• Cation exchange capacity (CEC) is the total capacity of a soil to hold positively charged exchangeable cations.
• CEC is an inherent soil characteristic and is difficult to alter significantly.
• CEC 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.

Cation exchange capacity is a measure of the soil’s ability to hold positively charged ions. It is a very important soil property influencing soil structure stability, nutrient availability, soil pH and the soil’s reaction to fertilisers and other ameliorants (Hazleton and Murphy 2007). CEC is an inherent characteristic of the soil, which means that it is largely outside a farmer’s control. It is strongly related to soil texture, the type of minerals in the soil as well as organic matter content and is mostly determined by the parent material of 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 (McKenzie et al. 2004) however, productive crops and pastures can be grown on low CEC soils.

The main ions associated with CEC in soils are the exchangeable cations calcium (Ca2+), magnesium (Mg2+), sodium (Na+) and potassium (K+) (Rayment and Higginson 1992) and are generally referred to as the base cations. In most cases, summing the analysed base cations gives an adequate measure of CEC (“CEC by bases”). However, as soils become more acidic these cations are replaced by H+, Al3+ and Mn2+, and common methods will produce CEC values much higher than what occurs in the field (McKenzie et al. 2004). This “exchange acidity” needs to be included when summing the base cations and this measurement is referred to as effective CEC (ECEC or CECe).

Different laboratories use various methods to measure CEC and can return contrasting results depending on the fraction of the soil measured. Some laboratories measure CEC directly and others calculate it as CEC by bases. Cation exchange capacity is commonly measured on the fine earth fraction (soil particles less than 2 mm in size). Measuring CEC involves washing the soil to remove excess salts and using an “index ion” to determine the total positive charge in relation to original soil mass. This involves bringing the soil to a predetermined pH before analysis. Methods, including pre-treatment, for measuring CEC and exchangeable cations are presented by Rengasamy and Churchman (1999) and described in detail by Rayment and Higginson (1992). It is recommended to ask the lab about the method used to determine CEC. The above method is considered the most accurate.

CEC is conventionally expressed in meq/100 g (Rengasamy and Churchman 1999) which is numerically equal to mg/kg or centimoles of charge per kilogram of exchange (cmol(+)/ kg). Conversion calculators can be used to assist with conversion from one unit of measurement to another (e.g. https://www.yaracanada. ca/crop-nutrition/tools-and-services/conversion-calculator/).

Soil type and CEC

The CEC of soils varies according to the clay %, the type of clay, soil pH and amount of organic matter. Pure sand has a very low CEC, less than 2 meq/100 g, and the CEC of the sand and silt size fractions of most soils is negligible. Clay has a great capacity to attract and hold cations because of its chemical structure. However, the different clay types occurring in Tasmanian soils have different CECs (Table 9). It is highest in smectite (e.g. montmorillonite) clay, found in black soils. It is lowest in heavily weathered kaolinite clay, found in Ferrosols and slightly higher in the less weathered illite clay. Humus, the end product of decomposed organic matter, has the highest CEC value because organic matter colloids have a large surface area and large quantities of negative charges. Humus has a CEC two to five times greater than montmorillonite clay and up to 30 times greater than kaolinite clay, so is very important in improving soil fertility. A higher CEC usually indicates more clay and organic matter is present in the soil and so high CEC soils generally have greater water holding capacity than low CEC soils. The addition of organic matter will increase the CEC of a soil, but this requires many years to take effect. Biochar also has a high CEC like humus, but raising CEC by adding biochar may not be economically viable in many production systems.

A generalised range of CEC by soil group is presented in Table 10.

Soils with a low CEC are more likely to develop deficiencies in potassium, magnesium and other cations while high CEC soils are less susceptible to leaching of these cations (CUCE 2007). A CEC above 10 meq/100g is preferred for plant production.

Soils with very low CEC will show relatively little residual effect from applications of potassium and magnesium fertiliser. This is because the K+ and Mg2+ ions are not buffered against leaching by adsorption on soil surfaces. Consequently, application of K fertiliser on sandy soils with low CEC should be split into multiple applications over the year to ensure a ready supply of K for plant growth.

Please refer to the comments on use of the base cation saturation ratio concept (BCSR) in Chapter 1.

The five major exchangeable cations are also shown in soil test results as percentages of CEC. The desirable ranges for them are (Blaesing 2017): calcium 65–80% of CEC, magnesium 10–20%, potassium 3–8%, sodium 0–1% and aluminium <1%. When Mg > 20% of the cations, it may cause an induced potassium deficiency. A sodium percentage of CEC > 6% indicates that the soil is sodic which is likely to result in dispersive soil that is difficult to manage. High Mg saturation can also lead to dispersive soils which are sticky when wet and hard setting when dry.

For plant nutrition, rather than the cation ratios, it is more critical whether the net amount of a cation

in the soil is adequate for plant growth. Calcium levels should be 6.0 – 7.5 meq/100g with levels less than 5 meq/100g indicating deficiency. Magnesium should be 1.6 – 2.0 meq/100g with levels less than 1 meq/100g indicating deficiency. Potassium should be 0.5 – 0.7 meq/100g. Sodium levels should be < 80 meq/100g. Plant testing provides good information on whether plant uptake and cation balances in plants are adequate.

Soils dominated by clays with variable surface charge (Ferrosols) are typically strongly weathered. The fertility of these soils decreases with decreasing pH which can be induced by acidifying nitrogen fertiliser, nitrate leaching and by clearing and agricultural practices (McKenzie et al. 2004). Soil pH change can also be caused by natural processes such as decomposition of organic matter and leaching of cations. The lower the CEC of a soil, the faster the soil pH will decrease with time. Liming soils to higher than pH 5.5water will maintain exchangeable plant nutrient cations.