Cation Exchange Capacity (CEC) is the ability of soil to hold positively charged ions (cations) essential for plant nutrition, influenced by factors such as clay content, soil pH, and organic matter. Higher CEC indicates more clay and organic material, promoting soil fertility by retaining essential nutrients. Additionally, Anion Exchange Capacity (AEC) represents the capacity to hold negatively charged ions (anions) and is also dependent on soil pH.

1. CATION EXCHANGE
CAPACITY (CEC)
Dr. Kiran Karthik Raj
Assistant Professor
COA,Vellayani

2. 2
Definition
 Cations are positively charged ions such as calcium (Ca2+
), magnesium
(Mg2+
), and potassium (K+
), sodium (Na+
) hydrogen (H+
), aluminum (Al3+), iron
(Fe2+
), manganese (Mn2+
), zinc (Zn2+
) and copper (Cu2+
).
 Cation exchange capacity (CEC) is the capacity of a soil to hold exchangeable
cations (hold positively charged ions) on its surface.
 Cations are held by the negatively charged clay and organic matter
particles in the soil through electrostatic forces (negative soil particles attract
the positive cations).
Units
 CEC is conventionally expressed in meq/100 g which is numerically equal to
centimoles of positive charge per kilogram of exchanger (cmol(+)/kg).
 The CEC of soils varies according the clay %, the type of clay, soil pH and
amount of organic matter.

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Why clay particles are negatively
charged ?
 pH Independent Charge / Isomorphous substitution: Net negative
charge because of the substitution of silica (Si4+
) by aluminum (Al3+)
in the silicon tetrahedral sheet of mineral structure of the clay.
 The negative charges associated with isomorphous substitution are
considered permanent, that is, the charges do not change with pH
changes.
 the negative charge of the clay particles is balanced by the
positive charge of the cations in the soil.
 pH Dependent Charge: Depends on the soil pH, de-protonation of
surface hydroxyl groups. CEC associated with soil organic matter is
called pH-dependent CEC. This means that the actual CEC of the
soil will depend on the pH of the soil.

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Cation exchange capacity (CEC)
• It influences the soil’s ability to hold onto essential
nutrients and provides a buffer against soil acidification.
• Soils with a higher clay fraction tend to have a higher CEC.
• Organic matter has a very high CEC.
• Sandy soils rely heavily on the high CEC of organic matter
for the retention of nutrients in the topsoil.
• It influences the soil’s ability to hold onto essential
nutrients and provides a buffer against soil acidification.
• Soils with a higher clay fraction tend to have a higher CEC.
• Organic matter has a very high CEC.
• Sandy soils rely heavily on the high CEC of organic matter
for the retention of nutrients in the topsoil.
• It influences the soil’s ability to hold onto essential
nutrients and provides a buffer against soil acidification.
• Soils with a higher clay fraction tend to have a higher CEC.
• Organic matter has a very high CEC.
• Sandy soils rely heavily on the high CEC of organic matter
for the retention of nutrients in the topsoil.

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Importance
 The higher the CEC the more clay or organic matter present in the soil.
 The clay mineral and organic matter components of soil have negatively
charged sites on their surfaces which adsorb and hold positively charged
ions (cations) by electrostatic force.
 This electrical charge is critical to the supply of nutrients to plants because
many nutrients exist as cations (e.g. magnesium, potassium and calcium).
 In general terms, soils with large quantities of negative charge are more
fertile because they retain more cations
 Acid soil: Exchange acidity due to exchangeable H+
, Al3+
and Mn2+
 The lower the CEC of a soil, the faster the soil pH will decrease with time.
Liming soils to higher than pH 6 will maintain exchangeable basic cations.

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Importance
 Soils with a low CEC are more likely to develop deficiencies of calcium,
magnesium and potassium.
 The higher the CEC, the larger the quantity of lime that must be
added to increase the soil pH; sandy soils need less lime than clay
soils to increase the pH to desired levels.

7. ANION EXCHANGE
CAPACITY (AEC)

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Definition
 Sum total of exchangeable anions that a soil can adsorb.
 In addition to predominantly negative charge sites which attract soil
cations, all soil clays and organic matter simultaneously have a relatively
small number of positive charge sites which retain anions in dynamic
equilibrium with the soil solution.
 Unit: milliequivalents per 100 grams of soil (meq /100 g soil) or
centimoles of negative charge per kilogram soil (cmol(-)/kg).
 Anion exchange capacity (AEC) represents the positive charge
available to attract anions in solution.
 In most soils CEC > AEC.

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Reason
 Soils that have an anion exchange capacity typically contain weathered
kaolin minerals, iron and aluminum oxides, and amorphous materials.
 Anion exchange capacity is dependent upon the pH of the soil and
increases as the pH of the soil decreases.
 Phosphorus, Sulphate and borate are held on anion exchange site.
 Govern fixation of phosphorus in soil.

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pH Dependent Charge