by Robert Paul Hudson
Lately there seems to be a renewed interest among hobbyists in using soil in freshwater planted aquariums. If you are considering doing this, one of the most basic things about soil science to take into account is CEC- cation exchange capacity. In agriculture and indoor gardening it is very important, and in the aquarium it is a factor in the overall success of using soil in a substrate.
What is CEC?
The Exchange Capacity of soil and growing mediums is a measure of its ability to hold and release various chemical elements and compounds that include nutrients for plant growth.
If you study chemistry you will learn that elements and compounds are ions, which means a charged particle. These particle ions can either be positively charged or negatively charged. Positive charged ions are called cations, and negatively charged ions are called anions.
Positive charged ions are attracted to the surface of a negatively charged medium and held there in a condensed layer where they may come into contact with plant roots. This “holding” prevents the cations from leeching out of the medium.
In agriculture a soil low in CEC would have its nutrients washed and carried away every time it rained. In the aquarium a low CEC substrate would have its nutrients leech into the water column.
Positive charged cations
Cations include some macro nutrients and all trace mineral nutrients. Other macro nutrients are anions. The most critical nutrients for CEC are Calcium, Magnesium, Potassium, Sodium, and trace minerals including iron.
Soil Organic Matter
Soil organic matter is the chief component of top soils and potting soil. This is decomposed organic material from compost, manure and humus. SOM is both positively charged and negatively charged so it will attract both cations and anions. Clay particles are almost always positively charged and attract only cations.
Cation Exchange Capacity is the measure of how many negatively charged sites are available in the soil or medium. Clay and other inert materials will continue their CEC indefinitely while SOM will last only until the organic material breaks down from further decomposition.
Cations and Anions
Common Cations: (ions grouped by charge)
|Anions from Organic Acids:|
The Calcium to Magnesium ratio determines how tight or loose a soil is. The higher the Calcium the looser it is and the higher the Magnesium, the tighter it is. A high Calcium soil will have more oxygen and support more aerobic breakdown of organic material, a high Magnesium soil will be more likely to have organic material ferment. If you have an extreme level of Calcium, the soil will loose its beneficial granulation and structure, which will interfere with the availability of other nutrients to plant uptake.
How plants get the nutrient cations
The roots and microorganisms get these nutrients by exchanging free hydrogen ions. The free hydrogen H+ fills the (-) site and allows the cation nutrient to be absorbed by the root or microorganism.
How to Increase CEC
Organic materials in soil usually have high CEC surfaces. This includes compost, peat, manure, and humus. Clays and other inert materials may also have high CEC surface areas and can be mixed with soil. Fired clay particles have much higher CEC than raw clay. This is what clay gravel aquarium substrates consist of. There are many different types of mineral clay and some are actually very low in CEC.
Here are some high CEC mediums that could be mixed with soil:
Cation Exchange Capacities for various growing media amendments and selected media.
Material/Cation Exchange Capacity meq/100g
Perlite/ 1.5 – 3.5
Silt/ 3.0 – 7.0
Clays/ 22.0 – 63.0
Pine Bark/ 53.0
Vermiculite/ 82.0 – 150.0
Sphagnum Peat/ 100.0 – 180.0
Peat moss : vermiculite 1:1/ 141.0
Peat moss : sand 1:1/ 8.0
Peat moss : perlite 1:3/ 11.0
Peat moss : perlite 2:1/ 24.0
Sources: see Bunt, A.C. 1988, and Landis, T. D. 1990.
The following is word for word from a University web site I saved several years ago. I have since lost the link:
Container growing mediums:
Sphagnum peat moss
Sphagnum peat moss, derived from the genus Sphagnum, contains at least 90% organic matter on a dry weight basis. In addition, this peat moss contains a minimum of 75% Sphagnum fiber, consisting of recognizable cells of leaves and stems.
Approximately 25 species of Sphagnum exist in Alberta, Canada and 335 species are present throughout the world. Sphagnum fuscum is an important species bearing many desirable traits. Sphagnum grows in northern cool regions and is also located in peat bogs found in Washington, Maine, Minnesota, and Michigan.
Many pores are present in the leaves of sphagnum; when used as growing media, as much as 93% of the water occupying this internal pore space is available for plant uptake (Peck, 1984). After draining, sphagnum peat can hold 59% water and 25% air by volume.
Sphagnum is usually characterized by an acidic pH, low soluble salts content, structural integrity, and the ability to serve as a nutrient reserve (Landis, 1990).
Although peat mosses are classified into four different groups, variation may exist within any one type of peat moss. Peats of the same classification often differ notably in quality, and even peats from the same bog taken from separate layers can possess different chemical and physical properties.
Sphagnum peat moss is classified as light or dark peat, based on its color. Light peats are characterized by a large amount of internal pore space, 15-40% of the pore space comprises aeration porosity. Dark sphagnum peat does not display the elasticity of light peat and is usually not as long lasting. Dark sphagnum peat moss maintains twice the cation exchange capacity of light peats, yet does not possess as much total or aeration porosity.
Materials such as vermiculite, perlite, and sand represent the inorganic fraction often used in container media formulations. These materials generally increase the aeration porosity and drainage yet decrease the water-holding porosity of media. Inorganic components are usually inert materials characterized by a low cation exchange capacity.
Vermiculite is a commonly used inorganic media component which is mined in the U.S. and Africa. This mineral, comprised of an aluminum/iron/magnesium/silicate mixture, is excavated as a material composed of thin layers. Processing includes heating the vermiculite to temperatures upwards of 1000 degrees C, which converts water trapped between the layers of the material into steam. The production of steam results in a pressure that expands the material, increasing the volume of the pieces 15 to 20 times their original size.
Vermiculite is sterile because of these high heating temperatures used during processing. Vermiculite is characterized by a high water-holding capacity as a result of its large surface area: volume ratio, a low bulk density, nearly neutral pH, and a high cation exchange capacity attributed to its structure. Because it compacts readily when combined with heavier materials, vermiculite is sometimes recommended more for propagating material than container media.
Vermiculite gradually releases nutrients for plant absorption; on average it contains 5-8% available potassium and 9-12% magnesium. This inorganic media component can adsorb phosphate – some of which remains in an available form for plant uptake – but cannot adsorb nitrate, chloride, or sulfate. Vermiculite can fix ammonium into a form that is not readily available for plant absorption. This fixed nitrogen is gradually transformed to nitrate by micro-organisms, making it available for plant uptake.
Vermiculite is manufactured in four different grades, differentiated by particle size. Insulation grade vermiculite and that which is marketed for poultry litter (which has not been treated with water repellents) has been used with some success. Vermiculite which has been treated with water repellent, such as block fill should not be used as growing media. Because vermiculite tends to compact over time, it should be incorporated with other materials such as peat or perlite to maintain sufficient porosity. It should not be used in conjunction with sand or as the sole media component, because as the internal structure of vermiculite deteriorates, air porosity and drainage decreases (Landis, 1990).
The particle size of vermiculite influences the water-holding and aeration porosity of the material. Although grade classification is based upon particle size, each grade is represented by a range of particle sizes. Note that grades consisting of larger particle sizes have a higher aeration porosity and lower water-holding porosity than grades consisting of a smaller range of particle sizes. Properties of the four vermiculite grades are shown in an associated table.
A mineral of volcanic derivation, perlite is a second inorganic component which may be used in formulating container mixes. This chemically inert material is extracted in New Zealand, the U.S., and other countries and is usually mined by scraping the earth’s surface. The processing method includes a grinding and heat treatment (up to 1000 degrees C) which results in very lightweight, white sterile fragments. As the ore is heated, internal water escapes as steam, resulting in the expansion of the material.
Perlite has a very low cation exchange capacity, low water-holding capacity (19%), and neutral pH. The closed-cell composition of perlite contributes to its compaction resistance, enhances media drainage, and heightens the aeration porosity of peat-based media (Bilderback 1982). Because perlite contains only minute amounts of plant nutrients, liquid feeding is a practical mode of fertilization. Be aware of possible aluminum toxicity in acidic media (pH < 5).
The very low levels of fluoride perlite contains is not likely to pose plant health problems. Any soluble fluoride present in a media characterized by 6.0 < pH < 6.5 will precipitate out of the media with excess calcium from sources such as gypsum, limestone, or calcium nitrate.
Although perlite has several positive attributes, it also has drawbacks. Perlite consists of many fine fragments which, when dry, can lead to lung or eye irritation. In addition, because water clings to the surface of perlite, it may tend to float in the presence of water (Landis, 1990).
Perlite contains, on average, 47.5% oxygen, 33.8% silicon, 7.2% aluminum, 3.5% potassium, 3.4% sodium, 3.0% bound water, 0.6% iron and calcium, and 0.2% magnesium and trace elements (Perlite Institute, 1983). Although a uniform categorization of perlite does not exist, individual producers of this inorganic component assign grade levels. This inorganic media amendment is sometimes recommended for use only in propagation media because of its low bulk density and tendency to compact.
In comparison with sand, polystyrene, or pumice, perlite has the greatest inner total porosity. Coarse perlite is characterized by approximately 70% total porosity, 60% of which is aeration porosity. Perlite can retain two to four times its dry weight in water, which is much greater than that of sand and polystyrene, yet much less than the water-holding capacity of peat and vermiculite (Moore, 1987).
Sand has been used as an inorganic media component to add ballast to containers. Some sands contain calcium carbonate which may raise media pH undesirably. A rise in pH may lead to nutrient deficiencies, particularly of minor elements such as iron and boron. A few drops of dilute hydrochloric acid or strong vinegar may be added to sand to test for carbonates; if bubbling and fizzing result, carbonate is present as a result of carbon dioxide production.
Sand used for container media should have a 6 < pH < 7. Sand maintains good drainage, a low water-holding capacity, and a high bulk density when used independently of other materials. Because of its shape and size, sand can obstruct pore spaces, decreasing drainage and aeration, instead of improving porosity.
Various sand particle sizes have been recommended for container media use, including ranges of 2-3 mm or 0.05 – 0.5 mm (fine sand) in size (Landis, 1990). In addition, another recommendation suggests that 60% of the particles be within 0.25-1.0 mm range, and 97% be greater than 0.1 mm and less than 2 mm (Swanson, 1989). Uniformity coefficients assigned to sand mixtures signify the amount of sand which is within a certain size range; a coefficient < 4 is evidence of a homogeneous sand mixture (Swanson, 1989). If the correct grade of sand is used, the wet ability of the media is enhanced.
When fired at high temperatures, some clays, fuel ash, and shales form stable compounds that possess low bulk densities and internal porosities of 40-50%. Though calcined clays alter the physical attributes of media in a positive way, they also decrease the level of water-soluble phosphorus in the mix.
Because calcined clays are characterized by a high cation exchange capacity, fertilizer application rates may need to be modified if calcined aggregates are incorporated into the media mixes (Bunt, 1988).
Pumice is produced as volcanic lava cools; escaping steam and gas contribute to its porous nature. This alumino-silicate material contains potassium, sodium, magnesium, calcium, and slight amounts of iron. Pumice can absorb K, Mg, P, and Ca from the soil solution and render it available for plant absorption later (Bunt, 1988).
Zeolite is a natural as well as synthetic mineral that has a honeycomb structure that provides a high CEC. It is inert, and in gravel form is suitable for the aquarium.
1. Top soil, potting soil is generally high in CEC, but has a limited life
2. Clay, depending on the type may be high in CEC and lasts forever
3. Perlite and Vermiculite float in water so are not suitable for the aquarium
4. Sand has low to zero CEC
Jamie Johnsons CEC and nutrient analysis
January 28th, 2012, Aqua Botanic Radio will be talking about soil in the aquarium