Cation Exchange Capacity (CEC)

Definition - What does Cation Exchange Capacity (CEC) mean?

Cation exchange capacity, or CEC, is a rating of how well soil or other types of grow media can hold plant nutrients. The plant nutrients are measured as cations, and examples of cations include potassium, calcium, and other positively charged ions.

Knowing the CEC of your soil or grow medium is important for several reasons, but the primary one is for fertilization. Higher CECs mean that fertilization needs to occur much less frequently, or you risk overfertilizing plants. Soil with a low cation exchange capacity is generally poor in nutrients.

MaximumYield explains Cation Exchange Capacity (CEC)

Soil varies considerably not only in its mineral makeup, but in its ability to store nutrients. This ability is called its cation exchange capacity, often abbreviated as CEC. It sounds complicated, but it’s really nothing more than the soil’s ability to store positively charged ions of plant nutrients.

A “cation” is a positively charged ion of a particular mineral. Negatively charged ions are called anions. The most commonly needed cations within soil (or other growing mediums) are calcium, phosphorus, magnesium, potassium, hydrogen, sodium, and ammonium.

Soil is able to hold cations because it generally has a negative charge. The higher that negative charge, the more nutrients the soil can hold (the greater the negative charge, and the stronger the attraction to positively charged ions/cations).

The more nutrients soil can hold, the healthier plants growing in that soil will be (if all other things are equal, of course). However, different soil types have different cation exchange capacities. For instance, light-colored sand only has a CEC of three to five millequivalents per 100 grams of soil. Dark-colored sand has 10 to 20. Light loam and silt loam also have 10 to 20, while dark loams have 15 to 25. Silty clay loams and silty clays have 30 to 40. Organic soils have the most, coming in at 50 to 100.

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