What is CEC or Cation Exchange Capacity? 

CBX increases the organic matter which increases the cation exchange capacity (CEC), then the biology of the soil creates aerobic conditions where plant residue is recycled into organic compounds to be preserved and available for plant life.

Want to know more? Here is a detailed report on what CEC is.

Here is how it works.

The clay and organic components of the soil have a negative charge. As a result of these charges, positively charged ions (cations) such as hydrogen (H+), potassium (K+), ammonium (NH+4), calcium (Ca2+), magnesium (Mg2+), aluminum (Al3+), etc. may be held at the surface of the clay or organic particles and exchanged with other ions in the solution or with ions at the plant root's surface.

The ability of a soil to hold cations is termed its cation exchange capacity (CEC). Since many cations are plant nutrients the cation exchange capacity is a measure of the soil's ability to hold such nutrients.

Calcium (Ca++) is normally the predominant exchangeable cation in soils, even in acid, weathered soils. In highly weathered soils, such as oxisols, aluminum (Al+3) may become the dominant exchangeable cation.

The energy of retention of cations on negatively charged exchange sites varies with the particular cation. The order of retention is: aluminum > calcium > magnesium > potassium > sodium > hydrogen. Cations with increasing positive charge and decreasing hydrated size are most tightly held. Calcium ions, for example, can rather easily replace sodium ions from exchange sites. This difference in replaceability is the basis for the application of gypsum (CaSO4) to reclaim sodic soils (those with > 15% of the cation exchange capacity occupied by sodium ions). Sodic soils exhibit poor structural characteristics and low infiltration of water.

The cations of calcium, magnesium, potassium, and sodium produce an alkaline reaction in water and are termed bases or basic cations. Aluminum and hydrogen ions produce acidity in water and are called acidic cations. The percentage of the cation exchange capacity occupied by basic cations is called percent base saturation.

The greater the percent base saturation, the higher the soil pH.

The CEC is the abbreviation for the cation exchange capacity of the soil. Any element with a positive charge is called a cation and in this case, it refers to the the basic cations, calcium (Ca+2), magnesium (Mg+2), potassium (K+1) and s odium (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 s oil 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 o f the total exchange sites (CEC) and magnesium occupies one-fifth 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 sample 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.

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 toxicity to plants.

So, why do we bother with the CEC, acidity and base saturation? Some consultants and farmers prefer to use the base saturation of the plant nutrients instead of the extractable amounts as a guide for maintaining optimum fertility. For Southeastern s oils with kaolinitic clays, a base saturation of 45 to 65 percent will be satisfactory for good plant growth. The following table gives the approximate base saturation for the soils of a given soil pH:

In South Carolina, if fertilizer and lime is applied to raise the base saturation of a kaolinitic soil to 85 percent as commonly done in the Midwest, the resulting pH would be between 7.1 and 7.5. Soil pH values in that range would result in a major problem with zinc and manganese deficiency. That is why the Clemson University fertilizer recommendations are determined by the amount of each nutrient extracted from the soil (expressed in pounds per acre) instead of using the percent base saturation as a guide. A favorable base saturation will be obtained if the soil pH is maintained between 5.8 and 6.5. The approach used by Clemson University is also used throughout the Southeast and Mid-Atlantic regions in determining soil fertilizer requirements. The CEC and base saturation is something that many farmers and consultants have asked for to better understand the soil and so it is now available in response to public demand.

The CEC of your soil directly affects the amount of fertilizer you should use and the frequency with which fertilizer should be applied. Based on your soil type, the percentage of organic matter in your soil, and the relative strength of positively and negatively charged nutrients in your soil, you can devise a sound soil treatment plan that accomplishes the most with the least.

The Soil is Like a Magnet
Imagine that your soil is a giant magnet for soil nutrients. Clay and organic matter in the soil have a negative charge. So naturally it attracts positively charged nutrients and repels negatively charged nutrients. This explains why cations, the positively charged nutrients, find an easy home in the soil while anions, negatively charged nutrients, are repelled and easily leached out of the soil.

Cations & Anions
Cations include everything from hydrogen, with a +1 positive charge, to aluminum, with a +3 positive charge. Anions include phosphate, nitrate, and other essential elements that hold a -1 or -2 charge. Table 1 shows the most common cations and anions along with their chemical formula and charge.

Competition among Cations
But just because a cation has a positive charge doesn’t mean it can’t be leached out of the soil. More strongly charged cations can be used to knock out others. For example, aluminum has three positive charges and would very easily displace sodium. This explains why gypsum (CaSO4) is so effective in correcting a sodium rich soil. The sodium (one positive charge) is pushed out by the calcium (two positive charges). See how CBX solves this problem by reviewing our solution for high salinity soils.

The CEC of soil is expressed as charges per 100 grams of soil (meq/100g).

Affect of pH on Soil CEC
In addition to clay and organic matter, pH also has an effect on CEC. And, of these three factors, usually only pH can be changed. Soil pH changes the CEC because the soil has exchange sites that become active as the pH increases. Soil CEC could be expected to increase up to 50% if the pH was changed from 4.0 to 6.5 and nearly double if the pH increased from 4.0 to 8.0.

In summary, your soil works like a giant magnet, attracting cations and repelling anions – positively and negatively charged nutrients. Knowledge of this process can be used to develop appropriate soil treatment and fertilization plans for the different types of soil.

1 dS/m = 1 mmhos/cm = 1000 mhos/cm
1 mg/L = 1 ppm
TDS (mg/L) = EC (dS/m) x 640 for EC < 5 dS/m
TDS (mg/L = EC) dS/m) x 800 for EC > 5 dS/m
TDS (lbs/ac-ft) = TDS (mg/L) x 2.72
Concentration (ppm) = Concentration (mol/m3) times the atomic weight

Sum of cations/anions
(meq/L) = EC (dS/m) x 10

Factors Affecting Soil EC
The conduction of electricity in soils takes place through the moisture-filled pores that occur between individual soil particles. Therefore, the EC of soil is determined by the following soil properties (Geonics Limited, 1980):
Porosity – The greater a soil’s porosity, the more easily electricity is conducted. Soils with high clay content have higher porosity than sandier soils.
Water content - Dry soils are much lower in conductivity than moist soils.
Salinity level – Increasing concentration of electrolytes (salts) in soil water will dramatically increase soil EC. The salinity level in most Corn Belt soils is very low.
Cation exchange capacity (CEC) – Soils containing high levels of organic matter (humus) and/or 2:1 clay minerals such as montmorillonite, illite or vermiculite have a much higher ability to retain positively charged ions (such as Ca, Mg, K, Na, NH4, or H) than soils lacking these constituents. The presence of these ions in the moisture-filled soil pores will enhance soil EC in the same way that salinity does.
Temperature – As temperature decreases toward the freezing point of water, soil EC decreases slightly. Below freezing, soil pores become increasingly insulated from each other and overall soil EC declines rapidly.
Electrical Conductivity (EC), Specific Conductance - simply defined this is a measure of a solution's ability to conduct electricity. It is the reciprocal of a solution's electrical resistance. The units of resistance are ohm-cm. The units for conductance is Seimens/ meter (S/m). Not too long ago EC was measured in units of mho/cm. It turns out that Seimen/meter equals mhos/meter (mhos/m), microSeimen/centimeter (uS/cm) is equivalent to umho/cm and milliSeimen/centimeter (mS/cm) is equivalent to millimho/cm (mmho/cm). No matter what the term appearing on our reports (Electrical Conductivity (EC), Specific Conductance, Conductance, Conductivity, etc,), we report uS/cm, or umhos/cm corrected to 25°C.
Total Dissolved Solids (TDS) - this is also a measure of the salts dissolved in water. Not surprisingly, there is a relationship between TDS and EC. Generally speaking the TDS in mg/L is about 2/3-3/4 of the EC measured in uS/cm. So, now you have a way of calculating TDS if you know the EC: TDS ~ 0.7 EC.
Category TDS (mg/L)
Fresh water < 1000
Brackish water 1,000 - 10,000
Saline water 10,000 - 100,000
Brine water > 100,000

Hardness - hardness is defined as the sum of the calcium (Ca) and magnesium (Mg) concentrations, both expressed as calcium carbonate in milligrams per liter (mg/L). To calculate hardness from the calcium and magnesium concentrations (mg/L), we must first convert these concentrations to milliequivalents/Liter (meq/L). This conversion in terms of concentration allows the calcium and magnesium to be added together.

Often water treatment technicians express hardness with units of grains per gallon. 1grain (gr) per gallon (gal) = 17.1 mg/L = 17.1 ppm. Conversely, 1 mg/L = 0.0585 gr/gal. So if you know your water's hardness in terms of grains per gallon you can convert that value to mg/L or ppm by multiplying by 17.1. In the reverse, if you have the value in terms of mg/L or ppm multiply that value by 0.0585 to obtain that value in terms of gr/gal.


REFERENCE: Bob Lippert's frequently asked questions regarding soil testing, plant analysis and fertilizers. (Department of Entomology, Soils and Plant Sciences - Clemson University Extension Service, South Carolina, U.S.A.) Also: Dr. Charles Mitchell - Soil Fertility Specialist, Dr. Owen Plank - Extension Specialist, Dr. Glen Harris - Soils and Fertilizer Specialist, Dr. Carl Crozier - Soil Science Specialist, Dr. Ray Tucker - Soil Testing Specialist, Dr. Jim Camberato - Soil Fertility Specialist, Dr. Bob Lippert - Soil Fertility Specialist, Dr. Kathy Moore - Extension Lab Director, Dr. Steve Donohue - Soils and Plant Analysis Specialist, Dr. Mark Alley - Agronomist