Why Cation Exchange Capacity (CEC) Matters

September 27, 2021

As fall approaches and crop harvest is underway, the replacement of removed nutrients through fertilizer applications is an important consideration. Soil fertility discussions are often about the soils ability to hold and exchange the important nutrients of hungry crops, particularly positively charged nutrients such as potassium (K), nitrogen (ammonium), and magnesium (Mg). A simple description of a soil’s ability to store a group of nutrients or cations, is called the soils Cation Exchange Capacity (CEC).

When thinking of CEC, it is important to understand the makeup of the soil in question. Soils are composed of a mixture of sand, silt, clay, and organic matter. Clay and organic matter are the most important components of the soil and have a net negative charge and therefore can attract and hold positively charged particles such as K. I often think of CEC as the soil bank because the cations on the soil's exchange sites (CEC) serve as a source of resupply (cash) for cations currently in soil solution which are being actively removed by plant roots. The higher the CEC (more clay and OM), the more cations can be stored and supplied (Figure 1).

Figure 1. Diagram of Cation Exchange Capacity with positive charged ions attaching to negatively charged soil particles. Adapted from International Plant Nutrition Institute (IPNI) Soil Fertility Manual 2006.
Figure 1. Diagram of Cation Exchange Capacity with positive charged ions attaching to negatively charged soil particles. Adapted from International Plant Nutrition Institute (IPNI) Soil Fertility Manual 2006.

Additionally, CEC is important to understand when lime is required to adjust soil pH. Lime recommendations will vary for soils with differing CEC capacities. Soils with a high CEC are desired because they have a greater buffering capacity against changing pH levels; however, when needed, they require more lime to adjust the pH.

In the same capacity, CEC also influences N and K fertility programs (Figures 2 and 3). Extra management is required on low-CEC soils (sands) as leaching of cations can occur, particularly with fall applications. In these situations, spring or as needed in-season applications can be more effective in supplying needed nutrients. Conversely, high CEC soils allow you to store or bank nutrients in the soil for a more dependable nutrient supply to the crop.

Therefore, as you begin to plan your fertility and lime programs, be sure to reference each fields CEC score and plan according to the soil’s ability to store and release (cation) nutrients.

Figure 2. Soybean plants showing symptoms of potassium deficiency.
Figure 2. Soybean plants showing symptoms of potassium deficiency.
Figure 3. Corn leaf showing symptoms of potassium deficiency.
Figure 3. Corn leaf showing symptoms of potassium deficiency.

Sources

Ketterings, Q., Reid, S., and Rao, R. 2007. Cation exchange capacity (CEC). Agronomy Fact Sheet Series. Fact Sheet 22. Cornell University Cooperative Extension. Cornell University. http://nmsp.cals.cornell.edu/publications/factsheets/factsheet22.pdf

Mengel, D.B. 1993. Fundamentals of soil cation exchange capacity (CEC). AY-238. Soils (Fertility) Agronomy Guide. Purdue University Cooperative Extension Service. Purdue University. https://www.extension.purdue.edu/extmedia/ay/ay-238.html.

Gelderman, R. 2013. Soil pH, soil CEC and root traffic. Basics of Soil Fertility. Nutrient, Soil, and Water Management Conference, Fergus Falls, MN. https://mawrc.org/downloads/Ron%20Gelderman,%20Soil%20pH,%20CEC%20and%20Root%20Traffic,%20Nutrient%20Mgt%20Conf%202-19-13.pdf.

International Plant Nutrition Institute (IPNI). 2006. Soil Fertility Manual.

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