Soil colloids, clay minerals and soil organic matter account for cation exchange properties of soils. See Chapter 5 of text for discussion of soil colloids, structural features, and other characteristics of soil colloids. Soil organic matter means the organic fraction of the soil but does not include undecayed plant and animal residues. Estimates of the average age of the carbon in soil organic matter, based on radiocarbon dating, varies from a few hundred years to more than 25,000 years. (See page 217 in text.) Soil organic matter does not consist primarily of recent crop residues. It is a recalcitrant mixture of organic "residuals" that resist decomposition. Generally, it takes many years to change the organic matter content of a soil from its current "equilibrium" value. Tillage is the major agronomic practice that affects soil organic matter content. Reducing tillage is the most effective way to maintain or attempt to build back organic matter content that has been severely depressed due to intensive or non conserving agricultural practices. However, climate exerts a natural control or limit as to the amount of soil organic matter that can be achieved and sustained.
Cation exchange capacity (CEC)
The CEC of a soil depends upon the amount and type of soil colloids present. The clay content, the type of clay minerals present, and the organic matter content determine a soil's CEC.
CEC, cmol(+) /kg*
3 - 15
20 - 40
60 - 100
soil organic matter, humus, etc.
100 - 300
*Unit is centimole of charge per kilogram of colloid; another common unit for expressing CEC is me/100g (milliequivalents/100 grams). Note that 1me/100g = 1cmolc /kg.
Kaolinite is the dominant clay mineral in soils of this region, but some soils contain significant amounts of montmorillonite, a clay mineral of the smectite group.
CEC for various soils: a typical range of CEC for soils in the state and the region is 3-5 me/100 g. See page 147 of text for a range of CEC for a variety of soils from various regions of the U. S. You may encounter a CEC expressed as "sum of the bases" (Ca+Mg+K+Na) or a CEC based on "sum of the bases and exchangeable acidity" (H+Al). For soils such as those in the Piedmont there is usually a large difference. Generally CEC is not used directly to manage soils and their fertility. The effect of CEC and its significance in managing soils is incorporated into management of pH and lime requirement. The larger the CEC the more buffering capacity a soil will have and the more lime that will be required to raise the soil pH by a specific amount, for example, from 5.5 to 6.5. By the same token soils with a large CEC will have more K supplying power for a given degree of K saturation. Following proven soil test methods and fertilizer recommendations is the best way to manage plant nutrient supplies.
Conversion of me/100 grams to pounds/acre and other facts relating to acreage:
1 me Ca/100 grams = 400 lbs /acre
1 me K/100 grams = 780 lbs/acre
1 me Mg/100 grams = 240 lbs/acre
1 me Na/100 grams= 460 lbs/acre
These conversions are based on an estimate that the surface 6 inch layer of soil over the area of an acre weighs 2,000,000 pounds. The actual weight depends upon the soil bulk density which commonly varies from about 1.3 to 1.7 g/cc. For example, a cubic foot of water weighs 62.4 lbs. If the bulk density of a soil were 1.3 g/cc (1.3 times heavier than water) then the soil would weight 62.4 x 1.3 or 81.1 lbs/cu. ft. on a dry weight basis. Soil properties such as clay content and gravimetric water content are always expressed on a soil dry weight basis.
Other useful numbers to remember and examples of how to use them:
1 acre = 43,560 sq ft
the weight of an acre foot of soil with a bulk density of 1.47
= 43,560 sq ft x 1 ft x 62.4 lbs/cu ft x 1.47 = 4,000,000 lbs
1 part per million (ppm) nitrate N in the top 12 inches of this soil = 4 lbs/acre
If you had a row spacing of 36 inches,
one row 14,520 feet long would be an acre (43,560 ft2/3 ft);
if the row spacing were 40 inches,
the row would be 13,068 feet long (43,560/(40/36) or 43,560/3.333).
- 454 grams = 1 pound; 1 ounce = 28.4 grams
- 1 acre inch of water = 3,630 cu ft = 27,154 gallons = 226,512 pounds
- 1 cu ft weighs 62.4 pounds
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Note differences in base saturation of kaolinite and bentonite over the pH range 5.5 to 6.5. Humic acid and illite have pH-base saturation ralationships similar to bentonite.
The figure "Base Saturation" is based on data published in Soil Testing and Plant Analysis, Edited by Leo M. Walsh and James D. Beaton and published by the Soil Science Society of America, Madison, Wisconsin in 1973. The original data was developed by A. Mehlich and published in 1943.
Base saturation is the amount of the CEC that is occupied by the basic cations such as Ca, K, Mg, and Na. The portion of the CEC that is occupied by acidic cations, primarily H, Al, and Fe, is called the "exchangeable acidity". Note differences in base saturation of kaolinite and bentonite over the pH range 5.5 to 6.5. Humic acid and illite have pH base saturation relationships similar to bentonite.
Unless one knows some specific characteristics about a soil such as the dominant clay minerals, the amount of clay in the soil, and the amount of organic matter, soil pH will not tell how much is the lime requirement. The pH is a measure of the amount of hydrogen ion in solution. As is true of all cationic species, acidic as well as bases, only a small amount of the total exchangeable ions are present in the soil solution at one time.
Soil test methods that have been developed to quickly measure exchangeable acidity must be relied upon to estimate the amount of lime required to raise the soil pH to a desired range.
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For most elements only a very small fraction of that present in soil is available to plants or other biological organisms. The soil solution, that is the water surrounding the soil particles which contains dissolved minerals and salts, typically contains only a few parts per million of the various elements. The natural abundance of elements in a surface soil is presented in Table 2.
Table 2. Elemental concentrations of surface soil from a location in the Piedmont of South Carolina; means of 33 samples.
* Estimate based on 1% organic matter content.
Note the large standard deviations. This is typical for soils. Also, soils in this region are naturally low in soil organic matter. A typical organic matter content for soils in South Carolina is 1%. Soils with a higher organic matter content would have a correspondingly higher total nitrogen content and higher nitrogen supplying power than most soils of the Southeast.
Nutrients are distributed between solid and liquid or water phases. The major portion of the various elements are part of the structure of amorphous and crystalline minerals, clay minerals, and organic matter. They are not available to plants or microorganisms except through dissolution and weathering processes. Exchangeable ions are held close to the colloidal surfaces. They are not free to move about as are ions or solutes in the soil solution but they can be replaced as a result of an ion exchange reaction.
The concentration of nutrients in the soil solution is constantly changing as a result of many reactions proceeding simultaneously, including growth cycles of soil microorganisms, decomposition of crop residues, dissolution and precipitation of solid phases, uptake of ions by plant roots, respiration of plant roots and release of metabolic products such as carbon dioxide and organic acids, and the cycling of ions between the various phases as a result of these reactions.
Measurement of the amount of nutrients in soil which are available to plants has been the subject of extensive research over the past 100 years. Most estimates are based on extraction of the soil with various solutions including acids, salts, and chelating agents. The amounts extracted are then compared with the amount which can be taken up by plants. The plant is the authority on what is available. Some plants are able to extract more nutrients from soil than other plants.
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Supply of Nutrients to Plant Roots
ForReferences: text, page 282
There are three main processes by which nutrients are supplied to plant roots:
1. Mass flow of soil solution to the plant root as a result of water uptake.
2. Diffusion of ions from solid phases or regions of high concentration towards the plant root as concentrations become depleted due to nutrient absorption.
3. Root interception as a result of the root growing and occupying more space.
The table below shows the relative amounts of several nutrients supplied to corn roots by the three processes.
From: Soil Testing and Plant Analysis. 1973. Editors, Leo M. Walsh and James D. Beaton. Soil Science Society of America. Madison, Wisconsin.
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Nutrient Availability and Mobility
See pages 261 274, 279 287, and 291 297 of the text for discussion of reactions that affect plant availability of N, P, and K in soil.
A. General mechanisms that affect availability of various elements and relevant terms.
Adsorption-desorption or ion exchange; Note: adsorption refers to an attraction to a surface while absorption refers to being incorporated into something. See diagram on page 144 of the text.
Mineralization: organic ---> inorganic
Precipitation-dissolution: when dissolved substances form solids (solid phases) and drop out of solution, the process is called "precipitation"; it is not the same as adsorption; dissolution is the opposite process whereby solids go into the solution phase and become "solutes".
Fixation: elements or certain ions such as NH4 become physically and chemically bound in a nonexchangeable form. An example is the entrapment of potassium between silica layers of clay minerals. Some clays also fix ammonium in the same way. Phosphorus can also become "fixed" in unavailable forms but the mechanism is different than ammonium or potassium fixation. Phosphorus is fixed by being bound chemically to iron and aluminum compounds.
Denitrification: biochemical reduction of nitrate or nitrite to gaseous nitrogen
Mineralization: conversion of an organic form of an element into an inorganic form.
Most of the nitrogen in soil is present as soil organic matter and is unavailable to plants. Organic matter is about 50% carbon and 5% nitrogen, thus it has a carbon/nitrogen ratio of about 10. Nitrogen is released very slowly from soil organic matter as a result of soil microbial activity. This process is affected by moisture, temperature, tillage or any physical disturbance such as soil wetting and drying. The amount of nitrogen released by this process is also dependent on the soil organic matter content. A soil with 1% organic matter will supply much less plant available nitrogen than one that is 5% organic matter, under similar conditions.
The decomposition of crop residues is usually limited by the C/N ratio of the residue as well as environmental conditions. Soil microorganisms have first choice of available nitrogen. Higher plants get what is left over. Alfalfa will decompose more quickly in soil than corn stalks because of its higher nitrogen content (lower C/N ratio).
Nitrate, being an anion, is the most mobile form of N in soil; it moves within the water as water percolates through soil and is easily lost through this leaching process. Ammonium and ammonia forms are cationic and held by cation exchange sites. However, ammonium and ammonia forms are converted to nitrate by soil microorganisms. The reaction is fast in warm moist soils. The reaction rate becomes slower as the soil becomes more acidic. From spring through fall most ammonium fertilizer is converted to nitrate within a few days to a week or two. Urea nitrogen is hydrolyzed to ammonium nitrogen by a soil enzyme and then quickly converted to nitrate provided conditions are right.
Phosphorus is strongly held by soil clays and iron and aluminum compounds associated with soil clays; also, phosphorus forms very sparingly soluble compounds, precipitates, in soil. These reactions proceed rapidly after fertilizer is applied. Most phosphorus in soil is precipitated, fixed, or adsorbed. These solid phases form phosphate reserves that can replenish the soil solution when phosphorus is taken up by plants, but the reaction is slow. Only small amounts of phosphorus are present in the soil solution. Because of these reactions with soil, P is very immobile, not subject to significant leaching losses. However, P is readily lost through erosion of surface soil and the associated P.
Potassium exists as exchangeable K and in K-bearing minerals. In soils that contain mica-type clay minerals and vermiculate some K exists as "fixed" K within the clay mineral structure. Fixed K is not exchangeable.
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Cations and Cation Exchange Capacity
- Cation exchange capacity (CEC) is the total capacity of a soil to hold exchangeable cations.
- CEC is an inherent soil characteristic and is difficult to alter significantly.
- It influences the soil’s ability to hold onto essential nutrients and provides a buffer against soil acidification.
- Soils with a higher clay fraction tend to have a higher CEC.
- Organic matter has a very high CEC.
- Sandy soils rely heavily on the high CEC of organic matter for the retention of nutrients in the topsoil.
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?
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 (McKenzie et al. 2004) however, productive crops and pastures can be grown on low CEC soils.
The main ions associated with CEC in soils are the exchangeable cations calcium (Ca2+), magnesium (Mg2+), sodium (Na+) and potassium (K+) (Rayment and Higginson 1992), and are generally referred to as the base cations. In most cases, summing the analysed base cations gives an adequate measure of CEC (‘CEC by bases’). However, as soils become more acidic these cations are replaced by H+, Al3+ and Mn2+, and common methods will produce CEC values much higher than what occurs in the field (McKenzie et al. 2004). This ‘exchange acidity’ needs to be included when summing the base cations and this measurement is referred to as effective CEC (ECEC).
Different laboratories use various methods to measure CEC, and can return contrasting results depending on the fraction of the soil measured. In Australia, some laboratories measure CEC directly and others calculate it as CEC by bases. Cation exchange capacity is commonly measured on the fine earth fraction (soil particles less than 2 mm in size). In gravelly soils the effective CEC of the soil as a whole is diluted, and if only the fine (clay) fraction is analysed, reported CEC values will be higher than actual field values.
Measuring CEC 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. Methods, including pre-treatment, for measuring CEC and exchangeable cations are presented by Rengasamy and Churchman (1999) and described in detail by Rayment and Higginson (1992).
CEC is conventionally expressed in meq/100 g (Rengasamy and Churchman 1999) which is numerically equal to centimoles of charge per kilogram of exchanger (cmol(+)/kg).
Soil type and CEC
The CEC of soils varies according the clay %, the type of clay, soil pH and amount of organic matter. Pure sand has a very low CEC, less than 2 meq/100 g, and the CEC of the sand and silt size fractions (2 µm/2 mm) of most soils is negligible. Claying sandy soils for managing water repellence increases the CEC of the surface layers by a small amount depending on type and amount of clay added. Typically CEC is increased by less than 1 meq/100 g.
The most commonly occurring clay in Western Australian soils, kaolinite, has a CEC of about 10 meq/100 g. Other clays such as 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 (Moore 1998). Because a higher CEC usually indicates more clay and organic matter is present in the soil, high CEC soils generally have greater water holding capacity than low CEC soils.
Soil pH and CEC
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 (McKenzie et al. 2004). Soil pH change can also be caused by natural processes such as decomposition of organic matter and leaching of cations. The lower the CEC of a soil, the faster the soil pH will decrease with time. Liming soils (see Soil Acidity fact sheet.) to higher than pH 5 (CaCl2) will maintain exchangeable plant nutrient cations.
Nutrient availability and CEC
Soils with a low CEC are more likely to develop deficiencies in potassium (K+), magnesium (Mg2+) and other cations while high CEC soils are less susceptible to leaching of these cations (CUCE 2007). Several factors may restrict the release of nutrients to plants. Some groups promote the controversial idea of managing cation ratios, claiming ideal ratios for Ca:Mg or Ca:K. For plant nutrition, a more critical factor is whether the net amount of Ca or K in the soil is adequate for plant growth. The addition of organic matter will increase the CEC of a soil but requires many years to take effect.
Figure 1 illustrates how CEC can change with depth. The sum of the base cations provides an estimate of the CEC of each soil layer. The surface 10 cm has a CEC of 4.6 meq/100 g because of a high organic content. At 10 – 30 cm depth, the organic content of the sand is very low, hence the low CEC. The CEC of the subsoil layers are governed by clay content, 61 %, 51 % and 34 % respectively. The dominant clay in this soil is kaolinite so CEC values remain low.
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.
Further reading and references
Cornell University Cooperative Extension (CUCE) (2007) Cation Exchange Capacity (CEC). Agronomy Fact Sheet Series # 22. Department of Crop and Soil Sciences, College of Agriculture and Life Sciences, Cornell University.
Hazelton PA, Murphy BW (2007) Interpreting Soil Test Results: What Do All The Numbers Mean?. CSIRO Publishing: Melbourne.
McKenzie NJ, Jacquier DJ, Isbell RF, Brown KL (2004) Australian Soils and Landscapes: An Illustrated Compendium. CSIRO Publishing: Collingwood, Victoria.
Moore G, Dolling P, Porter B and Leonard L (1998) Soil Acidity. In Soilguide. A handbook for understanding and managing agricultural soils. (Ed. G Moore) Agriculture Western Australia Bulletin No. 4343.
Rayment GE, Higginson FR (1992) Electrical Conductivity. In ‘Australian Laboratory Handbook of Soil and Water Chemical
Methods’ Inkata Press: Melbourne.
Rengasamy P, Churchman GJ (1999) Cation Exchange Capacity, Exchangeable Cations and Sodicity. In Soil Analysis an
Interpretation Manual. (Eds KI Peverill, LA Sparrow and DJ Reuter). CSIRO: Melbourne.
Author: Katharine Brown (The University of Western Australia) and Jeremy Lemon (Department of Agriculture and Food, Western Australia).
This soilquality.org.au fact-sheet has been funded by the Healthy Soils for Sustainable Farms programme, an initiative of the Australian Government’s Natural Heritage Trust in partnership with the GRDC, and the WA NRM regions of Avon Catchment Council and South Coast NRM, through National Action Plan for Salinity and Water Quality and National Landcare Programme investments of the WA and Australian Governments.
The Chief Executive Officer of the Department of Agriculture and Food, The State of Western Australia and The University of Western Australia accept no liability whatsoever by reason of negligence or otherwise arising from the use or release of this information or any part of it.
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