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What is CEC and Why Is It Important? Part.2

In part 1 of our discussion on cation exchange capacity (CEC), we defined CEC and its function in soil and soilless media. As we mentioned, CEC has a significant impact on fertilizer and pH management in clay soils. However, in soilless growing media, it has little impact on fertility and pH management.

In this article, we will discuss why CEC is not very important when it comes to soilless growing media.

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CEC expressed on a weight vs. volume basis

The typical CEC values of soil and various media components are listed in Figure 1 below.

Component / Mix CEC (meq / 100g)
Fine clay 56-63
Coarse clay 22-52
Silt 3-7
Sphagnum peat 100-120
Peat humus 200
Vermiculite 150
Perlite 1.5
Aged bard 40-60
50% Peat, 50% Vermiculite 141

Figure 1. Cation exchange capacities of soil, various growing medium components and a soilless medium.
Source: Bunt, A.C.  1988.”


When looking at Figure 1, it is apparent that the CEC of peat moss and vermiculite are higher than clay. This is true on a weight basis as the CEC is expressed as meq / 100 grams. However, containers are filled based on their volume and not the weight of the growing medium they hold. Expressing CEC on a volume basis is more accurate as containerized plants only have access to nutrients within the growing medium inside the container.

Values change on a volume basis

Therefore, when CEC is expressed on a volume basis, the numerical values change as seen in Table 1 below:

Measurement Field Soil 1 Peat: 1 Vermiculite
CEC (meq/100 g) 20 141
Bulk Density (g/cm3) 1.3 g/cm3 0.1 g /cm3
Container Volume (cm3/pot) 1250 cm3 1250 cm3
CEC (meq/pot) 325 176

Table 1. This table contrasts the CEC of a container filled with field soil and a 50% peat/50% vermiculite growing medium.
Notice that by weight (first row), CEC is higher for the peat/vermiculite growing medium, but when compared on a volume basis (last row), the field soil has a larger CEC capacity. Source: Biernbaum, J.A. 1992.”


Notice in the first row of Table 1 that field soil with a CEC of 20 meq/100g is much lower than a CEC of 141 meq/100g for a 50% peat/50% vermiculite growing medium. As stated, this is on a weight basis. The second row of the table shows that the bulk density of the field soil is 13 times heavier than the peat/vermiculite growing medium. If these two materials are added to individual pots of equal volume (1250 cm3), then the CEC of the field soil, on a volume basis, is 325 meq/pot, which is almost twice as high as a CEC of 176 meq/pot for the peat/vermiculite growing medium. It is often true that the CEC of a growing medium is lower than clay soil when expressed on a volume basis.

CEC shrouded by other variables

Research from John Biernbaum and Bill Argo showed that although the CEC of a soilless growing medium had some importance in nutrient retention and to help buffer against minor pH changes, other variables had greater influence on the pH and nutrient status of the growing medium.

Limestone charge

First, the pH of the growing medium is significantly influenced by the limestone charge in the growing medium, the potential acidity or potential basicity of the fertilizer applied, and the alkalinity (carbonate and bicarbonate content) of the water. Manipulating one or all three variables has greater influence on the long-term pH of the growing medium as opposed to the CEC.

Calcium and magnesium retention

Second, they found that calcium and magnesium retention by CEC sites may be considered significant in soilless media with high CEC, but it is not significant in comparison to the quantities of calcium and magnesium that need to be applied to avoid nutrient deficiencies.

In other words, unlike clay soil, which can hold enough magnesium and calcium on its exchange sites to provide them in sufficient quantities for crops throughout the growing season, soilless growing media simply cannot retain enough of these nutrients, so additional applications must be made through the crop cycle. As a result, growers do not depend on the CEC to help provide nutrients for crops grown in a soilless growing medium.

pH of sphagnum peat moss

Third, the pH of the growing medium influences the CEC. For example, if the pH of sphagnum peat moss increases from 3.5 to 8.0, the CEC increases by 140 meq/l. As the pH of sphagnum peat moss rises, the harder to displace hydrogen ions eventually come off cation exchange sites and allow other elements, chiefly calcium and magnesium, to occupy these exchange sites.

However, how many crops can be grown at a pH of 8.0? Typically, most crops do best when the pH of a soilless growing medium is between 5.5-6.2, so the actual CEC maybe lower than published sources.

Supply nutrients through fertilization

Although a soilless growing medium can have a reasonable CEC, it simply does not contribute much towards nutrient retention or prevention of pH change, unlike clay soil. If nutrients are needed, growers need to supply them through fertilization and not depend on the cation exchange sites. Likewise, the pH of a soilless growing medium can be manipulated rather easily with limestone, a fertilizer application and/or water alkalinity.

If you have questions about CEC and how this relates to our line of PRO-MIX and peat-based products, please contact us.

References:

  • Argo, W.R. and J.A. Biernbaum. 1997. "The effect of root media on root-zone pH, calcium, and magnesium management in containers with impatiens." J. Amer. Soc. Hort. Sci. 122(2):275-284.
  • Table 1. Biernbaum, J.A. 1992. "Root-Zone Management of Greenhouse Container-Grown Crops to Control Water and Fertilizer." Hort Technology 2(1):127-132.
  • Figure 1. Source: Bunt, A.C.  1988. "Media and Mixes for Container Grown Plants." Second edition. Unwin Hyman Ltd., London

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