium ions is 1 x 10-7 M.
• Thus, the pH of a solution of pure water is 7.
• The pH scale ranges from 0 to 14, where 7 is
  considered neutral (H3O OH-),

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pH
Hydronium ions Hydroxide ions Water

• Acid solutions are when pH is lt 7.0
• Alkaline solutions are when pH gt 7.0
• An acid can be defined as a proton donor, a
  chemical that increases the concentration of
  hydronium ions in solution.
• Conversely, a base is a proton acceptor, a
  chemical that reduces the concentration of
  hydronium ions in solution (and increases the
  concentration of hydroxide ions).

Neutral Solution
Slightly Acidic Solution
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Soil pH- Measure H in the Soil Solution

• pH - the negative log of the hydrogen ion(H)
  concentration in the soil water solution.
• pH - log H
• the pH scale is how we measure acidity and
  alkalinity of solutions.
  at neutral (pH 7) the
  number of H OH-
• Remember
• at pH of 6 there are 10x more H ions than at a
  pH 7
• and there are 100x more H ions between pH 7
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• NEED TO LOOK AT Cation Exchange before further
  discussion of soil pH

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Soil Cation Exchange

• Cation Exchange - the ability of the soil to hold
  onto nutrients and prevent them from leaching
  beyond the roots.
• Cations are charged ions Ca, Mg, K,
  NH4,
• The more cation exchange a soil has the more
  likely the soil will have a higher fertility
  level.

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Cation Exchange

• The interchange between a cation in solution and
  another cation on the surface of any negatively
  charged material such as clay or organic matter

H H
soil
Ca 2H
Ca
soil solution
soil
colloid solution
colloid
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Cation exchange influenced by

• 1) Strength of adsorptionAl3 gt Ca2 gt Mg2 gt
  K NH4 gt Na gtH
• held tight --------------------------gt easily
  replaced
• 2) the relative concentration of the cations in
  the Soil Solution the more ions in the soil
  solution the more ions on the soil exchange sites

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Cation Exchange Capacity

• 1) the number of cation adsorption sites per unit
  weight of soil or
• 2) the sum total of exchangeable cations that a
  soil can adsorb.
• CEC is expressed in milliequivalents (meq) per
  100 g of oven dry soil.
• Equivalent weight molecular or atomic wt (g)
• valence or
  charges per formula

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Milliequivalent (MEQ)

• 1 meq wt. of CEC has 6.02 x 10 20 adsorption
  sites
• MEQ of Common Cations
• Element Na K Ca Mg
• Valence 1 1 2 2
• Eq. Wt 23/123 39/139 40/220 24/2
  12
• MEQ wt .023 .039 .02 .012

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Back to Soil pH

• Active Acidity - due to the H ion activity in
  the soil solution at any given time
• Reserve Acidity (on sites)- represented by the H
  and Al3 that are easily exchanged by other
  cations (positively charged ion)
• H H H H H H
• H Ca H
• Mg Mg H
• Ca
  Ca H H
• H H H Na

soil
Reserve Acidity Active
Acidity -Soil solution
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Sources of acidity in Soil

• Hydrogen and Aluminum cations are responsible
  for soil acidity
• Exchangeable Hydrogen is the main source of H
  at pH 6 and above. Below pH 6 Aluminum is the
  main source of H due to dissociation of Al from
  clay minerals. Aluminum becomes more soluble at
  lower pHs
• Al3 H20 ----gt Al(OH) H
• Al(OH) H2O ---gt Al(OH)2 H
• Al(OH)2 H20 ---gt Al(OH)3 H

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Sources of acidity in Soil

• 1. Nitrification Ammonium to Nitrate
  (oxidation of NH4)
• NH4 2O2 ---gt NO3- H2O 2 H
• 2. O.M. decomposition
• organic acids ionized
• R-COOH---gt R-COO- H
• respiration CO2 H2O ----gt
  H2CO3 H HCO3-

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3. Acid rain

• Acid rain is caused by the burning of fossil
  fuels.
• Burning oil, gas and coal in power stations
  releases Sulfuric Dioxide (SO2) into the
  atmosphere.
• Burning oil and gasoline in motor vehicles puts
  nitrogen oxides (NOX) into the atmosphere.
• These gases mix with water droplets in the
  atmosphere creating weak solutions of nitric and
  sulfuric acids.
• When precipitation occurs these solutions fall as
  acid rain.

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Acid Rain in USA

• 3. Acid Rain
• SO2 OH --gt H2SO4 --gt SO4- 2 H
• NO2 OH --gt HNO3--gt NO3- H

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Sources of acidity in Soil (cont.)

• 4. Uptake of basic cations by plants. Basic
  cations are sources of OH- to the soil solution.
• Ca, Mg, K,
• Basic cations that are taken up by plants no
  longer contribute OH- to the soil solution.
• H ions are released to the soil solution.

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Leaching

• 5. Leaching of basic cations -
  as basic
  cations are removed from the soil solution by
  leaching they no longer contribute the OH- ions
  to neutralize the ever increasing amounts of H
• Ca 2 H20 ---gt Ca(OH)2 2H
• -----gt Ca 2OH-

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pH values for Midwestern Soils
NRCS Data
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Soil Acidity and Plant Growth

• Soil acidity is a major environmental stress
  factor which limits the growth of most crops.
• Acid soils are widely dispersed and comprise
  approximately 40 of the arable land in the
  world.
• In addition, acid rain also accelerates the
  acidification of soils.
• Aluminum (Al) ion is solubilized from soils at
  low pH. This is a major toxic factor for plant
  growth for low pH soils.

Acid soil (Yunnan Province, People's Republic of
China).
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Soil Acidity in Australia

• Soil acidification is a natural process that is
  part of all landscapes.
• Land clearing of native vegetation and their
  replacement with productive crops and pastures
  has accelerated acidification over the past 200
  years.
• In the past 50 years a significant lowering in
  soil pH due to
• greater use of fertilizers,
• increased production,
• greater use of legumes
• increase in irrigated agriculture.

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• An effective option for acid soils is a matter of
  applying lime. however
• The land use should be matched to
• capability of the land and soil,
• available rainfall,
• rate of acidification.

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Acid Sulfate Soils

• Acid sulfate soils form when pyritic (mineral of
  FeS2) estuarine sediments in the subsoil are
  exposed to air, oxidizing to form sulfuric acid.
• A variety of soil minerals react with the acid
  and release free aluminum, toxic to crops and
  marine life.

The red color of the water is caused by oxidized
iron
Pyrite is the most common iron disulphide
mineral in rock. It is found most often in
metamorphic and sedimentary rocks where it occurs
as either a primary mineral or a fine, widespread
impregnation of subsequent origin. Pyrite is
frequently found in association with coal and
shale deposits.
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• Acid sulfate soils are extremely acidic - times
  less than 3.0
• Acid soil horizons result from the aeration of
  soil materials that are rich in iron sulfides,
  (FeS)
• Acid sulfate soils are unique in that the impacts
  can be so severe that they can affect
• engineering works,
• agricultural productivity,
• water quality of estuarine systems

Acid soils have cost Australia 10 billion in
environmental damage as a result of coastal
development.
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• Iron staining is often a good indicator of
  disturbed acid sulfate soils.
• When acid sulfate soils are disturbed and undergo
  oxidation, the sulfuric acid produced mobilizes
  iron, aluminum and heavy metals present in the
  soil.
• Toxic amounts of dissolved iron can then be
  washed into waterways.
• This iron can precipitate when in contact with
  less acid water, such as rainwater or seawater.
• This results in a rust-colored iron oxide scum or
  floc which can smother vegetation and stain
  concrete and soil.
• From 'QASSIT, Qld Department of Natural
  Resources and Mines'

Green acid water leached from Acid Sulfate Soils
pours into a river
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Soil pH of Europe
The End