Cation Exchange Capacity (CEC)

The surface of clay particles and organic matter are negatively charged and as such are capable of storing and supplying plant nutrients, which are positively charged (i.e. cations). The process of uptake and removal of cations from soil solution by soil mineral particles and/or organic matter is termed cation exchange.

The cation exchange capacity (CEC) of a soil is defined as the total sum of exchangeable cations that it can adsorb at a specific pH. Cation exchange of exchangeable cations in reversible chemical reactions is a quality important in terms of soil fertility and nutritional studies.

The exchangeable cations of most importance in soil are

• Calcium (Ca²⁺),
• Magnesium (Mg²⁺),
• Sodium (Na⁺),
• Potassium (K⁺),
• Hydrogen (H⁺), and
• Aluminium (Al³⁺).

The four most abundant exchangeable cations in soil of humid regions are; hydrogen, calcium, magnesium and potassium. Of these, all except hydrogen are absorbed by plants in large quantities. Soil located in less humid regions (i.e. arid and semi-arid) generally contain little or no exchangeable hydrogen, and often contain large quantities of exchangeable sodium. This is the case in many of the soil profiles found in the irrigated cotton growing areas of south-east Australia.

The effective CEC of soil is calculated by the sum of the five most abundant cations present in soil. This includes the cations of calcium, magnesium, potassium, sodium and aluminium. Under alkaline conditions the sum of these exchangeable cations is approximately equivalent to the CEC.

CEC is expressed in terms of either milliequivalents of adsorbed cations per one-hundred grams soil (me/100g), or centimoles of charge per kilogram (cmol(+)/kg). The former unit is used in older publications. It should be remembered, however, that values presented in either me/100 g or cmol(+)/kg are equivalent.

In terms of soil particle size fractions, sand has virtually no CEC, whereas silt has a small CEC. Clay sized particles having a wide range of CEC values given the wide variety of minerals which are found in this size fraction (i.e. secondary silicate minerals and other secondary minerals). As a result only generalised levels of CEC can be suggested for various soil texture classes.

Soil	Texture	Sands	Fine Sandy Loams	Loams and Silt Loams	Clay Loams	Clays
cmol(+)/kg	CEC	1-5	5-10	5-15	15-30	>30

The capacity of soil to hold the major cations of calcium, magnesium, potassium and sodium (and hydrogen, aluminium and sometimes manganese in acid soil) is widely used for agricultural assessment. This is because it is a measure of the general fertility of soil.

If a soil has a large cation exchange capacity and this is satisfied by cations such as calcium, magnesium, potassium and sodium a large quantity of hydrogen is required to appreciably change the pH and hence the chemical and physical properties. In this case the soil is said to be highly buffered. Similarly, soil colloids largely dominated by hydrogen would require much calcium, magnesium and other base cations to change its characteristics.

The relative degree of weathering of soil can also be inferred from the CEC. This is because the major source of exchangeable cations in soil solution is from mineral weathering (i.e. primary minerals). A low CEC value indicates the disappearance of primary minerals through the process of weathering and the accumulation of secondary clay minerals of low CEC. Kaolinite, gibbsite and free oxides of iron (e.g. haematite) are the dominant clay minerals in highly weathered soil profiles.

Conversely, a higher CEC value is associated with less weathered soil, with primary minerals weathering to slowly release nutrients over a period of years. In this case they act as a plant nutrient reserve. Clay minerals such as illite and smectite characterize soil found in arid and semi-arid landscapes where weathering is slow.

Inferences as to the clay minerals present in soil can therefore be made from the CEC. Generally it has been found that clay minerals exhibit a range of values of CEC as measured by the ammonium acetate method (cmol(+)/kg).

Clay	Mineral	Gibbsite Goethite	Kaolinite	Illite and Chlorite	Smectite	Vermicullite
cmol(+)/kg	CEC	0-4	3-15	10-40	80-150	100-150

The sum of exchangeable calcium, magnesium, potassium and sodium in soil, also provides a rough index of the shrink-swell potential (resilience) of soil. A resilient soil is a soil with the ability to develop a desirable structure by natural processes after destructive forces (such as soil compaction under the wheels of heavy machinery) have been removed.

Soil that is capable of naturally enhancing the development of shrinkage cracks through the process of shrinking when dry and swelling when wet will aid the formation of stable vertical cracks into the soil which will enhance root growth and incorporation of organic matter and water into the subsoil. In addition, the activity of soil fauna such as ants and earthworms will be maintained.

As CEC increases, the soil generally tends to become more structurally resilient. Critical limits for soil resilience with respect to CEC values and appropriate management can be therefore be discerned from CEC. Low CEC values suggest poor shrink-swell potential and high values of CEC indicate good potential.

In general, CEC values less than 20 cmol(+)/kg indicate soil will have a poor shrink-swell potential. However, if the subsoil of a particular soil profile is more resilient than the topsoil, consideration can be given to invert the profile using a moldboard plough.

CEC values between 20 and 40 cmol(+)/kg suggest soil has moderate shrink-swell potential. In general natural shrink swell cycles loosen compacted soil, however, and in order to accelerate the process some form of tillage is required. The use of rotation crops (i.e. fibrous roots) which enhance biopore creation should also be considered.

CEC values greater than 40 cmol(+)/kg have good shrink-swell potential with natural shrink-swell cycles capable of loosening compacted soil horizons. Values of CEC in this range indicate the presence of naturally swelling clays such as smectite.

CEC	Shrink-swell potential	Very Poor	Poor	Moderate		Good
cmol(+)/kg	Structural Resilience	<10	10-20	20-30	30-40	>40