ACID, SALINE, AND SODIC SOILS

1
Chapter 3

• ACID, SALINE, AND SODIC SOILS

2
Why study acid, saline, and sodic soils?

• Acid, saline, and sodic soils have unique
  chemical and physical properties that influence
  how plants grow.
• Since availability of nutrient ions is determined
  by their chemistry, it is important to understand
  how nutrient availability will be influenced by
  the special chemical properties of these soils.
• What are acid soils?
• Acid soils, technically defined, are soils that
  have a pH less than 7.0, since by convention pH
  of 7.0 is neutral, above 7.0 is basic (or
  alkaline) and below 7.0 is acidic. From the
  standpoint of plant growth, soil management is
  usually not affected until the pH is less than
  about 6.2 for legumes and 5.5 for non-legumes.
• Understanding the concept of pH is fundamental to
  understanding and managing acid soils. Since pH
  is defined as the log H activity, a pH change
  of one unit (e.g. from pH of 6.0 to pH of 5.0)
  represents a 10-fold increase in acidity.
• What causes soil acidity?
• Acid soils are a natural phenomenon related to
  soil parent material and rainfall conditions
  under which the soil developed. Soils developed
  from limestone parent material, for example will
  often be neutral or alkaline in their pH (e.g. pH
  gt 7). Granitic parent material, on the other
  hand, will favor development of an acid soil.

3
Acid Soils

• Under high rainfall conditions (gt 30 inches/year)
  parent material that is permeable, such as
  sandstone, will likely become acidic because
  there is sufficient leaching over geological time
  (tens and hundreds of thousands of years) to
  remove even basic materials like limestone.
• Rainfall, by nature is slightly acidic because
  water and carbon dioxide form carbonic acid in
  the atmosphere (i.e. acid rain is normal).
  Thus, as basic materials are leached out of the
  parent material, H may remain to cause the soil
  to be acidic.
• CO2 H2O ?? H HCO3-
• atmosphere carbonic acid
• Two other factors, that contribute to soil
  acidity, are the removal of basic cations and use
  of N fertilizers associated with intensive crop
  production.

4
Basic and Acidic Cations

• The term basic cations is used to designate
  cations that, when combined with hydroxide (OH-)
  form a compound that would dissolve in water and
  create an alkaline solution
• The cations Na, K, Ca 2 , and Mg 2 are good
  examples. In contrast, the hydroxides of Al 3
  and Fe 3 are so insoluble the ions would not be
  present in solution unless the solution were
  acidified to dissolve them.
• Al 3 and Fe 3 , are usually referred to as
  acidic ions for this reason. Plants generally
  absorb nutrient cations in excess of nutrient
  anions. In this process, electrical neutrality
  or ion-charge balance may be maintained by
  simultaneous absorption of OH- or the exudation
  of H by the plant root.
• In either case the result is a contribution of
  acidity to the soil.
• Plant uptake of basic cations in excess of anions
  in a natural, non-agricultural environment
  contribute little to soil acidity because plants
  die and recycle the cations in-place.
• Intensive agriculture accelerates the
  acidification because the bases are generally
  removed from the field with harvest and are not
  recycled.

5
Intensive agriculture relies heavily on the use
of ammoniacal sources of N. These fertilizer
materials undergo biological oxidation to NO3-
according to the overall general reaction NH4
2O2 ? NO3- 2H H2O which produces two
protons for every mole of N oxidized
6
mZE 11H 42He E- elementm massz - atomic
number ( of protons in the nucleus) All hydrogen
atoms have one proton____________________________
______________11H 21H 31H_______________________
___________________ stable stable radioactive deu
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neutron 1 neutron 2 neutrons1 proton 1 proton 1
proton1 electron 1 electron 1 electron__________
________________________________126C 136C 146C__
________________________________________stable st
able radioactivemass12 mass13 mass146
neutrons 7 neutrons 8 neutrons6 protons 6
protons 6 protons6 electrons 6 electrons 6
electrons________________________________________
__
7
Plant Uptake and Exchange
NO3-
OH-
NH4
H
8
What is the nature of soil acidity and soil
buffer capacity?

• Soils behave as a system made up of the salt from
  a weak acid and strong base.
• Clay and soil organic matter, provide surfaces
  for adsorption of cations
• Clays have a net negative charge resulting from
  isomorphic substitution of divalent for trivalent
  ions (Mg 2 for Al 3 ) and trivalent for
  tetravalent ions (Al 3 for Si 4 ) within the
  mineral structure.
• Soil organic matter contributes to the net
  negative charge of soils because of dissociated
  H from exposed carboxyl and phenol groups.
• The cation exchange capacity (CEC) of organic
  matter is pH dependent, whereas most of the CEC
  from clays is not.
• A small contribution to soil CEC is from
  unsatisfied charges at broken edges of clays.
• The strength with which cations are adsorbed to
  cation exchange sites is directly proportional to
  the product of the charges involved and inversely
  proportional to the square of the distance
  between charges (Coulombs law). Consequently,
  the lyotropic series describing the adsorption of
  cations on clay particles in soils is generally
  considered being
• Al 3 ? H gt Ca 2 ? Mg 2 gt K ? NH4 gt Na.

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• The similarity in strength of adsorption for
  Al and H is because H, although only 1/3 the
  charge strength of Al, is much smaller in
  diameter, allowing it to get closer to the
  internal negative charge of clays than is
  possible for the larger Al.
• The electrostatic adsorption of cations on clay
  and organic matter surfaces creates a reservoir
  of these ions for the soil solution. The
  adsorbed ions are in equilibrium with like ions
  in the soil solution

10
Soil pH

• Relative amounts of each ion adsorbed and in
  solution varies depending upon their relative
  concentrations in the soil solution and how
  strongly the ion is adsorbed (lyotropic series).
  
• Amount of H in the soil solution is 1/100th the
  amount adsorbed on cation exchange sites
• We might expect the amount of Ca 2 and K to be
  present in the soil solution at about 1/50th and
  1/10th their amount adsorbed on cation exchange
  sites
• When soil pH is determined, only the H in the
  soil solution is measured.
• Soil pH referred to as active acidity, whereas
  the H adsorbed on exchange sites is called
  potential or reserve acidity.
• The buffer capacity of soils, that is, their
  ability to resist change in pH when a small
  amount of acid or base is added, is a function of
  their exchangeable acidic and basic cations.
• Soils with low CEC (e.g. sandy, low organic
  matter) have weak buffer capacity, while soils
  with high CEC (e.g. clayey, high organic matter)
  have strong buffer capacity.

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Effect of soil acidity on plants

• Plant species vary in their response to acidic
  soil conditions. Those which have evolved and
  are cultivated in humid regions (e.g., fescue,
  blueberries, and azalea) tolerate acidic soils
  better than other species (e.g., bermudagrass and
  wheat) grown in arid and semiarid climates.
• The chemical environment that plants must
  tolerate, or can benefit from, may be inferred
  from the relationship of percent base saturation
  and pH

Soil pH
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pH and pOH

• pH -log H
• pOH - log OH-
• pH pOH -log Kw 14
• Kw ion-product constant for water
• Kw HOH- 1 x 10-14
• Ka acid-dissociation constant
• Ka HA-/HA (A- conjugate base of the
  acid)
• Kb base-dissociation constant
• Kb OH-A/OHA (A conjugate acid of the
  base)
• Ka Kb Kw
• Ksp solubility-product constant
• -degree to which a solid is soluble in water
• -equilibrium constant for the equilibrium
  between an ionic solid and
  its saturated solution

13
Solubility

• Solubility of a substancequantity that
  dissolves to form a saturated solution (g of
  solute/L)
• Solubility product
• Equilibrium constant for the equilibrium
  between an ionic solid and its saturated solution

Solid AgCl is added to pure water at 25C. Some
of the solid remains undissolved at the bottom of
the flask. Mixture stirred for 2 days to ensure
an equilibrium is reached. Ag conc. Determined
to be 1.34x10-5M. What is Ksp for AgCl?
AgCl ?? Ag Cl- Ksp AgCl-
At equilibrium, conc of Ag 1.34 x 10-5
conc of Cl- 1.34 x 10-5
Ksp (1.34 x 10-5)(1.34 x 10-5) 1.80 x
10-10
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• The percentage base saturation identifies the
  proportion of the CEC that is occupied by cations
  like Na, K, NH4, Ca 2 , and Mg 2 compared to
  the acidic cations of H and Al 3 .
• This relationship is responsible for the fact
  that deficiencies of Ca, Mg and K are rare in
  soils with a pH near or above neutral.
• Aluminum oxides (Al(OH)3, also expressed as
  (Al2O3 ? 3H2O) are of such low solubility that Al
  3 usually is not present in the soil solution or
  on cation exchange sites until the soil pH is
  less than about 5.5.
• The apparent solubility product constant (Ksp)
  for Al(OH)3 in soils is about 10-30. From this,
  the concentration of Al in the soil solution
  and its change with change in pH can be
  calculated.

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Aluminum
Solving the above at pH of 5, OH- would be equal
to 10-9
The concentration of Al (10-3) is moles/liter.
Since the atomic weight of Al is about 27, a
mole/liter would be 27 grams/liter (g/L) and the
concentration of 10-3 is equal to 0.027 g/L, or
27 ppm. 27 ppm at a pH of 5
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Solubility

• Critical to the management and growth of plants
  in acid soils is the knowledge that Al in the
  soil solution increases dramatically with
  decrease in pH below about 5.5. When solved for
  a soil pH of 4.0 (OH- is equal to 10-10), we have

A concentration of 1.0 mole/L is equal to 27 g/L
or 27,000 ppm. While there may not be a
1000-fold increase in soil solution Al 3
concentration when pH changes from 5.0 to 4.0,
these calculations should make it clear why Al 3
concentrations may be significant at pH 4.5, for
example, and immeasurable at 5.5.
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Al toxicity

• Soluble Al is toxic to winter wheat at
  concentrations of about 25 ppm.
• Adverse effect of soil acidity on non-legume
  plants is usually a result of Al and Mn toxicity.
  
• In winter wheat, Al toxicity inhibits or prunes
  the root system and often causes stunted growth
  and a purple discoloration of the lower leaves.
• These symptoms are characteristic of P
  deficiency, and are likely a result of the plants
  reduced ability to extract soil P.
• Al toxicity versus P deficiency? Solubility
  diagram

Laboratory exercise, applying P to decrease Al
toxicity?
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pH preferences of common crops

• pH is not an essential plant nutrient, and
  plants obtain their large H requirement from H2O
  and not H.
• Thus, it is the chemical environment, for which
  pH is an index, that crops are responsive to
  rather than the pH itself.
• Non-legumes require a soil pH above 5.5 because
  more acidic soils tend to have toxic levels of Mn
  and Al present.
• Crops which grow well in soils more acidic than
  this can tolerate these metal ions and perhaps
  are ineffective in obtaining Fe from less acidic
  soils.
• Legumes usually grow best at soil pH above 6.0
  because the rhizobium involved in fixing
  atmospheric N2 seem to thrive in an environment
  rich in basic cations.

Plants split H2O
Mangroves
Mangroves 2
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How is soil acidity neutralized

• Most effective way to neutralize soil acidity is
  by incorporation of aglime.

Neutralization of acid soil using aglime (CaCO3)
resulting in increasing exchangeable Ca and
formation of water and carbon dioxide.
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Lime

• Aglime is effective because it is the salt of a
  relatively strong base (calcium hydroxide) and a
  weak acid (carbonic acid), and is therefore basic
  
• Ca(OH)2 H2CO3 ? CaCO3 H2O

carbonic acid
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Lime needed to neutralize soil acidity

• Exchangeable acidity must be neutralized in order
  to change soil pH because it represents most (99
  ) of the soil acidity. Since the amount of
  exchangeable acidity in the soil, at a given pH,
  depends on the soil CEC, the amount of lime
  required is a function of clay content, organic
  matter content, and soil pH.
• Lime requirements can be determined directly in
  a laboratory by quantitatively adding small
  amounts of a solution of known strength base
  (e.g. 0.1 normal NaOH), to a known amount of the
  acid soil mixed with water.

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pH and Lime

• By measuring pH as the base is added, the amount
  of base required to obtain any pH can be
  estimated

Buffer index of 6.2
pH scale of 14? Why?
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Lime

• Direct determination of lime requirement is very
  time consuming and is not usually done in the
  routine determination of lime requirement by soil
  testing laboratories.
• Direct determination identifies the amount of
  base, such as CaCO3, that must be applied if all
  the acidity is able to react with the base that
  is added
• In practice, this is virtually impossible because
  of size differences between clay and organic
  matter colloids (very small) and the finely
  ground (relatively large) lime particles.
• Field studies (calibration) can be conducted to
  develop the relationship between amounts of
  aglime identified by direct laboratory titration
  and crop response.

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Lime Requirements

• Most soil testing laboratories use an indirect
  method of determining aglime requirement.
• Involves adding a known quantity of a lime-like
  chemical solution (i.e., buffer solution of pH
  7.2) to an acid soil and water mixture.
• After equilibrium has been obtained (about two
  hours) the pH is measured.
• This pH is often called the buffer pH or
  buffer index. The buffer index, by itself,
  does not identify how much lime must be added to
  neutralize an acid soil.
• Field studies relating lime additions to soil pH
  are required to calibrate the buffer index, just
  as they would be in a direct titration approach.

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Lime Requirements
Why do we now lime to 6.0? (and not anything
above 6.0)
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Buffering Capacity

• Buffer capacity is a function of CEC (e.g. clay
  and soil organic matter content).
• Amount of lime required to neutralize acidity in
  a sandy soil (e.g. Meno fine sandy loam) and a
  fine textured soil (e.g. Pond Creek silt loam)
  will be quite different even when they have the
  same soil pH

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Amount of potential acidity that needs to be
neutralized
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How often should lime be applied

• The answer to this question will depend on how
  intensively the soil is managed and how large is
  the soil buffer capacity. For example, the
  amount of basic cations removed in a 30-bushel
  wheat crop in grain and straw is shown to be
  about the same as that removed by a ton of good
  quality alfalfa hay

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• Soil will become acidic faster, and require
  liming more often, if both grain and straw are
  harvested.
• If two fields are yielding at the same level, it
  might be expected that a sandy soil would need to
  be limed at lower rates, but more frequently,
  than a fine textured soil.

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Common liming materials

• Aglime. Any material that will react with, and
  neutralize, soil acidity may be considered for
  use to lime an acid soil. The most common
  liming material is aglime, a material that is
  primarily composed of calcium carbonate, mined
  from geological deposits at or near the earths
  surface.
• Some deposits are high in magnesium carbonate and
  are called dolomitic limestone. Dolomitic
  limestone is also a good source of Mg for deep,
  sandy, acid soils where this nutrient may also be
  deficient. The mined limestone is usually
  crushed and sieved to obtain material of a small
  enough particle size to be effective for aglime.
• Quick lime. Mined limestone may be processed to
  improve its purity and neutralizing strength.
  The term lime was initially used as a name for
  CaO, which may also be called unslaked lime,
  burned lime, or quick lime. It may be obtained
  by heating (burning) calcium carbonate to drive
  off carbon dioxide.

CaCO3 heat ? CaO CO2
Often used for stabilizing sewage sludge. When
added to the mixture of sewage solids and water,
it quickly reacts to raise the pH above 11
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Liming Materials

• Hydrated lime. Hydrated lime, which may also be
  called slaked lime or builders lime, is produced
  by reacting quick lime with water.

CaO H2O ? Ca(OH)2
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Special Formulations

• Liquid lime
• Formulated by mixing finely ground limestone with
  water and a small amount of clay.
• Clay is added to help keep the lime particles
  suspended in the water during application.
• Since the solubility of CaCO3 is low, most of the
  lime is present in solid form and will react like
  an application of solid lime. The ECCE of the
  formulation will be much less (depends on how
  much water was added) than that of the lime used
  in the mixture, even when the dry lime had a high
  ECCE.
• Typically the dry lime has an ECCE of nearly 100
  and the liquid lime is about 50 because about
  ½ of it is water.
• Pelleted lime
• Pelleted lime is created by compressing, or
  otherwise forming pellets out of finely ground,
  good quality CaCO3.
• Neutralizing effectiveness of liming materials
  depends upon being able to maximize their surface
  contact with soil colloids.
• The advantage of liquid lime and pelleted lime
  compared to conventional aglime is to minimize
  dust. The disadvantage is they are usually much
  more expensive, on a cost per ton of ECCE, than
  conventional aglime.

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Industrial by-products.

• Kiln dust from cement manufacturing plants,
• Fly-ash from coal burning power plants,
• Residual lime from metropolitan water treatment
  plants.
• Effectiveness of these materials will depend on
  particle size and neutralizing strength of the
  material.

Lime from Water Treatment

• History of Water Treatment

34
Plagiarism
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Mark 35 And when he had looked round about on
them with anger, being grieved for the hardness
of their hearts, he saith unto the man, Stretch
forth thine hand. And he stretched it out and
his hand was restored whole as the other
Nephi 218 But, behold, Laman and Lemuel would
not hearken unto my words and being grieved
because of the hardness of their hearts I cried
unto the Lord for them
35
How are the neutralizing values of liming
materials compared

• Effective Calcium Carbonate Equivalent.
• Effectiveness of the aglime identified as
  effective calcium carbonate equivalent, or ECCE.
  
• Expression of the active ingredient of the
  material for neutralizing soil acidity.
• ECCE of liming materials is expressed as a
  percentage of the material and takes into account
  the particle size and neutralizing strength of
  the material
• Chemical Equivalence.
• Equivalence of compounds relative to their acid
  neutralizing strength provides insight to their
  differences in neutralizing strength.
• Accomplished by calculating the equivalent weight
  of a liming material and comparing it to the
  equivalent weight of CaCO3.
• Only possible if the materials are pure
  chemically. This consideration is of interest,
  for example, when comparing the effectiveness of
  dolomitic lime (rich in MgCO3) to that of normal
  aglime (primarily CaCO3). The equivalent weight
  of each material is calculated, using the
  definition
• An equivalent weight is the mass of a substance
  that will react with one gram of H, or one mole
  (6 x 1023) of charge.

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Equivalent weights

• Equivalent weights are the chemists way of
  converting apples and oranges (etc.), all to
  apples.
• Atomic (or molecular) weight of an ionic species,
  divided by its charge is equal to its equivalent
  weight.
• For both CaCO3 and MgCO3 the charge of ions
  involved is two, and one mole of the carbonate
  ion will neutralize two grams of H, or two moles
  of charge.
• The molecular weight of CaCO3 is 100 and MgCO3 is
  84.
• Equivalent weights are ½ their molecular weights,
  or
• CaCO3 100/2 50
• MgCO3 84/2 42
• It only requires 42 g of MgCO3 to accomplish the
  same neutralizing as 50 g of CaCO3, the MgCO3 is
  50/42 or 1.19 times more effective than CaCO3.
• Applying the same comparison to CaO (eq. wt. 28)
  and Ca(OH)2 (eq. wt. 37) it is clear that these
  materials would be required at much lower rates
  than CaCO3 (eq. wt. 50)

37
Important considerations to improve success of
liming

• Soil Testing.
• Reliable soil test (representative)
• soil pH may be variable in the area (year to
  year?) (within year?)
• Amount of Lime. The buffer index from a soil
  test serves as a good guide for determining how
  much lime should be added,
• When non-legumes are grown successively in the
  same field, it is only necessary to apply enough
  lime to eliminate current and future Al and Mn
  toxicities.
• Lime recommendations for continuous wheat
  production in Oklahoma are to apply only ¼ the
  amount required to raise the pH to 6.8.
• This recommendation will raise the pH above 5.5
  and keep it below 6.5 to minimize the incidence
  of root-rot diseases.
• Occasionally the buffer index for sandy, low
  organic matter soils will be so high that no lime
  is recommended.
• In these cases a minimum of 0.5 ton ECCE/acre for
  non-legumes and 1.0 ton ECCE for legumes is
  recommended to assure the acidity will be
  corrected and the application is economical.
• When lime recommendations are extremely large the
  amount should be split into an initial
  application of 5 ton/acre (230lb/1000 ft2)
  followed by the remainder applied a year later.

38
Considerations

• Incorporation and Timing.
• Lime must be physically mixed with the soil.
• Pastures, perennial plantings, or no-till
  productions, may require three to five years
  before the lime causes a noticeable change in
  soil pH.
• Important to lime fields before they are planted
  to a perennial crop or managed as no-till.
• Systems where alfalfa is rotated with a
  non-legume annuals like corn or wheat, the field
  should be limed a year before the alfalfa is
  planted to take advantage of tillage operations
  related to corn or wheat production and allow
  more time for lime to react in the soil.
• When lime is incorporated well, and there is good
  soil moisture, it may still take a year or more
  before noticeable change in soil pH occurs.
• Tillage Depth. Lime recommendations are usually
  made assuming a six-inch tillage depth.
• Sandy soils are usually cultivated to eight or
  ten inches and a proportional increase in the
  lime rate should be made.
• For crops with a shallow root system, such as
  some vegetables, it may be important to reduce
  the lime rate to match a shallower depth of
  incorporation.

39
How can acid soils be managed without liming

• Liming Alternative.
• Acid tolerant varieties or different plant
  species.
• Karl and Custer are not acid tolerant whereas the
  variety 2163 is acid tolerant.
• Rye more acid tolerant than wheat.
• The Al and Mn toxicity that prevent normal
  seedling root development in wheat can be
  alleviated by adding phosphate fertilizer in a
  band with the seed at planting.
• Phosphate reacts strongly with Al to form
  insoluble aluminum phosphate, thus removing Al
  from solution and the exchange complex.
• Rate of 60 lb P2O5/acre is required to obtain
  normal fall pasture but only 30 P2O5/acre is
  needed if wheat is managed for grain only.
• If P is not deficient, the cost of applying the P
  for two or three years will usually equal the
  cost of an application of lime that would have
  lasted five to eight years.
• These alternatives allow normal or near normal
  production but do not cause a change in soil pH.
  
• Eventually the soil must be limed for long-term
  production.

40
What are saline soils

• Classified as saline when they contain a high
  enough concentration of soluble salts to
  interfere with normal growth and development of
  salt-sensitive plants.
• Soluble salts are compounds, like common table
  salt (NaCl), where ions that make up the salt are
  weakly bound and have a strong attraction for
  water.
• These ions hold water quite tightly, salty water
• (higher boiling point)
• (lower freezing point)
• Salt is added to water used in food preparation
  to raise the boiling point and hasten the
  process.
• Salt spread on icy sidewalks and roads to melt
  ice that would otherwise remain solid at
  temperatures below freezing.
• Soluble salts in soils soil water is held
  tightly enough by the ions that plants cannot use
  it (apparent moisture stress)
• Saline soils characteristically remain moist
  longer than the rest of the field
• Occupy poorly drained areas of the landscape
• White surface layer of salt after they become
  dry.
• Occur in semi-arid, temperate regions
• Saline soils are uncommon in the moisture
  extremes of deserts and tropical rain forests.

41
Saline Soils

• Saturating a soil sample with water (a paste
  condition) for about four hours,
• Extracting the water (and dissolved salts)
• Measuring its ability to conduct electricity.
• Ions in water allow electricity to pass through
  it
• More ions present the easier electricity is
  conducted
• Conductivity is expressed in mhos/cm.
• Conductivity of water is usually very low and
  expressed as mmhos/cm or micromhos/cm.
• Soils are classified as saline when the extract
  of a saturated paste has an electrical
  conductivity (EC) equal to or in excess of 4,000
  micromhos/cm.
• Concentration of soluble salts, expressed as ppm,
  is roughly equal to 0.65 times the conductivity
  expressed in micromhos/cm.
• Soil with an EC of 4,000 micromhos/cm will
  contain about 2600 ppm soluble salts in the
  saturated soil solution.
• Saline soils Reclamation
• leaching soluble salts out of the soil.
• create good surface and internal drainage.
• incorporating large amounts of organic matter
  (create large pores in the surface soil)
• Good quality irrigation water can be used to
  hasten the process.
• Deep tillage should be avoided once the organic
  matter is incorporated
• Salt tolerant species like bermudagrass or barley
  should be planted to provide a vegetative cover
• Practices to reduce surface evaporation and
  encourage water movement downward ???

42
What is a Sodic Soil

• Abnormally high levels of exchangeable sodium
  (Na).
• When enough Na is adsorbed, clay particles repel
  each other.
• Occurs when the exchangeable Na percentage (ESP)
  is equal to or exceeds 15
• Soil pH of sodic soils will often be above 8.
• Dispersed colloids become oriented as water moves
  into soil and eventually they plug soil pores.
• Poor internal drainage resulting in dry subsoil
  and a moist or wet surface layer. Crops fail
  because of excess surface water (drown out) or
  for lack of water (dry subsoil) even though there
  may have been adequate rainfall or irrigation.
• Reclaimed by improving surface and internal
  drainage and incorporating gypsum (CaSO4) in the
  surface.
• Gypsum dissolves to supply a high concentration
  of Ca in soil solution that replaces
  exchangeable Na, freeing it to be washed out of
  the soil
• Ca helps bind colloids into aggregates and
  restore soil permeability. Reclamation of sodic
  soils is similar to that of saline soils except
  that gypsum must be added to sodic soils.

43
What are Saline-Sodic Soils

• Contain salts in excess of 4,000 micromhos/cm and
  exchangeable Na in excess of 15
• Have all the features of the saline soil, and if
  reclamation procedures are used that do not
  include gypsum, they will become sodic soils when
  the salts are leached out.
• Many salt affected soils are saline-sodic because
  a primary soluble ion is Na.
• Reclamation takes several (2 or more) years,
  dependent upon the time required to get about two
  pore volumes of good quality water to pass
  through the soil.
• Most soils are about 50 pore space and so a
  pore volume-depth for a four foot profile would
  be about two feet and two pore volumes about four
  feet.
• Sandy soils in high rainfall regions may be
  reclaimed quite rapidly while clayey soils in
  semi-arid regions may take many years if rainfall
  is the only source of leaching water.

44
How Soluble is the Earths Crust

• The extent to which the earths crust dissolves
  over time depends upon solubility of rocks and
  mineral, abundance of elements in the rocks and
  minerals, and rainfall.
• Naturally occurring compounds containing either
  Na or Cl tend to be very soluble and, with time,
  end up in the oceans and seas of the world.

45
Turf

• Does soil acidity increase in turf situations via
  the application of N
• No continual removal of bases like in wheat and
  corn, thus soil acidity is diminished