MANAJEMEN KESUBURAN TANAH
KAPASITAS TUKAR KATION
&
HARA
TANAMAN
Cation Exchange Capacity (CEC)
Clay Particles and Humus
- affect chemical properties of soil
- complex structures with many negative charge
sites
- negative charge sites attract positive ions
called cations
KTK = CEC
Negative charge sites are referred to as . . .
Cation exchange sites
+ attract cations from soil solution+
KTK = CEC
Force of attraction is called:
Adsorption
similar to force of a magnet holding iron filings
ADSORPSI = JERAPAN KATION
KTK = CEC
Cations can move on and off particles . . .
when one leaves, another replaces it
This process is called cation exchange, and
cations involved are said to be exchangeable
http://www.une.edu.au/~agronomy/SSCATXCH.
dcr
KTK = CEC
The number of sites that a colloid (small
particle) of charged clay or humus (micelles)
contains is measured by the:
Cation Exchange Capacity expressed in
mEq/100g (older unit) or cmolc/kg
KTK = CEC
may range from:
2.0 mEq/100g for sand
to
> 50 mEq/100g for some clays
and
humus 100-300 mEq/100g
under certain soil conditions
KTK = CEC
How fertile can a soil be?
Does applying more fertilizer always provide
more nutrients to plants?
How much of the CEC is actually filled with
cations?
KTK = CEC
The proportion of the CEC occupied by basic
(+) nutrients such as Ca, Mg, K, Na, is called:
Percent Base Saturation and is an indication of
the potential CEC of a given soil
KTK = CEC
Estimations that > 99% of cations in soil solution
are adsorbed . . .
does not mean that percent base saturation is
99%
KTK = CEC
Example:
A soil with CEC of 10 mEq/100g has 6
mEq/100g of bases (Ca, Mg, K, Na) occupying
exchange sites
What is the percent base saturation of the
soil?
KTK = CEC
6 mEq/100g bases
10 mEq/100g sites
= 60 % base saturation
KTK = CEC
Cation Exchange is determined by:
1) strength of adsorption
2) law of mass
KTK = CEC
Strength of adsorption is as follows:
H+ and Al3+ > Ca2+ > Mg2+ > K+ > NH4+ > Na+
KTK = CEC
Law of Mass
the more of one ion available,
the greater the chance of adsorption
KTK = CEC
Cation Exchange Capacity (CEC) is the ability of the soil to hold
onto nutrients and prevent them from leaching beyond the roots.
The more cation exchange capacity a soil has, the more likely the soil
will have a higher fertility level. When combined with other
measures of soil fertility, CEC is a good indicator of soil quality and
productivity.
The cation exchange capacity of a soil is simply a measure of the
quantity of sites on soil surfaces that can retain positively charged
ions by electrostatic forces. Cations retained electrostatically are
easily exchangeable with other cations in the soil solution and are
thus readily available for plant uptake.
Thus, CEC is important for maintaining adequate quantities of plant
available calcium (Ca++), magnesium (Mg++) and potassium (K+) in
soils. Other cations include Al+++( when pH < 5.5) , Na+, and H+.
DIUNDUH DARI: http://www.swac.umn.edu/classes/soil2125/doc/s12ch2.htm ….. 17/9/2012
Cation exchange capacity (CEC)
The capacity of a soil to adsorb and exchange
cations (positively charge ions, Ca2+, Mg2+,
K+, Na+, NH4+ , Al[OH]2 +, Al3+, and H+).
This capacity is due to the net negative
charge of soil colloids (clays and organic
matter)
DIUNDUH DARI: http://www.swac.umn.edu/classes/soil2125/doc/s12ch2.htm ….. 17/9/2012
Cation Exchange Capacity (CEC)
Sources of charge on clays:
1.Ionizeable H+ on edges (pH-dependent, similar to charge on
OM), just as in the case of a weak acid.
2.Isomorphous substitution in clays:
•
Substitution of Al3+ for Si4+ in the tetrahedral layer of
clays
•
Substitution of Mg2+ for Al3+ in the octahedral layer of
clay
•
This type of CEC is often referred to as permanent
charge CEC because it is not affected by pH.
Cation Exchange Capacity (CEC)
Sources of charge on clays:
1. Both ionizable H+ and isomorphous substitution
impart CEC to clays.
2. Total CEC of the soil is dependent upon the
amount of these sources and also upon the surface
area of clays exposed (lower when they clamp shut)
3. See swarm of cations in diffuse double layer
Silicate Clay Types
Amorphous silicate clays (allophane):
1. Mixtures of Al and Si that have not crystallized. May
contain other oxides like Fe.
2. Often present where weathering ins not complete, as in
from volcanic ash (present in Andisols)
3. CEC: variable to high, can have AEC (amphoteric), high
affinity for P
4. Shrink-swell: low
Silicate Clay Types
Kandites (kaolinite, nacrite, halloysite):
1. Kaolinite is most common
2. Secondary mineral formed in soil; prevalent in highlyweathered soils
3. Structure: 1:1 , 0.7 nm spacing
4. Interlayer: hydrogen bonds between sheets (no water or
cations)
5. CEC: low
6. Shrink-swell potential: none
Silicate Clay Types
Smectities (montmorillonite, saponite):
1. Secondary mineral formed in soil; prevalent in less highly
weathered soils
2. Structure: 2:1 , 1-2 nm spacing
3. CEC: very high (highest of all clays)
4. Interlayer: water molecules and miscellaneous cations
5. Shrink-swell potential: very high
Silicate Clay Types
Hydrous mica and illite:
•Poorly defined group, 2:1 clays
•Micas:
1. Primary mineral (Important in igneous and
metamorphic rocks)
2. Structure: 2:1 , 1 nm spacing
3. Interlayer: K+
4. CEC: low
5. Shrink-swell potential: none
Silicate Clay Types
Hydrous mica and illite:
1. Fine-grained micas (formerly called illite):
2. Weathered mica (smaller particle, less interlayer
K+)
3. Structure: 2:1, 1 nm spacing
4. CEC: intermediate
5. Interlayer: K
6. Shrink-swell potential: none
Silicate Clay Types
Vermiculite:
1. Secondary mineral formed in soil; prevalent in
less highly weathered soils
2. Like fine-grained mica but no interlayer K+
3. Structure: 2:1, 1.0 to 1.5 nm spacing
4. CEC: high
5. Interlayer: water molecules and miscellaneous
cations, especially Mg
6. Shrink-swell potential: high, but less than smectite
Silicate Clay Types
Chlorite
1. Secondary mineral formed in soil
2. Structure: 2:1, 1.4 nm spacing
3. CEC: intermediate
4. Interlayer: Mg hydroxide octahedral sheet,
firmly bonded
5. Shrink-swell potential: very low
Silicate Clay Types
Sesquioxides
1. Mixtures of Al, Fe oxides and hydroxides left
after extensive weathering (hot humid soils -
Oxisols, Ultisols)
2. Shrink-well: none
3. CEC: low, amphoteric can have AEC, high
affinity for P
Silicate clays: permanent charge CEC
Vermiculite (High CEC,
Mica (Primary mineral)
1.0
nm
expands/contracts somewhat)
SiO 4
Al(OH) 3
≈1.4
nm
K K K K K K K
Ca Mg H
2
O Ca H
2
O
Smectite (or Montmorillonite
Illite (Med. CEC)
(High CEC, expands/contracts a lot)
≈1.8 to 4.0
1.0
nm
H+ K K K H+ H+
nm
Ca Mg H
Chlorite (Low-Med CEC)
2
O Ca H
Kaolinite
0.72
0.93
nm
nm
H+ bonding
2
O
KATION TUKAR
The replacement of one adsorbed cation for
another from solution.
A simple example: Ca2+ exchange displaces exchangeable Na+
-
..Na+
2+
..Ca
-
[Ca2+]
..Na+
Dissolved in soil solution
Negatively-charged clay
XNa
2
+
+ Ca2+ 
XCa
X = exchangeable
2+
+ 2Na+
[Na+]
[Na+]
Cation Exchange Capacity (CEC)
1. Quantity of exchangeable cations per unit weight of
soil
2. Strongly affect soil solution (through cation
exchange) and are available to plants
3. Units: centimoles of charge refers to charge; so for
example 1 centimole of Ca2+ has 2 centimoles of
charge, whereas one centimole of K+ has 1
centimole of charge.
1.
2.
3.
4.
Cation Exchange
Strength of cation adsorption (lyotropic series):
Na+ < K+ = NH4+ < Mg2+ = Ca2+ < Aln+ < H+
Adsorption depends on charge density (charge/vol), so increases with valence
and decreases with size.
Not all exchangeable ions are Aln+ and H+ because mass action allows the
others to be present; but at equal soil solutoin conc's, this will be the order.
DIUNDUH DARI: http://www.swac.umn.edu/classes/soil2125/doc/s12ch2.htm ….. 17/9/2012
Note: Al3+ is a weak acid and combines with water to form
various ions depending on pH:
pH < 4.5
pH 4.5-6.5 (mostly monovalent form)
pH 6.5-8 (gibbsite)
pH 8-11
3+
2+
+
0
Al(H2O)6 <->Al(H2O)5(OH) <-> Al(H2O)4(OH)2 <-> Al(H2O)3(OH)3 <-> Al(H2O)2(OH)4-
DIUNDUH DARI: http://hubcap.clemson.edu/~blpprt/acid1.html ….. 17/9/2012
AKSI MASA
1.
2.
3.
Displacement of one adsorbed/exchangeable cation by another by competition
for sites when the second has a high number of ions in solution (high
concentration)
This is why fertilization with K, Mg and liming (Ca2+) work - they flood
exchange sites and drive off other even more strongly adsorbed cations (like
H+ and Al).
Also, sodic soils (10-20% exchangeable Na) are cured by gypsum in the same
way.
Diunduh dari: http://www.fao.org/docrep/field/003/AC172E/AC172E05.htm .... 17/9/2012
Ca2+ Displaces Al3+ by Mass Action even though
Al3+ is more strongly absorbed
Ca2
Al3+
Al3+
Al3+
+
2+
2+
Ca
Ca Ca2+
Ca2+
Ca2+
Ca2+
Ca2+
Ca2+ Ca2+
2+
Ca
Al3+ Ca2+Ca2+
2+
Ca
Ca2+
Ca2+
Ca2+
Ca2+
Ca2+
Silicate clays: permanent charge CEC
Vermiculite (High CEC,
Mica (Primary mineral)
1.0
nm
expands/contracts somewhat)
SiO 4
Al(OH) 3
≈1.4
nm
K K K K K K K
Ca Mg H
2
O Ca H
2
O
Smectite (or Montmorillonite
Illite (Med. CEC)
(High CEC, expands/contracts a lot)
≈1.8 to 4.0
1.0
nm
H+ K K K H+ H+
nm
Ca Mg H
Chlorite (Low-Med CEC)
2
O Ca H
Kaolinite
0.72
0.93
nm
nm
H+ bonding
2
O
HUMUS
TANAH
1. Temporary (will ultimately decompose)
2. Nearly insoluble in water, but soluble in base (high pH)
3. Contains 30% each of proteins, lignin, complex sugars 50% C
and O, 5% N
4. Very high CEC on a weight basis
5. Develops a net negative charge due to the dissociation of H+ from
fenolic (-OH), carboxyl (-COOH), and phenolic (
-OH)
groups as pH increases (solution H+ concentration decreases):
pH-dependent CEC on Organic Matter
No charge
CEC and exch. K+ (could be any cation)
R-OH0 + OH- --------> R-O- …K+ + H2O (R stands for one of the
above groups)
This leaves a net negative charge on the organic colloid (R-O-)
which attracts cations just as the net negative charge on an
isomorphously-substituted clay does.
Organic matter is the most important source of pH-dependent
CEC in soils.
Organic matter : pH-dependent CEC
OH
O
-
K +
+ OHOH
Low pH, sites protonated
no CEC
+ H2O
OH
High pH (depronotated,
cation exchange site)
Measurement of Cation Exchange Capacity (CEC) and Base
Saturation (%BS)
1. CEC is measured by applying concentrated ammonium
chloride (NH4Cl) or ammonium acetate (NH4OAc) to the
sample to exchange all exchangeable cations with NH4+
by mass action
2. The extractant solution is analyzed for Ca2+, Mg2+, K+,
Na+, and in some cases Al to determine what was on the
exchanger.
3. At that point, one measure of CEC can be made (see 1
below). Then the NH4+ is displaced by another cation
(typically Na+ or K+ ) by mass action, and NH4+ is then
measured to obtain another estimate of CEC.
Measurement of CEC and %BS
1. The usual assumption is that NH4+ constitutes a negligible
proportion of CEC.
2. Exchangeable NH4+ is often measured separately using
concentrated KCl extractant.
3. H+ (pH) is not measured on this extractant, either;
exchangeable H+ is measured another way.
4. Some soil scientists argue that there is no exchangeable H+
on mineral soils; all H+ that becomes absorbed onto clay
minerals quickly enters the lattice structure and causes
clay decomposition to hydrous oxides.
There are three ways to measure CEC (two from one
method and one from another method):
1. Sum of cations Method:
•
•
•
The sum of Ca2+, Mg2+, K+, Na+, and Al after
extraction with 1M NH4Cl (a neutral salt which does
not buffer pH).
CEC by sum of cations, CECsum, and is measured in the
first extractant in Figure 1.
In a pure clay system (no organic matter Fe, Al hydrous
oxides, of allophane; i.e., no pH-dependent CEC) this
represents CEC and cations on the clay minerals
(permanent charge CEC).
Step 2. Displace exchangeable
NH4+
+
+
with Na or K
Step 1. Displace exchangeable
cations with NH4+
1 M NaCl of KCl
1 M NH 4Cl
Soil Sample
Soil Sample
+
Na +or K
displaces
exchangeable
NH +4
+
NH 4displaces
exchangeable
cations
Extractant
Extractant
2+
Analyze for Ca, K, Mg,
3+ gives
Na, + and Al ; this
exchangeable cations.
Sum of these cations =
CEC sum
-
-- Ca
-- Mg
-- K
-- Na
-- Al
-- H
2+
2+
+
+
3+
+
+ NH
+
4
+
2+
+
Analyze for NH ; this4
gives CEC
eff
-
-- NH
-- NH
-- NH
-- NH
-- NH
-- NH
+
4
+
+4
4+
4
+
4+
4
Extractant
(Exchangeable
Cations,
CEC sum)
Figure 1. Measurement of exchangeable cations and CEC
using neutral salt. (KCl)
+ Na
+
-
-- Na
-- Na
-- Na
-- Na
-- Na
-- Na
Extractant
(CEC eff)
+
+
+
+
+
+
2. Effective CEC (CECeff) at existing soil pH.
1. This includes the permanent charge CEC plus that
portion of pH-dependent CEC that is in effect at
existing soil pH.
2. It is determined from the second extractant in Figure 1,
After the 1M NH4Cl extraction, the soil is washed with
ethanol to remove soluble NH4+ , and then extracted
with 1M NaCl to displace the exchangeable NH4+.
3. The extractant is analyzed for NH4+ .
Step 2. Displace exchangeable
NH4+
+
+
with Na or K
Step 1. Displace exchangeable
cations with NH4+
1 M NaCl of KCl
1 M NH 4Cl
Soil Sample
Soil Sample
+
Na +or K
displaces
exchangeable
NH +4
NH 4+displaces
exchangeable
cations
Extractant
Extractant
2+
Analyze for Ca, K, Mg,
Na,+ and Al ; 3+
this gives
exchangeable cations.
Sum of these cations =
CEC sum
-
-- Ca
-- Mg
-- K
-- Na
-- Al
-- H
2+
2+
+
+
+
3+
+
+ NH 4
+
2+
+
Analyze for NH ; this
4
gives CEC eff
-
-- NH
-- NH
-- NH
-- NH
-- NH
-- NH
+
4
+
+4
4+
4
+
4+
4
+ Na
Extractant
(Exchangeable
Cations,
CEC sum)
Figure 1. Measurement of exchangeable cations and CEC
using neutral salt. (KCl)
+
-
-- Na +
-- Na +
-- Na +
-- Na +
-- Na +
-- Na +
Extractant
(CEC eff )
3. Ammonium acetate CEC (CECOAc).
1. This includes permanent charge CEC + all pHdependent CEC. Is is measured by extracting the
soil with either ammonium acetate (NH4OAc,
buffers pH at 7.0). (Figure 2).
2. Then the same produre is followed as for the
neutral salt CEC.
3. Note: exchangeable Al should be measured
separately because Al precipitates as Al(OH)3 at
high pH
Step 2. Displace exchangeable NH4+
with Na +
Step 1. Displace exchangeable
cations with NH4+
1 M NaCl
1 M NH4 OAc
Buffers pH at 7
Soil Sample
Soil Sample
Na + displaces
exchangeable
+
NH 4
NH 4+displaces
exchangeable
cations
Extractant
Extractant
2+
2+
+
Analyze for Ca, K, Mg,
+
and Na ; this gives
exchangeable cations
except for Al. 3+
-
-- Ca 2+
-- Mg 2+
-- K +
-- Na +
-- Al 3+
-- H +
+
+ NH 4
+
Analyze for NH ; this
4
gives CEC pH 7
-
-- NH
-- NH
-- NH
-- NH
-- NH
-- NH
Extractant
(Exchangeable
Cations)
+
4
+
+4
4
+
4
+
4+
4
+ Na
+
-
-- Na +
-- Na +
-- Na +
-- Na +
-- Na +
-- Na +
Extractant
(CEC)
Figure 2. Measurement of exchangeable cations and CEC buffering pH
at 7 using ammonium acetate.
Figure 3. Types of CEC depend on how it is measured
CECOAc
CECeff
CECsum
Permanent Charge CEC
pH-dependent CEC
CECsum: Measured as the sum of Ca + Mg + K + Na + Al extracted with ammonium
chloride in the first extraction in Figure 1
CECeff: Measured with ammonium chloride, neutral salt, after second extraction in Fig 1
CECOAc: Measured with ammonium acetate at pH 7 in Figure 2
Base Saturation.
Base Cation Saturation Percentage (BCSP)
(often stated as simply base saturation) BCSPis
defined as the sum of exchangeable base cations
(Ca2+, Mg2+, K+, and Na+) divided by CEC. It is
usually expressed as a percentage of CEC thus:
BCSP (%) (or %BS) = Ca + Mg + K + Na
x100
CEC
KEJENUHAN BASA
1. Since CEC can be measured in different ways, BCSP will
vary with the method used, and must be specified.
2. For a soil with a given amount of exchangeable bases, %
Base saturation calculated from CECsum will be greater
than that calculated from CECeff which will be greater
than that calculated from CECtot because more of the
potential acidity on the pH-dependen CEC is counted as
CEC (i.e., CECsum < CECeff < CECtot).
3. The example in Figure 4 shows how this might occur. In
each case, the base cations are the same (6 cmolc kg-1);
only the measure of CEC (the deminator) changes.
Figure 4. BCSP value depends on which CEC measure is used
CECOAc= 10 cmolc kg-1
CECeff = 8 cmolc kg-1
CECsum = 7 cmolc kg-1
Ca2+
Base Cations
+ Mg2+ + K+ + Na+ = 6 cmolc kg-1
Ca2+ + Mg2+ + K+ + Na +
BSCPsum=________________________
CECsum
=
X 100
2+ + Mg2+ + K+ + Na +
Ca
__________________________
Ca2+
+
Mg2+
+
K+
+
Acid cations
Aln+ = 1 cmolc kg-1
H+ = 3 cmolc kg-1
X 100
6
7
=
Na++ Aln+
Ca2+ + Mg2+ + K+ + Na +
BSCPsum=________________________ X 100
CECrff
Ca2+ + Mg2+ + K+ + Na +
BSCPOAc=________________________
CECOAc
=
X 100 =
6
8
X 100 = 85%
X 100 = 75%
6
10
X 100 = 60%
Anion adsorption and retention on soils:
Negatively-charged ions adsorbed on positively-charges
sites.
•In general, anion adsorption is associated with
allophane and the hydrous oxides of Fe and Al in
soils.
•H2PO4- >> SO4-2- >> NO3- > Cl- (the latter being nil
in all but the most sequoixide-rich soils)
•Anion adsorption on these surfaces is highly
dependent upon pH.
•Usually much lower than CEC in temperate, nonvolcanic ash soils.
Allophane, Fe and Al hydrous oxides are
amphoteric
: they take
on different charges depending upon pH.
+
OH 2
Al
Cl -
-
OH
Al
OH
Low pH (protonated,
anion exchange site)
O
K+
Al
OH
Zero Point of Charge
OH
High pH (depronotated,
cation exchange site)
pH
pH is the negative log of the H+ activity = -log (H+);
therefore,
10-pH = (H+) (in moles L-1)
Soil reaction, or pH is taken in a paste of water or
0.01 CaCl2.
The latter gives a lower pH than the former, in most
cases, because the Ca displaces exchangeable H and
Al by mass action.
pH
pH decreases as base saturation decreases (recall
that you must keep the methods constant, that is by
sum, eff, or Oac; the soil in Figure 4 has only one pH
although base saturation value differs by method).
pH has a strong effect on plant growth (Fig 4-10)
and nutrient availability (Fig 4-11)
It not only changes the solubility of many nutrients
(will be reviewed later), but may also cause direct
toxicity (Al, usually) to plant roots.
Buffering capacity:
•Ability of the soil (or whatever else) to resist
changes in pH.
•In soils, this is a function of exchangeable H and Al
in acid soils and carbonates in alkaline soils.
•CEC always plays a major role in buffering.
Potential
Acidity
Buffering
Active
Acidity
•Total acidity on solid phase > 10,000 x that in
soil solution
HARA TANAMAN
There are at least 17 elements recognized as
essential nutrients for plants;
we will recognize 18 elements:
C, H, O, P, K, N, S, Ca, Fe, Mg,
Mn, Mo, Cl, Cu, Zn, B, Co, Ni
HARA TANAMAN
Nutrients grouped into 2 categories according to the
relative amount used by plants:
Macronutrients – major elements; large amounts
Micronutrients – minor elements; small amounts
Both are essential for optimal plant production
PENYERAPAN HARA OLEH BULU AKAR
Diunduh dari: http://www.waldeneffect.org/blog/Cation_exchange_capacity/ ….. 17/9/2012
PERTUKARAN KATION HARA
HARA TANAMAN
Plants obtain
some mineral
nutrients through
ion exchange
between the soil
solution and the
surface of clay
particles.
Diunduh dari: http://bcs.whfreeman.com/thelifewire8e/content/cat_010/3601001.htm?v=chapter&i=36010.01&s=36000&n=00010&o=|01000| ….. 17/9/2012
HARA TANAMAN
Cation exchange in soil. Clay soils
are usually alkaline and bind
positively charged minerals (cations
such as Ca2+).
Hydrogen ions (H+) help make
nutrients available by displacing the
cations.
Plants secreting H+ by cellular
respiration:
CO2 reacts with H2O to form
carbonic acid (H2CO3) in the soil,
which dissociates to add H+ to the
soil.
Diunduh dari: http://bio1903.nicerweb.com/Locked/media/ch37/soil_availability.html ..... 17/9/2012
HARA TANAMAN
Except for C, H, O . . .
- Nitrogen (N) is present in
greatest concentrations;
- Plants respond readily to
Nitrogen (N)
KTK TANAH





In most soils, 99% of soil cations
can be found attached to
micelles (clay particles &
organic matter) and 1% can be
found in solution.
Cations in the soil (mainly Ca++,
Mg++, K+ and Na+) maintain an
equilibrium between adsorption
to the negative sites and
solution in the soil water.
This equilibrium produces
exchanges -- when one cation
detaches from a site (leaving it
free), another cation attaches to
it.
Therefore the negatively
charged sites are called cation
exchange sites.
The total number of sites is the
Cation Exchange Capacity or
CEC
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
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
23/1=23 39/1=39 40/2=20 24/2 = 12
MEQ wt .023 .039 .02 .012
Sample calculation for equivalent
weight for lime or CaCO3
CaCO3 - formula wt. = 40 + 12 + 48 = 100
charges involved = 2
eqwt. = 50
meq = .05 grams
Or one meq of Lime = .05grams
Calculation of CEC with % clay and % OM
Assume Avg CEC for % OM = 200 meq/100g
Assume Avg CEC for % clay = 50 meq/100g
CEC = (% OM x 200) + (% Clay x 50)
From soil data: soil with 2% OM and 10% Clay
200 x .02 + 50 x .1 = 4 + 5 = 9 meq/100 g
Predicting CEC
1) sum of cations : remove all cations
and total the amount
2) NH4+ saturation: soil is saturated with
NH4+ - the NH4+ is replaced by Ca++
and the NH4+ removed is measured.
3) Estimation based on texture:
Sand = 0-3 meq/100 g
LS to SL = 3-10
Loam = 10 - 15
Clay Loam = 15-30
Clay = > 30 (depends on kind of clay)

A high CEC value (>25) is a
good indicator that a soil
has a high clay and/organic
matter content and can
hold a lot of cations.

Soil with a low CEC value
(<5) is a good indication
that a soil is sandy with
little or no organic matter
that cannot hold many
cations.
http://www.spectrumanalytic.com/support/library/ff/CEC_BpH_and_percent_sat.htm
Base Saturation vs pH
÷ CEC x 100
- meq H ÷ CEC x100
% Base Saturation - meq bases
% Hydrogen Saturation
Example: Ap Soil Horizon
Cations-- H+
Ca++ Mg++ K+
Na+
9.4
14
3
0.5
CEC = 27 meq/100g (sum of cations)
0.1
÷ 27 x 100 = 65%
% hydrogen sat = 9.4÷27 x100 = 35%
% base sat = 17.6
pH vs. Base Saturationan approximate relationship
Buffering Capacity

The ability of soil to resist change in
pH.
 The amount of H+ in the soil solution
is small compared with the “H+, Al
+
3 ” adsorbed on the soil colloids
(reserve)

Neutralization (by the addition of
bases) of the solution H+ (H+ is
removed from the system) results in a
rapid replacement of H+ from the
exchangeable H+ on the soil colloid.

CaCO3 when added to soil will
neutralize H+.
 CaCO3 = Lime
(dolomitic = MgCO3 & CaCO3
Sample % BS Problem

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
Calculate the amount of CaCO3 which must be added to an acre
furrow slice of this soil to raise the soil’s base saturation to 90%
SOIL = CEC of 17meq/100g and BS = 32%
(hint = takes 1000 lbs CaCO3/acre to neutralize 1 meq of H+/100 g 90% 32% = 58% change in BS
0.58 x 17 meq/100g = 9.86 meq/100g of H+ to neutralize
or 9.86/100 X 1000 lbs CaCo3/100g = 9860 lbs
OR
9.86 meq x .05g/meq = .493g/100g and
.493/100g is to X / 2,000,000lbs or X = 9860 lbs.
Divided by 2000 lb/ton = 4.9 tons

SOIL = CEC of 27meq/100g and BS = 32%

90-32=58%change in base
or .58x27=15.66 me of H+ to neutralize

15.66x1000lbs=15660/2000lbs/ton= 7.8tons
In the Southeast US, if
fertilizer and lime is applied
to raise the base saturation
of a kaolinitic soil to 85
percent as commonly done
in the Midwest, the resulting
pH would be between 7.1
and 7.5- due to low CEC from
Kaolinite
 Soil pH values in that range
would result in a major
problem with zinc and
manganese deficiency.
 Thus, soils are only limed to
60-70% BS.

Tifton soils formed in loamy
sediments of marine origin.
Cotton, peanuts,soybeans, and
corn are the principal crops
grown on these soils in Georgia

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CEC and Soil Testing:
Because the CEC of a soil is relatively constant unless large
amounts of organic matter are added, it is not measured or reported
with a routine soil test.
Ca : Mg Ratio and Soil Testing
Some soil testing labs will report ideal calcium to magnesium ratios
for plant growth.
However, most plants tolerate a very wide range of soil calcium to
magnesium ratios.
Adjusting the ratios of calcium and magnesium on the exchange
complex by adding gypsum (calcium sulfate) or Epsom salts
(magnesium sulfate) has not been shown to significantly benefit
plant growth.
Gypsum is primarily used as a soil amendment to improve water
penetration and increase the level of calcium in the soil.