Download Soil pH and Cation Exchange

Soil Chemistry
Colloids
Clay minerals and humus and complexes
 The most chemically active part of soil

Large surface area
 Electrical charge (usually net negative)

 Some

+, some –
Nutrient cations (+ions) and anions (- ions)
are held on colloid surfaces, in reserve for
plants
Why are colloids negative?
1.Clay:

Oxygen ions along edges of micelles
2. humus:
H+ ions tend to migrate away from
organic compounds in humus, to soil
solution, leaving net negative charge (OH-)
Ion Exchange
What is the exchange?
exchange of ions on soil surfaces with ions
in soil solution.
Cations and anions are involved
Where does exchange take
place?

Organic colloids (humus) and inorganic
micelles (clays) and complexes of both

Where do ions in soil come from?
 Release
from organic matter
 Rain
 Weathering
of parent material
Leaching

ions (on soil surfaces) cannot be removed
by leaching.

ions (in solution)
can be removed by leaching.

When soil is dried…
…the ions on soil surfaces STAY ON
adsorption sites
…the soluble ions (in soil solution) precipitate
or crystallize (come out of solution) as salts.
Examples of soluble cations precipitating
Ion exchange
Exchangeable ions on soil surface trading
places with ions in solution.
On soil surfaces, there are
exchangeable and nonexchangeable
ions
Exchangeable:
weakly held,
in contact with soil solution,
ready for quick replacement,
available for plants
“outer sphere complex”
Nonexchangeable:
adsorbed by strong
bonds or held in
inaccessible places



(e.g., the K+ between
layers of illite)
“inner sphere complex”
not part of ion
exchange !
Cation exchange capacity (CEC)
Sum total of exchangeable cations that a
soil can adsorb.
If a soil has a high CEC,
it prevents nutrients from
being leached away from roots
CEC
Expressed in:
milliequivalents per 100 g (meq/100g)
Dynamic equilibrium
Strive for equivalent proportions of solution
and exchangeable ions.
Upset equilibrium by:
removal by plants
leaching
fertilization
weathering
Initiate ion exchange
Ion exchange example
Add K fertilizer
K+
Ca+2
+
Ca+2
K+
Ca+2
K+
Ca+2
+
K+
K+
K+
K+
K+
exchangeable
solution
exchangeable
solution
Rules of ion exchange

Process is Reversible

Charge by charge basis

Ratio Law:

ratio of exchangeable cations will be
same as ratio of solution cations
K+
Ca+2
+
Ca+2
K+
Ca+2
K+
Ca+2
+
K+
K+
K+
K+
K+
1 Ca : 2 K
1 Ca : 2 K
Same ratio
Energy of adsorption
The more strongly a cation is attracted to the
exchange surface, the greater the chance
of adsorption.
Depends on:
1. charge
2. hydrated radius
Radius
Unit
Nonnm
hydrated
Hydrated nm
Na+
K+
Mg2+
Ca2+
Al3+
0.095
0.133
0.066
0.099
0.050
0.360
0.330
0.430
0.410
0.480
Strong --------------------------------------Weak
Al+3 > Ca+2 > Mg+2 > [K+ = NH4+ ] > Na+ > H+

The less tightly held (lower energy of
adsorption) ions are the ones furthest from
the soil surfaces and can be leached more
easily and are further down the soil profile.

The strongly held ones are closer to the
soil particle surfaces and tend to move
more slowly down profile
How do plants get nutrients?
Nutrients on the colloids are kept within
root zone of plants.
 H+ from roots exchange with cations on a
charge-by-charge basis

Two videos:

CEC

CEC
Treating a sodic soil

Sodic: too much sodium (Na)

Add gypsum (CaSO4) : increases calcium
concentration; Ca is adsorbed at expense
of sodium
Al+3 > Ca+2 > Mg+2 > [K+ = NH4+ ] > Na+ > H+
Base saturation
% of exchange sites occupied by basic
cations (cations other than H+ and Al+3)
Base saturation
+ H+/Al ion saturation
should equal 100%
Base saturation and pH
relationship
(for midwest US soils)
Notice neutral pH (7.0)
requires a base sat
of 80%.
Soil pH
Soil pH importance

Determines solubility
of nutrients


Before plants can get
nutrients, they must be
dissolved in soil
solution
Microbial activity also
depends on pH
pH
negative log of the hydrogen ion concentration
(also a measure of OH- concentration)
If H+ concentration > OH- : acidic
If OH- > H+ : basic
Soil pH is pH of solution, NOT exchange complex
General soil pH conditions:
“Slightly acid”
6.0 – 6.6
“Slightly basic”
7.4 – 8.0
“Moderately acid”
5.0 – 6.0
“Moderately basic”
8.0 – 9.0
“Strongly acid”
< 5.0
“Strongly basic”
> 9.0
In soil, both H+ and Al+3 ions produce acidity
Al+3 produces H+ ions when it reacts with
water.
(when pH below 6: Al+3 is the cause of acidity)
Causes of soil basicity
1.
2.
Hydrolysis of basic cations
Hydrolysis of carbonates
1. Hydrolysis of basic cations:
(especially Ca+2, Mg+2, K+, NH4+, Na+)
(also called exchangeable bases)
Extent to which exchangeable bases will hydrolyze
depends on ability to compete with H+ ions for
exchange sites.
Na
Na
Na
Na
Na
Na
+
Na
H2O
H
Na
Na
+
Na
+ OH-
K+ and Na+ are weakly held compared to
Ca+2 and Mg+2.
 Recall
energy of adsorption
So, K+ and Na+ are hydrolyzed easily and
yield higher pHs .
2. Hydrolysis of carbonates
(especially CaCO3, MgCO3, Na2CO3)
•
As long as there are carbonates in the soil,
carbonate hydrolysis controls pH.
•
•
Calcareous soils remain alkaline because H+ ions
combine with OH- to form H2O.
For those soils to become acid, all carbonates
must be leached.
•
Basic cations replaced by Al+3 and H+
CaCO3 + H2O
Na2CO3 + H2O
Ca+2 + HCO3- + OHNa + HCO3- + OH- (higher pH because Na more soluble)
Causes of soil acidity
1.
2.
Accumulation of soluble acids
Exchangeable acids (Al+3, H+)
1. Accumulation of soluble acids
at faster rate than they can be neutralized or
removed
a.
Carbonic acid
(respiration and atmospheric CO2)
b. Mineralization of organic matter
(produces organic, nitric, sulfuric acids)
Precipitation increases both a and b
2. Exchangeable acids
Dissociation of exchangeable H+ or Al+3
Al+3 ties up OH- from water, releases an
equivalent amount of H+ ions.
Al+3 + H2O
AlOH+2 + H+
CEC and pH
Only 2:1 silicate clays do not have pH-dependent CECs.
Others are pH-dependent:
1:1 kaolinite:
low pH: low CEC
high pH: high CEC
Oxidic clays
pH dependence of humus