Cations and Cation Exchange Capacity – Queensland

Key Points

Cation exchange capacity (CEC) is a measure of a soil’s ability to hold exchangeable cations. It provides an indication of a soil’s fertility and ability to resist acidification.

CEC is an inherent soil characteristic and is difficult to alter significantly.

Soils with a higher clay and organic matter content tend to have a higher CEC.

Background

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 (Hazleton and Murphy 2007).

What are exchangeable cations?

Clay minerals and organic matter have negatively charged sites on their surfaces which adsorb and hold positively charged ions (cations) by electrostatic force. The number of negatively charged sites determines how many cations the soil is capable of attracting, and is referred to as the soil ‘cation exchange capacity’ or CEC.

The main ions associated with CEC in soils are the exchangeable cations calcium (Ca²⁺), magnesium (Mg²⁺), sodium (Na⁺) and potassium (K⁺). A soil’s CEC is critical to the supply of plant nutrients, because many of these cations are also essential for plant growth. That is why, in general terms, soils with high CECs are considered to be more fertile or potentially more fertile.

Measuring CEC

In most cases, CEC is measured by summing the number of base cations (Ca²⁺, Mg²⁺, Na⁺ and K⁺) present on the soil exchange sites (CEC by bases). However, as soils become more acidic, these cations may be replaced by hydrogen (H⁺), aluminium (Al³⁺) and manganese (Mn²⁺). This ‘exchangable acidity’ must also be taken into consideration when measuring CEC in acidic soils. Methods for measuring CEC and exchangeable cations are described in detail by Rayment and Lyons (2011).

CEC is conventionally expressed in milliequivalents of charge per 100 g of soil or meq/100 g. This is numerically equivalent to centimoles of charge per kilogram of soil (cmol+/kg).

Management Implications

Soil type and CEC

The CEC of soils varies according to clay content, the type of clay present, soil pH and organic matter content. Pure sand has a very low CEC, less than 2 meq/100 g. Clays such as kaolinite have a CEC of about 10 meq/100 g, while illite and smectite have CECs ranging from 25 to 100 meq/100 g. Organic matter has a very high CEC, ranging from 250 to 400 meq/100 g. In most soils, CEC ranges from around 50 in high clay content soils to 1 in pure sands.
Figure 1 illustrates how CEC can change down a soil profile as clay content and organic matter change. In this soil, the sandy surface 10 cm has a CEC of 4.6 meq/100 g because of a high organic content. At 10–30 cm depth, however, organic matter content decreases and the soil also has a low clay content, and hence a low CEC. The CEC increases in the subsoil layers due to an increase in clay content. The dominant clay in this soil is kaolinite, however, so CEC values remain low compared to many clay soils.

Figure 1: Sandy duplex soil, with clay at 40 cm. Note the high CEC of the clay below 40 cm, and the impact of organic matter on the sand’s CEC.

CEC can influence a number of soil characteristics, and knowing your soil’s CEC can be a valuable management tool (table 1). Soils with higher CECs tend to be higher in clay and/or organic matter, have a greater water holding capacity, greater capacity to store and hold cations against leaching, and greater capacity to resist changes to soil pH.

Table 1: Characteristics of low and high CEC soils.

LOW CEC (1 – 10 meq/100 g)
MODERATE TO HIGH CE)

(11 – 50 meq/100 g)

High sand content
Higher clay content

Nutrient leaching like to be a problem
Greater capacity to retain nutrients against leaching

Low water holding capacity
Higher water holding capacity

Low capacity to resist changes to soil pH
Greater capcity to resist changes to soil pH

A soil with a high CEC will generally be more fertile than one with a low CEC. However, it should be noted that once the nutrient reserves or ability of a high CEC soil to resist acidification have been exhausted, high rates of fertiliser or lime will be required to restore soil fertility.

Soil mineralogy and CEC

If clay content is known, and organic matter is low or removed before measuring CEC, CEC values can also be used to infer the type of clay minerals present in the soil (table 2). Clay mineral types can strongly influence many soil properties, such as:

•Their water holding capacity—soil dominant in smectite minerals can hold more water than those dominant in kaolinite.

•Their ability to shrink and swell on wetting and drying—a characteristic of smectitic soils.

•Their fertility—clays containing smectite, vermiculite, illite and mica minerals tend to produce highly fertile soils and have a constant CEC regardless of pH.

Table 2: Relationship between the clay:CEC ratio and the clay mineral composition of a soil (Shaw et al. 1998).

CLAY:CEC RATIO
CLAY MINERAL)

< 0.02
Kaolinite

0.2 – 0.35
Illite and kaolinite

0.35 – 0.55
Mixed clay mineralogies

0.55 – 0.75
Mixed clay mineralogies with a higher proportion of smectite

0.75 – 0.95
Dominantly smectite with the possibility of feldspars

> 0.95
Smectite, plus feldspars or CEC from other than the clay fraction (e.g. organic matter)

Soil pH and CEC

For many soils, CEC remains the same regardless of soil pH. However, some highly weathered soils vary their CEC as pH changes. The ferrosol soils common in the inland Burnett region are an example of this type of soil. As soil pH decreases these soils become positively rather than negatively charged and can start to attract negatively charged anions such as sulphate, phosphate and nitrate. For variable charge soils like these, proper management of pH and the maintenance of soil organic matter is crucial in order to provide sufficient amounts of the nutrient cations. The lower the CEC of a soil, the faster the soil pH will decrease with time. Liming soils, increasing soil organic matter, and ensuring the appropriate use of nitrogen fertilisers can help maintain exchangeable plant nutrient cations in these soils (see Soil Acidity—Queensland fact sheet).

Fact Sheets Cations and Cation Exchange Capacity - Qld