Understanding Plant Nutrition: Nutrient Sources: Media Cation Exchange Capacity

Understanding Plant Nutrition: Nutrient Sources: Media Cation Exchange Capacity

Figure 1. Examples of how cation exchange capacity buffers media pH and
nutrient concentrations. (A) is an example of an acidic sphagnum peat particle.
The peat particle contains several negative charges (exchanged sites) at the surface.
In unlimed peat, these exchange sites are usually filled with hydrogen ions.
(B) is an example of the same peat after the application of some limestone. The
CO3-2 of the limestone has neutralized most of the hydrogen ions in
the soil solution as well as at the exchange sites, causing the pH to increase.
Some of the calcium and magnesium (both cations) from the neutralized lime are
attracted to the exchange sites. (C) is an example of the same limed peat, but after
the application of an acidic fertilizer. The acidic fertilizer has produced excess hydrogen ions.
If these hydrogen ions remained in solution, then the solution pH would be reduced.
In this case, the excess hydrogen ions replaced a calcium and magnesium ion on the
exchange site, which then entered the soil solution in their place. Because the net
result was that no additional hydrogen ions entered the soil solution,
the pH did not change, and therefore, the pH was buffered by the soil CEC.

Every university course on soil fertility discusses the importance of the “cation exchange capacity” (CEC) of the soil for buffering nutrients and pH. In this article, however, we debunk the myth that CEC is important in soilless media. Why then should you care that container media has low CEC? Because understanding that nutrient levels and pH in the medium can change very quickly (within days or hours) can help you better manage pH and fertility levels in the greenhouse or nursery.

CEC And Buffering
CEC has been used historically to describe the buffering capacity of soil (mainly based on crops grown in field soils). Buffering means the resistance to change in pH or nutrient concentration in the soil solution. Therefore, a medium that is high in CEC would help growers maintain a stable pH or nutrient concentration over time.Media particles, such as soil or peat, have negatively charged “exchange sites” that allow the particles to loosely hold onto positively-charged “cations” (Figure 1). Cations include acid (hydrogen H+), fertilizer cations (e.g. ammonium NH4+, calcium Ca2+, magnesium Mg2+, potassium K+), and other waste cations (e.g. sodium Na+). Media with high CEC have the ability to absorb and release large amounts of cations from the soil solution, which evens out high and low concentrations of nutrients available for plant uptake.

Let’s take an example of how CEC affects liming rates of acidic sphagnum peats. The exchange sites before liming are mainly loaded with acid (H+). The higher the CEC of the batch of peat, the more lime that must be added to raise pH up to around pH 6. That is why the amount of lime needed to reach a target pH may vary between sources, or why finer peats (higher bulk density with more exchange sites per unit volume) have greater lime requirements than coarser peats (lower bulk density with less exchange sites per unit volume).

CEC also helps explain how lime reacts with the growing medium. During the dissolution of limestone, some of the CO3-2 from the limestone neutralizes the H+ contained in the soil solution to form carbon dioxide and water. As the H+ is reduced in the soil solution, additional H+ moves from the exchange sites to the soil solution and is neutralized by the lime. Finally, calcium (Ca2+) and magnesium (Mg2+) from the neutralized lime moves from the soil solution onto the peat exchange sites. The net result is that the exchange sites become saturated with mostly calcium and magnesium, and the concentration of H+ in the soil solution is reduced (higher media pH).

Figure 2. Effect of root media on medium pH. The grower grade peat was
long fibered with little dust and the consumer grade peat was a
more degraded fine peats with large amounts of dust. All media
were blended with 30 percent perlite and the lime source
was hydrated lime (low residual). The crop was impatiens grown
with a water-soluble fertilizer containing 50 percent
ammoniacal nitrogen and RO purified water. Acceptable pH ranges from
5.8 to 6.2. Research by Bill Argo and John Biernbaum.

How will CEC of the peat buffer pH during crop production? Let’s assume a grower uses an acid-reaction fertilizer (e.g. 20-10-20). This fertilizer adds acid (H+) into the soil solution, which lowers the pH. However, some of that H+ in the soil solution is exchanged with the Ca2+ and Mg2+ on the peat. The acidity is removed from the soil solution, and the pH goes back up. In the process, Ca2+ and Mg2+ are also added into the soil solution.

How can CEC buffer nutrient concentrations? A growing medium can exchange nutrient cations back and forth between the exchange sites and the soil solution, in the same way as it can exchange H+ acid. Therefore, the exchange sites act as a back up “pool” of nutrients to recharge the soil solution when nutrient levels are low.

Media that have high CEC (more buffered) can resist a change in pH or nutrient concentrations for long periods of time, whereas pH or nutrient concentrations can change very rapidly in media that have low CEC (less buffered). 

Bulk Density And CEC

Organic materials like peat are often shown to have CEC values as much as 10 times greater than that of a typical field soil, based on weight. So why do media-pH and fertilizer level change so quickly in greenhouse production?

Anyone who has picked up a pot containing field soil knows that it weighs much more than a pot containing a peat-lite medium. Peat, bark, perlite, expanded vermiculite or any other material used to produce container media typically have a very low bulk density compared to soil.

In other words, you can get a lot more soil into a pot than you can a peat-lite medium. Consequently, the effective CEC of organic materials like peat measured on a volume basis (i.e. per pot) is about 40 percent to 50 percent less than that of a field soil. On balance, then, CEC of soilless container media is low and provides little buffering. 

CEC And pH And Nutrition Management

Research has shown that the CEC of soilless media has little effect on resisting change in pH, or in supplying nutrients to the crop. Several experiments were completed at Michigan State University that tested the effect of CEC on long-term pH and nutrition management using impatiens as the test crop. The media tested ranged in buffering capacity from one considered very low (5 meq/liter, a 70 percent rockwool and 30 percent perlite mix) to one that would be considered highly buffered (76 meq/liter, a mix of 70 percent highly degraded consumer grade sphagnum peat and 70 percent perlite).

Hydrated lime was used as the lime source to increase the initial pH to about 6 in all media because it reacts quickly and completely and did not influence long-term pH management. The amount of hydrated lime needed ranged from 0 lbs/yd3 with the rockwool/perlite media, 0.8 lbs/yd3 with the coir perlite media, 2.5 lbs/yd3 with the grower grade sphagnum peat/perlite and 4.5 lbs/yd3 with the consumer grade sphagnum peat/perlite media.

When an acidic fertilizer solution was applied to the impatiens grown in the different media, pH of the rockwool medium tended to be higher than for the other media. In all media, however, the pH dropped very quickly to a low of about 4 (Figure 2) regardless of the CEC of the media.

When shoot-tissue calcium was tested after four, eight, 12 or 17 weeks of growth, there was little difference between plants grown in the media with low CEC (rockwool perlite) or relatively high CEC (consumer grade peat/perlite). The media-CEC therefore did not act as a buffer to nutrient levels available for plant uptake.

The conclusion of these and other experiments was that CEC from peat has little or no effect on either pH management or calcium and magnesium management in container grown crops. 

Other Media Components

Secondary components (used at less than 40 percent of the total volume) for container media have little effect on buffering because they either have almost no CEC (perlite, polystyrene, rockwool, sand) or have such a low bulk density that its effect is minimal (vermiculite).

Calcined clay is an exception because it has a fairly high bulk density (about ½ that of the typical field soil) and CEC. But because of cost, calcined clay is typically added to container media at 5 percent or less of the total volume. Although calcined clay may affect buffering, the combination of lower bulk density and much lower incorporation rate means that its effect is limited.

CEC can be increased when a field soil is added to a container medium, which increases buffering of pH and nutrient levels because the medium has a high bulk density (weight). However, because of problems obtaining consistent and uncontaminated field soil, and the problems for freight and retail from having a heavy soil, most growers are no longer using soil in container media. 


In contrast to field soils, CEC of soilless media has little effect on resisting change in pH or in supplying calcium or magnesium to a crop. However, buffering does exist in soilless root media. In the next articles we explain how limestone can be used to buffer pH, calcium and magnesium.

Leave a Reply

More From Fertilization...

April 1, 2015

Philadelphia Flower Show Draws More Than 250,000 Attendees With Disney Pixar Movie Theme

With more than 250,000 consumers attending the prestigious Philadelphia Flower Show in March each year, it's a great opportunity to get flowers and gardening products into the public eye. This year's show displays took on family favorites at the movies, with a focus on Disney and Pixar films. Check out some of the highlights in our slideshow.

Read More

April 1, 2015

Peace Tree Farms Grows Its Customer Base

Over the past five years, Peace Tree Farms in Kintnersville, Pa., has concentrated on growing its business by providing plant material for the displays at the illustrious Philadelphia Flower Show. We caught up with Peace Tree Farms’ Lloyd Traven to ask about how the Flower Show figures into his business plan.

Read More
protecting bees and pollinators video

March 31, 2015

New Video On Protecting Bees And Pollinators Educates Horticulture Industry Professionals

A new educational video that provides information on the horticultural industry’s essential role in bee and pollinator stewardship is one result of industry collaboration by the Horticultural Research Institute, AmericanHort, Society of American Florists and the American Floral Endowment. “Protecting Bees & Pollinators: What Horticulture Needs to Know,” narrates the current state of bee and pollinator health, provides information on factors that impact pollinators and the environment and underscores the beneficial role horticulture plays in providing healthy pollinator ecosystems.

Read More
Latest Stories

January 9, 2015

6 New Fertilizer Products For Healthy Plants

These five products add even more options for delivering nutrients to the root zone.

Read More

January 7, 2015

Fertilizers And The Future

As growers look for new ways to cut costs and conserve resources, fertilizer and equipment companies are offering products that strive to save water, reduce toxic runoff and keep chemicals out of the equation.

Read More

December 31, 2014

Gain Greater Control Of Fertilizer With Automated Ferti…

University researchers look at integrating irrigation and fertilization with the help of water sensors to reduce fertilizer treatments and improve application timing.

Read More

October 30, 2014

Basics & Beyond: Comparing Substrate Fertilizer Ame…

Cornell University researcher determines if substrate-incorporated slow-release fertilizers can be used to replace or reduce the need for liquid fertilizer for four spring crops.

Read More

July 24, 2014

Using Controlled Release Fertilizers To Produce Garden …

Researchers determined whether or not garden mums can be grown with controlled-release fertilizer, and if it reduces fertilizer leaching, as compared with water-soluble fertilizers.

Read More

March 14, 2014

New Foliage Pro Fertilizer Offers Complete Nutrition Pl…

Dyna-Gro Nutrition Solutions has developed a process it says is capable of keeping all 16 essential plant nutrients in solution form.

Read More

January 30, 2014

OASIS Grower Solutions Introduces New One-Bag Hydroponi…

The new 16-4-17 Hydroponic Fertilizer from OASIS Grower Solutions (OGS) is a one-bag solution that replaces two-part systems traditionally used by commercial hydroponic growers. It is specifically formulated for commercial hydroponic production of lettuce, herbs and vegetables.

Read More

December 30, 2013

Fertilizer Changes Growing Mix pH

When considering a fertilizer's influence on media pH, you need to know its acid or basic reaction.

Read More

December 30, 2013

Basics & Beyond: Fundamentals Of Phosphorus Nutriti…

Phosphorus is an essential element, after all.

Read More
Everris Liquid S.T.E.M.

December 30, 2013

New Fertilizers For 2014

New fertilizer products not only deliver optimum nutrition, they also provide for easier application and increased efficiency. Check out these new products to help your operation produce a healthy crop in 2014. Click through on the pages below.

Read More

December 18, 2013

Focus On Fertilizer: Micronutrients And Organics

New fertilizer products are focusing on micronutrients and providing efficient options for organic production.

Read More

August 27, 2013

BioWorks Adds EcoVita To The Verdanta Family Of Biofert…

EcoVita, a homogeneous granular organic fertilizer, has been added to Bioworks Inc.’s Verdanta biofertilizers product family. This fertilizer will be manufactured and supplied to BioWorks by DCM Corporation of Belgium, a producer of natural and organic-based fertilizers in Western Europe. EcoVita is suitable for a wide variety of crops with its gentle release curve including:• Organic fertilization as a base nutrition in potting mixes• Leafy vegetables• Fruiting vegetables (s a top dressing) • Roses and other ornamentals The new fertilizer offers long-lasting and continuous action for 75 to 100 days and contains organic phosphorus (5 percent P2O5) for fast rooting. Nutrients in EcoVita are gradually released by the soil microbes, in addition to producing humus for better rooting and less leaching. EcoVita is OMRI Listed, making it suitable for use in organic production. “We’re pleased to introduce EcoVita 7-5-10 as our organic NPK product to complement our organic products: K-Vita 2-0-20 […]

Read More

April 29, 2013

Rockwell Farms Introduces Ready-To-Pour Container Ferti…

Rockwell Farms has introduced Rockwell Farms Plant Food, a bottled liquid fertilizer that does not need to be diluted before use. Always looking for ways to help the consumer succeeed, Jason Roseman of Rockwell Farms says the operation is also always looking for ways to get consumers to come back and buy more plants and flowers. “We feel like fertilization is one of those things that can be very confusing,” Roseman says. “Not everyone does it, and not everyone knows what they’re supposed to do.” The solution: Rockwell Farms Plant Food. The formulation is 150 ppm of a 20-10-20 fertilizer and is sold in 24-ounce bottles. Rockwell recommends that one bottle be used to fertilize 1.5- to 3-gallon outdoor plant and flower containers every 21 days. The product’s signage shows a young patio gardener pouring the bottled fertilizer on a windowbox with the tagline, “Just pour on your way out […]

Read More

April 29, 2013

Plant Products To Be Purchased By MGS Horticultural And…

MGS Horticultural Inc., a full-service supplier of fertilizers, pest control products, seeds and substrates in North America, together with Haifa Chemicals, a global supplier of potassium nitrate for agriculture and industry, specialty plant nutrients and food phosphates, today announced a plan to purchase Canadian fertilizer and pesticides supplier Plant Products Co. Ltd. The deal is anticipated to close by the end of June 2013. MGS will acquire Plant Products’ Canadian distribution business, sales force and name. MGS plans to use both names (MGS Horticultural and Plant Products) in all communications going forward. MGS will maintain locations in Leamington, ON; Brampton, ON; Laval, QC; St. Hyacinthe, QC; and Detroit, Mich. As part of the deal, MGS has signed multi-year agreements with Haifa to maintain exclusive distribution of Plant-Prod Soluble Fertilizers, Acer Controlled Release Fertilizer, Stim-Root and potting soil premix fertilizers for distribution in Saskatchewan, Manitoba and Eastern Canada. “MGS is excited […]

Read More

March 19, 2013

Irrigation And Fertilizer Tips For New Vegetable Grower…

Compared to other variable costs, fertilizers are not the largest part of the budget in conventional greenhouse production. Nevertheless, over the course of the season, mistakes in fertilizer use can lead to significant damages or crop losses. That makes this an important topic for ornamental growers who are experimenting with growing vegetable crops. This article will emphasize major differences between fertilizers used in vegetable production and in ornamentals production. The different nutrition strategies, monitoring and water volume per plant will also be explained. Fertilizer In ornamental production, nutrients are delivered using various water-soluble fertilizers through a fertilizer injector, through the use of controlled-release fertilizers, or a combination of the two. There are numerous fertilizer mixes available with all the needed nutrients already included. The fertilization rate is often given in parts per million (ppm) of nitrogen (N), which is a way of expressing the fertilizer concentration. At younger stages, plants will […]

Read More

March 11, 2013

Daniels Plant Food Rebranded As Nature’s Source

Ball DPF has announced the launch of Nature’s Source, a rebranding in name and packaging for its seed extract-based plant fertilizer product Daniels Plant Food. The new brand will make its industry debut at California Spring Trials. “While we are proud of our heritage, we made this bold decision because we’re expanding sales to our existing market segments and entering new ones. It was a good opportunity to start with a fresh name and a modern look for all our products and packaging,” says Chance Finch, general manager for Ball DPF. “We wanted to make it obvious, beginning with our Nature’s Source brand name, to know our products are sourced from nature. Growers, contractors and gardeners can be confident that our effective and unique formulations remain unchanged. We’re excited about launching updated packaging for all our products, and especially the new ready-to-spray plant food for home gardeners.” The Nature’s Source brand […]

Read More

March 6, 2013

Everris Introduces E-Max Release Technology Coating Che…

Everris has introduced its new E-Max Release Technology, a proprietary coating chemistry for use on a wide variety of nutrient components that are incorporated into controlled-release fertilizer products. According to Chris Buchheit, marketing manager for Everris’ ornamental horticulture products, E-Max will help Osmocote and the company’s other existing brands deliver even better performance and value. “This coating will aid in the development of products that both complement and enhance our Osmocote portfolio and other fertilizer lines. It will increase Everris’ flexibility to create customized nutrition programs designed for horticulture growers,” Buchheit says. E-Max Release Technology is a durable, cutting-edge, reacted polymer coating for use on a variety of essential macro- and micronutrients. Nutrients coated with E-Max Technology are produced to the same performance standards as Osmocote. It will allow for continued expansion of the Everris portfolio, the ability to further enhance nutritional values and an efficient use of a grower’s […]

Read More

February 4, 2013

BioSafe Systems And Daniels Plant Food Company Will Par…

BioSafe Systems has entered into a strategic partnership with Daniels Plant Food Company, a wholly owned subsidiary of Ball Horticulture Inc. BioSafe and Daniels have worked closely together for the past two years and will now look to further develop liquid plant food serving both the conventional and organic markets. In conjunction, BioSafe Systems will be introducing its own branded line of plant food products focusing on turf, landscape and agricultural markets. “Liquid plant food is a natural progression for our company” says Rob Larose, CEO of BioSafe Systems. “It fits perfectly into our current line of green and sustainable products, and we are excited about partnering with Daniels.” Daniels manufactures and markets both conventional and organic liquid fertilizers, using botanical extracts to provide high-value nutrition to plants. BioSafe Systems develops and markets effective and sustainable products to a wide variety of industries, including fruit and vegetable production, turf and […]

Read More