Fundamentals Of Understanding Cec (cation Exchange Capacity) Of Your Soil

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What are Cation Exchange and Cation Exchange Capacity?
Soil clay minerals and organic matter tend to be negatively charged, thus attracting positively charged ions (cations) on their surfaces by electrostatic forces. As a result, the cations remain within the soil root zone and are not easily lost through leaching. The adsorbed cations may easily exchange with other cations in the soil solution, hence the term "cation exchange." The adsorbed cations replenish the ions in the soil solution when concentrations decrease due to uptake by plant roots.

Cation exchange capacity (CEC) is a measure of the total negative charges within the soil that adsorb plant nutrient cations such as calcium (Ca2+), magnesium (Mg2+) and potassium (K+). As such, the CEC is a property of a soil that describes its capacity to supply nutrient cations to the soil solution for plant uptake. Figure 1 illustrates cations retained on soil clay minerals that can exchange with those in the soil solution. Plant roots can remove nutrients from the soil solution, which results in nutrients moving away from the clay particles. Addition of fertilizer to soil causes an initial increase in nutrient concentration in the soil solution, which results in nutrients moving toward clay particles.

The nutrient cations plants use in the largest amounts are potassium (K+), calcium (Ca2+) and magnesium (Mg2+). Other cations adsorbed on exchange sites are ammonium (NH4+), sodium (Na+), hydrogen (H+), aluminum (Al3+), iron (Fe2+ or Fe3+), manganese (Mn2+), copper (Cu2+) and zinc (Zn2+). Micronutrient cations such as zinc, copper, iron and manganese are typically present at very low concentrations in soils. Ammonium concentrations are also typically very low because microorganisms convert ammonium to nitrate in a process called nitrification.

How CEC changes with Soil pH
Figure1.JPG
Figure 1. Schematic diagram showing exchange of cations between the soil surfaces and the soil solution, and the movement of these cations from soil solution to roots (rhizosphere) for uptake.
The CEC of soil organic matter and some clay minerals varies with pH. Generally, the CEC is lowest at soil pHs of 3.5 to 4.0 and increases as the pH is increased by liming an acid soil, as shown in Figure 2. Because CEC may vary considerably with soil pH, it is a common practice to measure a soil's CEC at a pH of 7.0. Also note that some positive charges may occur on specific soil mineral surfaces at low pH. These positive charges retain anions (negatively charged ions) such as chloride (Cl-) and sulfate (SO42-).

Calculating the Cation Exchange Capacity from a Routine Soil Test
The CEC value included on typical soil testing laboratory reports is calculated by adding together the concentrations (expressed as milliequivalents of charge per 100 grams of soil) of potassium, magnesium, calcium, sodium and hydrogen, which are extracted from soils using an appropriate extraction method. The University of Georgia Soil Testing Laboratory uses the Mehlich I procedure, based on a double acid (0.05 N HCl + 0.025 N H2SO4) extracting solution. This method is appropriate for acidic, low CEC soils, which are commonly found in Georgia. The CEC of soils containing large amounts of clay or organic matter, or that are alkaline, cannot be satisfactorily analyzed using the Mehlich I extract. Other soil extraction methods should be used on these types of soils.

Figure2.JPG
Figure 2. Influence of pH on the surface charge of soil and its components.

Typical CEC Values in Soils
In most soil reports, CEC is expressed as milliequivalents (meq) of charge (number of charges) per 100 grams of soil (meq/100 g or as cmol/kg when using International Scientific Units). The number of milliequivalents is used rather than a weight (pounds, grams, etc.) of adsorbed cations because CEC represents the total number of charges, which is a better standard of comparison of different soils because each cation species has a different weight and soils differ in the proportions of the different cation species.

Table 1 shows the typical cation exchange capacities of soil clay minerals and soils of various textures. Because soil is a mixture of different particle sizes (sand, silt and clay), clay mineral types and organic matter in various proportions, the dominant components and soil pH dictates the soil’s CEC.

Table 1. Cation exchange capacities at pH 7.0 of different soil types, textures and soil organic matter.
Soil and Soil Components CEC (meq/100 g)
Clay Type

Kaolinite 3-15
Illite 15-40
Montmorillonite 80-100
Soil Texture
Sand 1-5
Fine Sandy Loam 5-10
Loam 5-15
Clay Loam 15-30
Clay >30
Organic Matter 200-400
CEC and Fertility Characteristics of Georgia Soils
For practical purposes, Georgia soils are grouped into four major categories: (1) Coastal Plain, (2) Piedmont, (3) Mountain and Limestone Valley and (4) soils from landscapes, golf greens, greenhouses and flower beds. These categories make for easier evaluation of fertility. The chart below describes the CEC and general fertility characteristics of each soil group (Soil Test Handbook for Georgia, 2008).

Soil Group CEC and Soil Fertility Characteristics
Coastal Plain (includes Atlantic Flatwoods and Sand Hills) Soils have sandy surfaces and a CEC of 6 meq/100 g or less. Soils in their native state can be acid and infertile. Soils will vary in clay content, drainage characteristics and color. Soils will vary in productivity, ease of handling and adaptation to row crop production. Typical soil types are Norfolk, Lakeland, Lynchburg and Tifton.
Piedmont soils Soils are predominately upland, well-drained red soils with a CEC of 6 to 12 meq/100 g. Soils in their native state are acid and low in phosphorus but higher in potassium than the Coastal Plain soils. Major soil series are Cecil, Madison and Davison.
Mountain and Limestone Valley soils Soils may have a gray, sandy surface underlain with a heavy red sandy clay or clay texture soil. The alluvial terraces and river bottoms are gray to light brown in color with yellow to dark red sandy clay loam subsoil. Soils are acid and low in fertility. The average CEC value of these soils is 9 meq/100 g. The major soil types are Porters, Hayesville, Talladega, Fannin, Congaree, Clarkesville, Fullerton, Dewey and Decatur.
Soils from landscapes and golf greens These soils are frequently maintained differently than crop soils and are usually artificially constituted and maintained. Many are erratic in fertility and cannot be easily placed in one of the three categories given above.
Percent Base Saturation
Percent base saturation (BS) is the percentage of the CEC occupied by the basic cations Ca2+, Mg2+ and K+. Basic cations are distinguished from the acid cations H+ and Al3+. At an approximate soil pH 5.4 or less, Al3+ is present in a significantly high concentration that hinders growth of most plant species, and the lower the soil pH, the greater the amount of toxic Al3+. Therefore, soils with a high percent base saturation are generally more fertile because:

  1. They have little or no acid cation Al3+ that is toxic to plant growth.
  2. Soils with high percent base saturation have a higher pH; therefore, they are more buffered against acid cations from plant roots and soil processes that acidify the soil (nitrification, acid rain, etc.).
  3. They contain greater amounts of the essential plant nutrient cations K+, Ca2+ and Mg2+ for use by plants.
The percentage base saturation is expressed as follows:

%BS = [(Ca2+ + Mg2+ + K+)/CEC] × 100

Depending on soil pH, the soil's base saturation may be a fraction of CEC or approximately equal to CEC. In general, if the soil pH is below 7, the base saturation is less than CEC. At pH 7 or higher, soil clay mineral and organic matter surfaces are occupied by basic cations, and thus, base saturation is equal to CEC. Figure 2 illustrates the relative amount of cations retained on soil surfaces at various soil pH levels.

Significance of CEC and BS
A soil's CEC affects fertilization and liming practices. For example, soils with high CEC retain more nutrients than low-CEC soils. With large quantities of fertilizers applied in a single application to sandy soils with low CEC, loss of nutrients is more likely to occur via leaching. In contrast, these nutrients are much less susceptible to losses in clay soils.

Crop production releases acidity into soil. Soil pH will decrease more due to crop production on low CEC soils. High CEC soils are generally well buffered such that pH changes much less from crop production. Therefore, sandy soils low in CEC need to be limed more frequently but at lower rates of application than clay soils. Higher lime rates are needed to reach an optimum pH on high CEC soils due to their greater abundance of acidic cations at a given pH.

http://extension.uga.edu/publications/detail.cfm?number=C1040
 
Seamaiden

Seamaiden

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Elaine and her husband are awesome people. Dr Ingham is a nematode specialist and right nice bloke. Elaine, just wow, her energy is awesome
That's fantastic! She's certainly opened up soil science. I do believe, however, that this book is written by Jeff Lowenfels though, right? I'll have to double check.

Another I can speak to is one of the Hussey family, a younger member. Extremely helpful and forward thinking.
 
Patanjali

Patanjali

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That's fantastic! She's certainly opened up soil science. I do believe, however, that this book is written by Jeff Lowenfels though, right? I'll have to double check.

Another I can speak to is one of the Hussey family, a younger member. Extremely helpful and forward thinking.
It is written by Jeff. If I'm not mistaken Jeff has a relationship to Elaine - her lawyer, friend, something. Tad will know better though. ;)

P-
 
Ecompost

Ecompost

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That's fantastic! She's certainly opened up soil science. I do believe, however, that this book is written by Jeff Lowenfels though, right? I'll have to double check.

Another I can speak to is one of the Hussey family, a younger member. Extremely helpful and forward thinking.
yes and wayne lewis, elaine really only wrote the forward.
Oregon has me, I should have been born among them. I get to Elaine via Paul Stamets. I am a fungi freak, we have huge pine forests here with some really great fungi types.
 
Seamaiden

Seamaiden

Living dead girl
23,596
638
yes and wayne lewis, elaine really only wrote the forward.
Oregon has me, I should have been born among them. I get to Elaine via Paul Stamets. I am a fungi freak, we have huge pine forests here with some really great fungi types.
I love the wild mushroom group on FB I belong to. Extremely educational! We had a wee bit of warmer weather, one of my Russian friends, who is AMAZING at his mushrooming skills (and he's like 22yo!) has already found morels locally. We had the Butte Fire last year, so there're going to be tons of new morel sites locally. :D
 
Ecompost

Ecompost

5,134
313
I love the wild mushroom group on FB I belong to. Extremely educational! We had a wee bit of warmer weather, one of my Russian friends, who is AMAZING at his mushrooming skills (and he's like 22yo!) has already found morels locally. We had the Butte Fire last year, so there're going to be tons of new morel sites locally. :D
the old ladies here can spot an edible fungus at 50 paces in a moving vehicle. The knowledge the old Russians have here about the forests are overwhelming. The know what and where, they know it works for xyz, but they havent done the science bit.
I have a concoction of forest plants that one of them gave me while muttering in Orthodox Christian Russian verse, she told me to rub it on my forehaed when I have a migraine. I am having it analyzed, i will say no more but we can take from this its worth the money to find out whats in it.
I have told my business partner that we might have to divert a few percent in to her church so we can find out what else she has tucked away in her head of wisdom.
 
Ecompost

Ecompost

5,134
313
I love the wild mushroom group on FB I belong to. Extremely educational! We had a wee bit of warmer weather, one of my Russian friends, who is AMAZING at his mushrooming skills (and he's like 22yo!) has already found morels locally. We had the Butte Fire last year, so there're going to be tons of new morel sites locally. :D
added, we are on facebook too as Better organix, i will get Al to add the group when he defrosts his head after being outside
 
Patanjali

Patanjali

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Maybe this will help some...

SeaMaiden,

Hopefully the Horticulture School at Clemson University can help answer your question(s) (Dr. T.L. Senn, et al) – one of the oldest horticulture schools in the world, i.e. not just the USA

Quote:
What is the use for the Cation Exchange Capacity (CeC) and the Percent Base Saturation on the soil test reports?

The CEC is the abbreviation for the Cation exchange Capacity of the soil. Any element with a positive charge is called a cation (pronounced cat eye on and not ka-shun) and in this case, it refers to the the basic cations, Calcium (Ca+2), Magnesium (Mg+2), Potassium (K+1) and Sodium (Na+1) and the acidic cations, Hydrogen (H+1) and Aluminum (Al+3). The amount of these positively charged cations a soil can hold is described as the CEC and is expressed in milliequivalents per 100 grams (meq/100g) of soil.

The larger this number, the more cations the soil can hold. A clay soil will have a larger CEC than a sandy soil. In the Southeast, where we have highly weathered soils, the dominant clay type is kaolinite which has very little capacity to hold cations. A typical CEC for a soil in the Coastal Plains region is about 2.0 meq/100g of soil and the typical CEC for a soil in the Piedmont region is about 5.0 meq/100g of soil. The CEC gives an indication of the soils potential to hold plant nutrients. Increasing the organic matter content of any soil will help to increase the CEC since it also holds cations like the clays. Organic matter has a high CEC but there is typically little organic matter in our soils.

The Percent Base Saturation tells what percent of the exchange sites are occupied by the basic cations. If Calcium has a base saturation value of 50% and Magnesium has a base saturation value of 20% as shown above, then Calcium occupies half of the total exchange sites (CEC) and Magnesium occupies 20% of the total exchange sites (CEC). In our example where the soil has a CEC of 5 meq/100g, 2.5 meq/100g of the CEC is occupied by Calcium and 1 meq/100g of the CEC is occupied by Magnesium. If all the exchangeable bases (Ca, Mg, K and Na) total 100%, then there is no exchangeable acidity.

The acidity on the report is the amount of the total CEC occupied by the acidic cations (H+1and Al+3). The acidity, like the CEC, is expressed as meq/100g of soil. If the CEC is 5 meq/100g of soil and the acidity is 1 meq/100g of soil (see sa mple above), then one-fifth of the exchange sites in the soil are occupied by acidic Hydrogen and aluminum ions. The remaining 4 meq/100g of soil (or 80% of the CEC) will be occupied by the basic cations. The more acidic a soil is and the lower the soil pH value, the closer the acidity number will be to the CEC number.

You can see a detailed explanation of how the CEC, exchangeable acidity, and percent base saturation are calculated from the routine soil test data.

Sodium is included among the bases to indicate if sodium levels are getting too high. This happens in situations where industrial by-products are applied to the soil or where soils along the coastal region are irrigated with water high in sodium. The acceptable base saturation limit for sodium is 15%. This is also called the Exchangeable Sodium Percent or ESP. Sodium levels higher than 15% on the exchange site could result in soil dispersion, poor water infiltration, and possible sodium toxi city to plants.
The next step to understand this is the ‘why’ that Calcium is written as Ca++ whereas Magnesium is written as Mg+ – this has to do with the ‘exchange sites’ meaning that a single Calcium ion (Ca++) can be exchanged for 2 Hydrogen ions (H+)

That is how pH is maintained in the rhizosphere – microbial activity breaks the bonds on Calcium Carbonate releasing Carbon and Oxygen (O3) into the soil thereby freeing the elemental Calcium.

That’s how the boar runs through the buckwheat here in the Shire……

Bazinga!

LOL

CC
 
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