Cation Exchange Capacity (CEC): What Does It Mean?

​You might remember seeing cation exchange capacity in soil test reports. Recently, I have been asked about what it means. This is one of the important soil characteristics that we need to understand. 

Firstly, we need to know what cations are. Cations are positively charged ions. There are several. The ones very important for plant growth are calcium (Ca2+), magnesium (Mg2+) and potassium (K+). Soil particles are negatively charged. They can hold positive charged cations. When plants absorb nutrients from the soil solution, these bound nutrient cations are exchangeable with other cations in the soil solution and become plant available. Therefore, CEC describes the soil’s capacity to supply nutrient cations to the soil solution for plant uptake.

Sands do not have the quantity of negative charges that clays and organic matter do. Thus sandy soils generally have the lowest CEC. Soils with lower CEC have less ability to retain cation nutrients, and have higher risk for nutrient leaching. As a result, vegetables grown on sandy soils are more likely to show potassium and magnesium deficiencies. Applying fertilizers a little at a time, and multiple times can reduce the problem.

CEC is expressed as meq/100 g (milli-equivalents per 100 grams of soil) or cmol/kg (centimol positive charge per kg of soil). The two expressions are numerically the same. It is not surprising to see CEC less than 5 for sandy soils and approaching 100 for organic soils. To improve CEC, the most effective way is to enhance soil organic matter, which can be done by growing cover crops or adding compost.

Another important point is that soil pH decrease due to crop production is faster with lower CEC soils. So sandy soils need to be limed more frequently than clay soils. Remember, it takes 6 to 12 months for the lime to dissolve completely, so fall is a good time to consider adding lime. I will explain liming in another article.

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