6. Exchange

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6. Exchange
SOIL 5813 Soil-Plant Nutrient Cycling and
Environmental Quality Department of Plant and
Soil Sciences Oklahoma State University Stillwater
, OK 74078 email wrr_at_mail.pss.okstate.edu Tel
[phone]
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Absorption interception of radiant energy or
sound waves Adsorption adhesion in an extremely
thin layer of molecules to the surfaces of solid
bodies or liquids with which they are in
contact.   Soils containing large amounts of
mineral clay and organic matter are said to be
highly buffered and require large amounts of
added lime to increase the pH.   Sandy soils with
small amounts of clay and organic matter are
poorly buffered and require only small amounts of
lime to change soil pH, (Tisdale, Nelson, Beaton
and Havlin, p.94)   Buffering capacity (BC)
represents the ability of the soil to re-supply
an ion to the soil solution.
H ion Buff.
CEC
BC
BC
pH
CEC
BC
pH
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• pH independent charge (permanent)
• Isomorphic substitution substitution of one
  element for another in ionic crystals without
  changing the structure of the crystal
• Substitution of Al for Si in tetrahedral
• Mg, Fe, Fe for Al in octahedral
• Leaves a net negative charge (permanent)
• pH dependent charge positive charge developed at
  low pH and excess negative charge formed at high
  pH
• Gain or loss of H from functional groups on the
  surface of soil solids.
• Hydroxy (-OH)
• Carboxyl (-COOH)
• Phenolic (-C6H4OH)

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pH dependent charge (cont.) Only 5-10 of the
negative charge on 21 layer silicates is pH
dependent whereas 50 or more of the charge
developed on 11 minerals can be pH dependent.
1. Kaolinite 11 At high pH, these H are
weakly held and may be exchanged. At low pH, H
are held very tightly and are not exchanged.
Deprotonation or dissociation of H from OH-
groups at the broken edges of clay particles is
the prime source of negative charge in the 11
clay minerals. High pH values favor this
deprotonation of exposed hydroxyl groups. This
creates some confusion since high pH is seldom
associated with weathered soils. Fifty percent
or more of the charge developed on 11 clay
minerals can be pH dependent. In a weathered
soil, hydrous oxides (Fe and Al oxides) are a
more important source of pH dependent charge. Si
(as silicic acid H4SiO4) is weathered and
leached. 21s ?become ?11s become ? Fe-Al
oxides Where does the negative charge come from
in an acid soil? 2. Organic Matter Most soils
have a net negative charge due to negative
charges on layer silicates and organic matter,
however, some highly weathered soils dominated by
allophane and hydrous oxides may actually have a
net positive charge at low pH.
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SiO- permanent charge (will form SiOH but at pH
2.0) pH dependent charges Al-OH pH 6 7.5
(pure surface) Al-O- pH goes up (negative
charge) AlOH2 pH goes down (positive
charge) Same for Fe
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Hydroxy (-OH) Carboxyl (-COOH) Phenolic (-C6H4OH)
higher pH
pH dep. charge
COO-
COOH
CEC
Perm. charge
4.0
7.0
pH
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CEC, Ca, Mg, KAEC, SO4 High CEC at High pH How
much CEC do we need? Zeolites (CEC 100
meq/100g) Does the soil need added CEC? CEC
holds nutrients, keeps them from being leached
If pH is low, and a soil has small CEC and high
AEC If pH is high, and soil has small AEC and
high CEC Total of sites stays constant With 21
clays pH independent charge can be significant
regardless of pH
Soil with pH dependent Charge
Soil with little pH dependent Charge
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21
- 0
pH dependent charge
pH independent charge (IS)
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Soil pH
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Should not use a buffered solution (fixed pH) for
CEC. If a 1 N NH4OAc ( pHgt7.0 ) solution were
used to displace cations on the exchange complex
of a soil with a pH of 5.0, CEC would be
overestimated as pH dependent charge sites would
be included (specifically organic matter) that
would not have been present at the soils natural
pH. Calcareous soil, REVERSE Ions must exist
in soils as solid compounds or adsorbed to
cation/anion exchange sites. Can be described by
the ratio of the concentrations of absorbed (D Q)
and solution (D I) ions BC D Q/D I The BC in
soil increases with increasing CEC, organic
matter and other solid constituents in the
soil. For most minerals the strength of cation
adsorption or lyotropic series is AlgtCagtMg
gtKNH4gtNa ions with a higher valence are held
more tightly than monovalent cations (exception,
H) AlgtHgtCagtMggtKNH4gtNa
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Replaceability of an ion decreases as its
dehydrated radius increases. Cations are
attracted toward, and anions are repelled from,
negatively charged soil colloids. These
interactions follow Coulomb's law
where Fqq'/Dr2 F is the force of attraction or
repulsion q and q1 are the electrical charges
(esu, equal to 2.09 x 109 individual electronic
charges) r is the distance of charge separation
(cm) D is the dielectric constant (78 for water
at 25C) Strength of ion retention or repulsion
increases with increasing ion charge, with
increasing colloid charge and with decreasing
distance between the colloid surface and either
the source of charge or the soluble
ion. Interaction between ions increases with
concentration and with the square of the ion
charge. The parameter embracing the
concentration and charge effects is the ionic
strength (I) of the solution. I ½ sum Mi
Zi2 where M is the molarity, Z is the charge of
each ion i. Ionic strength measures the
effective ion concentration by taking into
account the pronounced effect of ion charge on
solution properties. A solution has only one
ionic strength but each of its constituent ions
may have a different activity coefficient.
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Exchangeable cations Ca Mg K and
Na Exchangeable acidity 1. H ions obtained
from the hydrolysis of exchangeable, trivalent
Al 2. Hydrolysis of partially hydrolyzed and
non-exchangeable Al 3. Weakly acidic groups,
mostly on organic matter 4. Exchangeable H In
the early days of soil science there was no
agreement on the pH of the soil at which
exchangeable acidity was to be determined.
Bradfield, 1923 noted that the usual substance
used to increase the pH of acid soils is CaCO3
and that the maximum pH obtainable with CaCO3 is
pH 8.3. Therefore base saturation is defined as
the quantity of base adsorbed by a soil in the
presence of CaCO3 equilibrated with air having a
CO2 content of 0.03 (Thomas, 1982).
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• Cation Exchange Capacity (CEC)
• Sum total of exchangeable cations on the exchange
  complex expressed in meq/100g (Ca, Mg, K,
  Na, H, Al)
• Quantity of readily exchangeable cations
  neutralizing negative charge in the soil
• Exchange of one cation for another in a solution
  phase
• Soils capacity to adsorb cations from an aqueous
  solution of the same pH, ionic strength,
  dielectric constant and composition as that
  encountered in the field.
• Extract sample with neutral 1 N ammonium acetate.
  (NH4OAc)
• exchange complex becomes saturated with NH4
• extract same soil with 1N KCl (different salt
  solution), K replaces NH4
• quantity of ammonium ions in the leachate is a
  measure of CEC
• example
• -filtrate has 0.054 g of NH4
• (20 g of soil extracted)
• 1 meq of NH4 (144)/1000
• 0.018g/meq or 18g/eq
• 0.054/0.018 3 meq
• 3 meq/20g 15meq/100g

AlgtHgtCagtMggtKNH4gtNa
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increase clay, increase CEC increase OM, increase
CEC increase 21, increase CEC 11 clays 1-10
meq/100g 21 clays 80-150 meq/100g
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Effective CEC Extraction with an unbuffered salt
which would give a measure of the CEC at the
soils normal pH. Use of neutral N ammonium
acetate (7.0) will result in a high CEC on acid
soils because of the adsorption of NH4 to the pH
dependent charge sites. Why? 1.At high pH, H
are weakly held and may be exchanged pH
dependent charge 2.Deprotonation (dissociation of
H from OH groups at the broken edges of clay
particles which is the prime source of negative
charge in 11 clay minerals) occurs only at high
pH (7.0 and up) Kamprath unbuffered salt
solution, 1.0 N KCl will extract only the cations
held at active exchange sites at the particular
pH of the soil. The exchangeable acidity is due
to Al and H.
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CEC Methods 1.Polemio Rhoades (1977) arid soils
containing carbonates, gypsum and zeolites.
Saturation of exchange sites with Na (pH 8.2)
0.4N NaOAc 0.1N NaCl Extraction with 0.5N
MgNO3 Na determined (soluble Na from saturation
step deducted from total Na to obtain
exchangeable Na) Method will determine CEC as a
result of permanent charge but not for variable
charged soils (pH) adding NH4OAc to a
calcareous soil would result in NH3
volatilization at pH 7, using NH4OAc, NH4 will
not displace all of the Ca in a calcareous soil
(underestimate CEC) at pH 8.2 (or higher),
CaCO3 will not dissolve anymore 2. Gillman
(1979) acid soils (Ba has a higher charge density
than does NH4 (more charge per volume)) Saturation
of exchange sites with BaCl2 (solution of a
concentration approximately equivalent in ionic
strength to the soil solution) Extraction with
MgSO4 to replace Ba with Mg (MgSO4 concentration
is adjusted to achieve an ionic strength
comparable with that of the soil solution) Ba
determined The use of unbuffered solutions
throughout ensures that natural soil pH is not
significantly altered. SCS (has largely
determined benchmark methods simply due to volume
of samples over the years) NH4OAc at pH 7.0
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CEC Problems Presence of CaCO3 and/or CaSO4
(dissolution) and the presence of salt in arid
type soils. Dissolution of CaCO3 and/or CaSO4
will cause Ca to exchange for Mg, K and Na
instead of NH4 replacing all of these. When 1 N
KCl is then added to displace the NH4 (from
NH4OAc) less NH4 is detected in the filtrate than
what should have been present. Variable charge
soils (high content of more difficulty
exchangeable aluminum-hydroxy "cations").
Exchangeable Al and its hydroxy forms are not
readily exchanged with monovalent cation
saturation solutions. This error results in an
underestimation of CEC. The underlying factor
which has caused various researchers to develop
alternative methods for determining CEC was how
to deal with pH dependent charges (pH of the
saturating solution and replacement solution).
This is important considering the pH is a
logarithmic function of H where 10 times as much
H occurs in solution at pH 5 as pH
6. Schollenberger (1936) chose NH4 because NH4
levels were low in soils Ba was not used because
the emission line for Ba is very close to K
(766.5nm) Flame photometers were used from 1950
to 1970 Atomic absorption did not have the
interference (could now use Ba to extract Al)
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Base Saturation Why is it important to know Base
Saturation? Should probably use exchangeable
acidity (K is supplied via the CEC, so should we
be more interested in exchangeable K)If BS is
high (gt70), dont worry about Ca, Mg and KIf BS
is low (30), worry about AlBASE SATURATION
USED in MORPHOLOGYBS of 35 or more at a depth
of 0.75 to 1.25m (Alfisol)BS of lt 35 0.75 to
1.25m (Ultisol)
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BASE SATURATION Reflects the extent of leaching
and weathering of the soil?Could have high BS
and high Na. What does this mean? Drainage? It
is the percentage of total CEC occupied by
cations, Ca, Mg, Na and K, where each is
determined separately from the NH4OAc extract
(Atomic Absorption - interception of radiant
energy) Ca 0.03gMg 0.008gNa 0.021gK 0.014gCa
0.03/0.02 1.5Mg 0.008/0.012 0.66Na
0.021/0.023 0.91K 0.014/0.039
0.36 3.43meq/20g 17.15 meq/100g CEC 20
meq/100g BS 17.15/20 85.85 BS CEC - (H
Al) / CEC remember this is exchangeable H
and Al
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Anion Exchange (Kamprath) Adsorption of anions to
charged sites in hydrous oxide minerals where
the hydrous oxides are amphoteric (have - and
charge depending on pH and therefore have AEC and
CEC). Order of adsorption strength H2PO4- gt SO4
gtNO3- Cl- pH lt 7.0 More in weathered soils
(11) containing hydrous oxides of Fe and Al
(exposed OH groups on the edges of clay minerals)
Soils which have pH dependent charges. Anion
exchange of 43meq/100g at an acidic equilibrium
pH of 4.7. Can a soil have a net positive charge?
(highly acid lt 5.0, FeO, Oxisols) Is H2PO4-
adsorption on soils anion exchange? only
physically adsorbed initially but soon
precipitate as Ca-P in alkaline soils and Fe or
Al-P in acid soils. Can P applications induce S
deficiencies in acid soils? Acid soil S levels
low --gt P exchange for S on exchange complex
(anion exchange) and SO4 can be leached. 90 of
all water soluble bases will be leached as
sulfate (Pearson et al, 1962)
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Kamprath et al. (1956) Increased P concentration
in solution reduced the amounts of SO4 adsorbed
by the soil. Amount of sulfate adsorbed decreased
as the pH of the soil suspension increased (4 to
6). Aylmore et al. (1967) Sulfate adsorption on
clays possessing positive edge charges oxides
of Fe and Al (highly resistant to leaching and
less available for plant growth) Sulfate adsorbed
on kaolinite clay is weakly held and easily
released Fox et al. (1964) Ca(H2PO4)2 best
extracting solution for S AEC negatively
correlated with Base Saturation
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Discussion pH and BS are positively
correlatedWhy would pH and BS be positively
correlated if pH and CEC were not? All are
positively correlated (acid soils would be the
exception) If CEC, Base Saturation, Buffering
Capacity, Hydrogen ion buffering capacity are all
positively correlated, why dont we just use one
procedure for all of them? Are CEC and AEC
negatively correlated?