Cation Exchange Capacity and Base Saturation

Page reviewed: 15/06/2026

Soil particles are electrically charged and can bind ions from the soil solution. The binding is non-specific, meaning that all ions with an opposite charge can bind. The ions are not fixed to a specific site but are exchangeable and can be replaced by other positively charged ions.

Cation exchange capacity

Cations (+) are bound to negatively charged surfaces and anions (−) to positively charged surfaces. The soil’s ability to bind cations to particle surfaces is called cation-exchange capacity (CEC). The corresponding concept for anions is called anion exchange capacity (AEC).

In Sweden, most soils have a substantially higher CEC than AEC. One consequence is that cations are retained more effectively in soils than anions. Examples of exchangeable cations include calcium (2⁺), magnesium (2⁺), potassium (1⁺), ammonium (1⁺), hydrogen (1⁺), and aluminium (3⁺). Examples of anions include sulfate (2⁻), nitrate (1⁻), chloride (1⁻), and phosphate (3⁻). The numbers in parentheses indicate the ion charge.

The amount of bound ions depends on charge

The unit for exchange capacity is cmol_c*kg⁻¹ (centimoles of charge per kg of soil). However, it is often expressed as mmol_c*kg⁻¹ (millimoles of charge per kg of soil), or meq*kg⁻¹. A mole is an SI unit for amount of substance; one mole contains a fixed number of entities (atoms, molecules), equal to Avogadro’s constant (approximately 6.022 × 10²³).

The number of ions that can be bound per unit of surface charge depends on the charge of the ion. A given surface charge can bind twice as many potassium ions as calcium ions because potassium is monovalent while calcium is divalent. For this reason, exchange capacity is expressed in millimoles of charge rather than number of ions.

Clay minerals and humus determine ion-binding capacity

The soil’s capacity to bind ions is primarily associated with the smallest particles, especially clay minerals and humus. Humus particles have a large internal surface area where ion exchange processes can occur. Clay-rich and humus-rich soils therefore generally exhibit high values of both CEC and AEC. Soils rich in precipitated iron and aluminium oxides have particularly high AEC. Organic matter in the humus layer can have a CEC of around 1000 mmol_c*kg⁻¹, while clay minerals can range between 100 and 1500 mmol_c*kg⁻¹.

The maps presented below show the cation exchange capacity (extractant: 1 N NH₄OAc solution buffered to pH 7.0) in the humus layer (O horizon), the eluviated horizon (E horizon), the illuvial horizon (B horizon), the BC horizon (45–55 cm below the soil surface), and the C horizon (55–65 cm below the humus layer), based on the sampling and measurements carried out during 1993–2002.

In the maps, the values are expressed in meq/kg dry matter, which is equivalent to mmolc/kg dry matter.

A more detailed description of how cation exchange capacity has been determined is provided above under the heading “Determination of cation exchange capacity.”

Horizon	Total	Mor	Mull	Peat	Comparison
O	map	map	map	map	maps
B	map	map			maps
BC	map	map			maps
C	map	map	map		maps

The maps presented below show the base saturation (BS_pH=7) in the humus layer (O horizon), the eluviated horizon (E horizon), the illuvial horizon (B horizon), the BC horizon (45–55 cm below the soil surface), and the C horizon (55–65 cm below the humus layer), based on the sampling and measurements carried out during 1993–2002.

Base saturation (BS_pH=7) is expressed as a percentage (%).

A more detailed description of how base saturation (BS_pH=7) has been determined is provided above under the heading “Base saturation.”

Horizon	Total	Mor	Mull	Peat	Comparison
O	map	map	map	map	maps
B	map	map			maps
BC	map	map			maps
C	map	map	map		maps

The maps presented below show the effective base saturation (BS_eff) in the humus layer (O horizon), the eluviated horizon (E horizon), the illuvial horizon (B horizon), the BC horizon (45–55 cm below the soil surface), and the C horizon (55–65 cm below the humus layer), based on the sampling and measurements carried out during 1993–2002.

The effective base saturation (BS_eff) corresponds to the base saturation at the soil’s actual pH and is calculated as follows:

BS_eff = (Ca²⁺ + Mg²⁺ + K⁺ + Na⁺) / (Ca²⁺ + Mg²⁺ + K⁺ + Na⁺ + Al³⁺)

where Ca²⁺, Mg²⁺, K⁺ and Na⁺ are the concentrations of the respective exchangeable base cations (extracted with a 1 N NH₄OAc solution buffered to pH 7.0), and Al³⁺ is the concentration of exchangeable aluminium (extracted with a 1 M KCl solution).

The effective base saturation (BS_eff) is expressed as a percentage (%).

Horizon	Total	Mor	Mull	Peat	Comparison
O	map	map	map	map	maps
B	map	map			maps
BC	map	map			maps
C	map	map	map		maps

Determination of cation exchange capacity

By summing the concentrations (in mmol_c*kg⁻¹) of the exchangeable base cations (Ca²⁺, Mg²⁺, K⁺ and Na⁺) and total acidity (TA), the sample’s cation exchange capacity (CEC) is obtained:

CEC = ∑(Ca²⁺ + Mg²⁺ + K⁺ + Na⁺ + TA)

The magnitude of cation exchange capacity increases with pH. Therefore, CEC can be expressed in two different ways. In the expression above, the concentrations of exchangeable cations and total acidity are determined at pH 7. The resulting value of cation exchange capacity is sometimes referred to as potential cation exchange capacity or CEC_pH7.

The cation exchange capacity at the soil’s actual pH is called effective cation exchange capacity (CEC_eff). This value can be determined using an unbuffered extractant (e.g. NH₄Cl) and is sometimes used when the soil pH is clearly below 7.

An estimate of CEC_eff can be obtained by adding the exchangeable base cations (extracted with 1 N NH₄OAc solution buffered to pH 7.0) to exchangeable aluminium (extracted with 1 M KCl solution):

CEC_eff = ∑(Ca²⁺ + Mg²⁺ + K⁺ + Na⁺ + Al_KCl)

Base saturation

The total base saturation (determined at pH 7.0) is a measure of the proportion of the cation exchange capacity (CEC) occupied by base cations, usually expressed as a percentage (%):