Cation exchange capacity is an inherent soil characteristic and is difficult to alter significantly.
Cation exchange capacity (CEC) 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.
- Cation exchange capacity influences the soil’s ability to hold onto essential nutrients and provides a buffer against soil acidification.
- Soil with a high level of sand content has low holding capacity for cations compared to clayey and silty soils which have negatively charged sites for holding cations.
- Soil with increasing clay content have increasing cation exchange capacity.
- Organic matter has a very high cation exchange.
- Sandy soils rely heavily on the high cation exchange of organic matter for the retention of nutrients in the topsoil.
What are exchangeable cations?
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. However, productive crops and pastures can be grown on low CEC soils.
The main ions associated with cation exchange capacity in soils are the exchangeable cations calcium (Ca2+), magnesium (Mg2+), sodium (Na+) and potassium (K+) and generally referred to as the base cations. In most cases, summing the base cations (‘CEC by bases’) gives an adequate measure of cation exchange. However, as soils become more acidic these cations are replaced by H+, Al3+ and Mn2+, and common methods will produce exchange values much higher than will occur in the field. This ‘exchange acidity’ needs to be included when summing the base cations and this measurement is referred to as effective CEC (ECEC).


Measuring cation exchange capacity
Different laboratories use various methods to measure cation exchange capacity and can return contrasting results depending on the fraction of the soil measured. In Australia, some laboratories measure cation exchange capacity directly and others calculate it by sum of the bases.
Cation exchange capacity is commonly measured on the fine earth fraction (soil particles less than 2 mm in size). In gravel soil, the effective cation exchange capacity of the soil as a whole is diluted by the gravel particles which are not included in the determination of exchange. Reported values for the resultant exchange capacity of the fine earth (< 2 mm) fraction analysed will be higher than actual field values.
Measuring cation exchange capacity 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. Cation exchange capacity is conventionally expressed in meq/100g which is numerically equal to centimoles of charge per kilogram of exchanger (cmol(+)/kg).
Soil type and cation exchange capacity
Cation exchange of soil varies according to the clay percentage, clay mineralogy, soil pH and the amount of organic matter. Pure sand has a very low exchange capacity, less than 2 meq/100g, and the cation exchange capacity of the sand and silt size fractions (2 µm/2 mm) of most soils is negligible. Claying sandy soils to manage water repellence increases the exchange capacity of the surface layers by a small amount depending on type and amount of clay added. Typically, the cation exchange is increased by less than 1 meq/100 g.
The most commonly occurring clay in Western Australian soils, kaolinite, has a cation exchange capacity of approximately 10 meq/100g. Other clays such as illite and smectite range from 25 to 100 meq/100g.
Organic matter has a very high cation exchange capacity ranging from 250 to 400 meq/100g but is generally present in relatively low concentrations in Australia soil. Because a higher soil cation exchange capacity usually indicates higher clay and organic matter content, soils generally have greater water holding capacity than low cation exchange capacity soils.
Indicative cation exchange capacity for different soil textures
Soil texture | Cation exchange capacity (meq/100g) |
---|---|
Sand | 1-5 |
Sandy loam | 2-15 |
Silt loam | 10-25 |
Clay loam/Silty clay loam | 15-35 |
Clay | 25-100 |
Organic matter | 40-200 |
Humified organic matter | 250-400 |
In gravel soil, the gravel and soil fractions also contain positively charged surfaces. Unlike most soil types, gravel soil possesses anion exchange capacity in addition to cation exchange capacity, adding complexity to how nutrient interactions are controlled. These positively charged surfaces, can slow nitrogen leaching by retaining nitrate on surfaces for longer (or in greater volume).

Ion exchange capacity of gravel soil
From Soil Quality: 9 Gravel Soil (Brailsford et al. 2023). Animation: Science with Style.
Soil pH and cation exchange capacity
Soils dominated by clays with variable surface charge 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.
Soil pH change can also be caused by natural processes such as decomposition of organic matter and leaching of cations. The lower the cation exchange capacity of a soil, the faster the soil pH will decrease with time. Liming soils to higher than pHCaCl2 5 will maintain exchangeable plant nutrient cations.
Soil nutrient availability and cation exchange capacity
Soils with a low cation exchange capacity are more likely to develop deficiencies in potassium, magnesium and other cations while high exchange capacity soils are less susceptible to leaching of these cations. The addition of organic matter will increase the cation exchange capacity of a soil but requires many years (decades) to take effect.
PAGE REFERENCES AND ACKNOWLEDGEMENTS
Material on this page adapted from:
- Hoyle FC (2007). Soil Health Knowledge Bank.
- Cations and Cation Exchange Capacity | Fact Sheets | soilquality.org.au
- Soil Quality ebook series. SoilsWest, Perth, Western Australia.
Last updated July 2024.