Cation Exchange Capacity (CEC)

Capacity of a soil to exchange cations that a soil can absorb. Roots can exchange cations with the water around it. If a soil has a high CEC it prevents nutrients from being leached away from roots. Closely related to soil fertility.

1 octa to 1 tetra 1:1 ratio, hydrogen bonds in interlayer space (very strong bonds), Non-expandable bc of strong bonds. Low CEC bc of small surface area due to sheets of tetra octa bonds being large. Nutrient Cations include: Mg, Ca.

1 octa to 2 tetra 1:2 ratio, weathering product thats always negative charged and expandable (shrink swell) clays which are highly present in vertisols. weakly held together layers leading to more surface area and high CEC.

Ultimate weathering product found in Ultisols and Oxisols. Very stable and can exist indefinitely in pedagogic time scale. Low CEC so don't expand and the soil does not have a negative charge meaning cations are not retained.

Effect of CEC on soil

Low CEC means high sand and low clay soils that do not resist changes in pH or chemical changes, and they have a low water holding capacity. High CEC usually means the opposite of this.

Extremely low base saturation making it toxic to plants.

Resistance of soils to changes in pH of the soil solution. Increases with the amount of clay and OM present.

Adsorbed by the soil and clings to it, plants absorb these and take them in.

Alkali (base) cations

Ca, Mg, Na and K. Make the soil more basic

Hg, Al and NH4. Make the soil more acidic.

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CEC Quiz

Question	Answer
Cation Exchange Capacity (CEC)	Capacity of a soil to exchange cations that a soil can absorb. Roots can exchange cations with the water around it. If a soil has a high CEC it prevents nutrients from being leached away from roots. Closely related to soil fertility.
What are types of cations
adsorb	is stuck on
absorb	is taken into
Kaolinite	1 octa to 1 tetra 1:1 ratio, hydrogen bonds in interlayer space (very strong bonds), Non-expandable bc of strong bonds. Low CEC bc of small surface area due to sheets of tetra octa bonds being large. Nutrient Cations include: Mg, Ca.
Smectite	1 octa to 2 tetra 1:2 ratio, weathering product thats always negative charged and expandable (shrink swell) clays which are highly present in vertisols. weakly held together layers leading to more surface area and high CEC.
Sesquioxides	Ultimate weathering product found in Ultisols and Oxisols. Very stable and can exist indefinitely in pedagogic time scale. Low CEC so don't expand and the soil does not have a negative charge meaning cations are not retained.
Effect of CEC on soil	Low CEC means high sand and low clay soils that do not resist changes in pH or chemical changes, and they have a low water holding capacity. High CEC usually means the opposite of this.
Phytotoxicity	Extremely low base saturation making it toxic to plants.
Buffering Capacity	Resistance of soils to changes in pH of the soil solution. Increases with the amount of clay and OM present.
Cations	Adsorbed by the soil and clings to it, plants absorb these and take them in.
Alkali (base) cations	Ca, Mg, Na and K. Make the soil more basic
Acidic Cations	Hg, Al and NH4. Make the soil more acidic.
Primary Minerals
Secondary minerals
inorganic colloids
colloids
Flocculation
Sesquioxide/Oxide clays
Silicate clays (crystalline)

Created by: user-1870303

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