The soil cation exchange capacity (CEC) is the ability of soils to bind and store a particular group of nutrients by electrical attraction, those that form positively charged cations, such as calcium (Ca2+), magnesium (Mg2+), potassium (K+), and nitrogen in the form of ammonium (NH4+).
Soils are composed of a mixture of sand, silt, clay and organic matter. The particles of clay and organic matter in soil carry an electrical charge, they have a net negative charge (-). This allows them to attract and hold positively-charged particles (+), because ‘opposites attract’, much like how opposite poles of a magnet attract each other. They also repel other negatively-charged particles (-), because ‘like repels like’, much like how similar poles of a magnet repel each other.
Soils with a low CEC are not able to bind much nutrient molecules to their particles, while those with a high CEC are able to bind a larger amount of nutrient molecules to the surface of the soil particles.
What Are Anions and Cations?
Chemical elements (such as calcium, magnesium, iron) don’t normally have an electrical charge, but when they lose or gain electrons, they then gain an electrical charge, and are called ions.
- Positively-charged ions (+) are called cations
- Negatively-charged ions (-) are called anions
A great mnemonic to remember which is which, is to substitute the letter ‘t‘ with a ‘+‘ symbol in the word ca+ions.
Some common soil cations (with their chemical symbol and charge) include calcium (Ca2+), magnesium (Mg2+), potassium (K+), ammonium (NH4+), hydrogen (H+) and sodium (Na+)
Some common soil anions (with their chemical symbol and charge) include chlorine (Cl-), nitrate (NO3-), sulfate (S042-) and phosphate (PO43-)
Note: from the list above, we can see that some cations have more than one (+) positive charge, while some anions can have more than one (-) negative charge and also be combined with oxygen.
How Soil Cation Exchange Capacity Works
The cations that are bound to the particles of clay and organic matter in soils can be replaced by other types of cations, they are exchangeable. For example, cations of potassium (K+) can be replaced (exchanged) by cations of calcium (Ca2+) and vice versa.
The soil cation exchange capacity (or CEC) is the total number of cations that a soil can hold, which is its total negative charge.
The higher the CEC, the higher the number of negative charges, and the more cations (soil nutrients with positive charge) such as calcium (Ca2+), magnesium (Mg2+), potassium (K+), ammonium (NH4+) that can be held.
What Is the Measurement Unit for CEC?
Soil CEC is measured in millequivalents per 100 grams of soil (meq/100g) which is a measure of concentration, the amount of something present per unit volume.
One meq is a measure of the number of ions that are required to total up to a quantity of approximately 6 x 1020 positive electrical charges.
The amount of nutrient ions required to make up one meq (6 x 1020 positive charges) decreases as the amount of charge they carry increases.
- Potassium requires 6 x 1020 potassium (K+) ions (the same number) to make up one meq, because each ion only carries one positive charge.
- Calcium requires 3 x 1020 calcium (Ca2+) ions (half as much) to make up one meq, because each ion carries two positive charges.
- Aluminium (which is not a plant nutrient but present in the soil) only requires 2 x 1020 aluminium (Al3+) ions (one-third as much) to make up one meq, because each ion carries three positive charges.
Listed below are common soil nutrient cations, with the amounts (weight per acre) required to equal a concentration of 1 meq/100g:
- Calcium (Ca++): 180 kg/acre (400 lb/acre)
- Magnesium (Mg++): 110 kg/acre (240 lb/acre)
- Potassium (K+): 350 kg/acre (780 lb/acre)
- Ammonium (NH4+): 160 kg/acre (360 lb/acre)
Soil Types and CEC
The CEC of a soil is directly related to the soil’s composition. Soils that have a high sand content have low CEC values. As the clay, silt and organic matter levels of a soil increase, the soils have an increasingly higher CDC.
- Sand particles are the largest particles in the soil, so they have the lowest surface area relative to their volume, and can only provides a limited number of sites where molecules can bind on the relatively small surface area.
- Clay particles are much finer, so for the same volume, there will be much more particles, which can provide a much greater surface area and more molecular binding sites, increasing the soil reactivity, creating a higher CEC.
- Organic matter particles are even finer yet again, providing a greatly increased surface area, much more molecular binding sites, and an even higher CEC.
Since a soil’s CEC comes from its clay and organic matter content, it can be estimated from looking at the soil texture and colour.
Normal Range of CEC Values for Common Colour/Texture Soil Groups
- Soil groups Common CEC in meg/100g
- Light coloured sands……………………..…..…….3-5
- Dark coloured sands…………………………..…10-20
- Light coloured loams and silt loams……………..10-20
- Dark coloured loams and silt loams………………15-25
- Dark coloured silty clay loams and silty clays……30-40
- Organic soils…………………………….………..50-100
Soil pH Stability and Buffer Capacity
In chemistry, a buffer (buffering agent) is a weak acid or weak base (alkali) in aqueous solution (dissolved in water) that has a highly stable pH, so if an acid or a base is added to a buffered solution, its pH will not change significantly.
Similarly, soil also resists changes in pH to maintain stable conditions. The buffer capacity of soil is defined as a soil’s ability to maintain a constant pH level when an acidifier or alkalizer is added to it.
A soil’s buffering capacity (its ability to maintain a stable soil pH) is related to its CEC (cation exchange capacity). How this works is explained below.
Soil CEC and Buffer Capacity of Soils
As previously mentioned, a buffer (buffering agent) which stabilises the soil pH and protects it from extreme changes is either a weak acid or weak base (alkali).
Cations can be classified as either acidic (acid-forming) or basic (alkaline-forming).
- Common acidic cations are hydrogen and aluminium
- Common basic cations are calcium, magnesium, potassium and sodium
Soil particles have exchange sites on their surface which which bind cations. If the cations in the soil water (which are free and not bound) are taken up by plant roots, or lost through leaching, the cations that are bound on the soil’s exchange sites can act as a source that can resupply them back into soil water.
The higher the soil CEC, the more acid-forming and alkaline-forming cations it is able to supply to which carry out the buffering function, and this is referred to as the soil’s buffer capacity.
The Effect of Soil CEC on Herbicides
Many herbicides such as glyphosate, 2,4-D, dicamba, and others are weak acids, they have a hydrogen (H+) ions into their molecular structure. Other herbicides, under certain conditions, also can incorporate hydrogen ions (H+) into their molecular structure. Atrazine for example, is neutral in charge when the soil pH is above 7, but when the soil pH falls below 7, it can pick up hydrogen (H+) ions from the soil solution and take on a positive charge.
Since these positively charged molecules are also cations, they too can be bound to the negatively charged soil particle of organic matter and clay, much the same way that soil nutrients are.
As the soil CEC increases, more herbicide is bound to soil particles, leaving less available in the soil solution that plants can take up to be poisoned by.
As such, many herbicide application rates are also CEC dependent, varying with the type of soil. That is why the labels suggest using lower application rates on coarse-textured (sandy) soils, and higher application rates on fine-textured (clay and silty) soils.
Some herbicides are just not used on soils high in organic matter because the high CEC of organic soils binds the herbicide so tightly that it becomes unavailable and is rendered ineffective.
The other problem with herbicides binding to soil particles is the increased potential for herbicide carryover. Herbicides tied up in the soil are not taken up by plants, and show decreased losses through leaching soil water and volatilisation into the atmosphere.
This means that more of the herbicide is held in reserve in the soil, and depending on the half-life of the herbicide, may possibly be released at a later point in time, to injure susceptible/sensitive crops planted in that soil in the future.
Generally, medium and fine-textured (silty and clay) soils that contain more than 3% organic matter (in other words, rich, fertile soils that can hold the most nutrients, and best support plant growth) have the greatest potential to bind or hold herbicides that can injure future herbicide-sensitive crops. This puts farmers who choose to use toxic herbicides in a quite a dilemma, as the costly ongoing expense of herbicide use for convenience also means greater expenses in chemical fertilisers to artificially supplement poor soils with low nutrient-holding ability, and without a healthy soil ecosystem (which requires organic matter to function), plants are more prone to pests and diseases, requiring further expenditure in pesticides and fungicides.
By understanding the soil science, we can see how it’s easier to build healthy soil, and work with the soil chemistry and ecology to grow healthy plants. That’s what nature has been doing for over 460 million years, it’s evidence-based horticulture, so we know it works.
References
- Purdue University, Cooperative Extension Service, Agronomy Guide AY-238 Soils (Fertility), Fundamentals of Soil Cation Exchange Capacity (CEC) by David B. Mengel <https://www.extension.purdue.edu/extmedia/AY/AY-238.html>
- Michigan State University Extension, What is your soil cation exchange capacity? November 7, 2011 by Ron Goldy, <https://www.canr.msu.edu/news/what_is_your_soil_cation_exchange_capacity>
- Pennsylvania State University Extension, Persistence of Herbicides in Soil, Updated: January 1, 1999 <https://extension.psu.edu/persistence-of-herbicides-in-soil>