Spectrum Agronomic Library

CEC, as reported by nearly all soil testing laboratories, is a calculated value that is an estimate of the soils ability to attract, retain, and exchange cation elements. It is reported in millequivalents per 100 grams of soil (meq/100g).

In order for a plant to absorb nutrients, the nutrients must be dissolved. When nutrients are dissolved, they are in a form called “ions”. This simply means that they have electrical charges. As an example table salt is sodium chloride (NaCl), when it dissolves it becomes two ions; one of sodium (Na⁺) and one of chloride (Cl^-). The small + and - signs with the Na and the Cl indicate the type of electrical charges associated with these ions. In this example, the sodium has a plus charge and is called a “cation”. The chloride has a negative charge is called an “anion”. Since, in soil chemistry “opposites attract” and “likes repel”, nutrients in the ionic form can be attracted to any opposite charges present in soil.

Soil is made up of many components. A significant percentage of most soil is clay. Organic matter, while a small percentage of most soil is also important for several reasons. Both of these soil fractions have a large number of negative charges on their surface, thus they attract cation elements and contribute to a higher CEC. At the same time, they also repel anion nutrients (“like” charges).

Some important elements with a positive electrical charge in their plant-available form include potassium (K⁺), ammonium (NH₄⁺), magnesium ( Mg⁺⁺), calcium (Ca⁺⁺), zinc (Zn⁺), manganese (Mn⁺⁺), iron (Fe⁺⁺), copper (Cu⁺) and hydrogen (H⁺). While hydrogen is not a nutrient, it affects the degree of acidity (pH) of the soil, so it is also important. Some other nutrients have a negative electrical charge in their plant-available form. These are called anions and include nitrate (NO₃^-), phosphate (H₂PO₄^- and HPO₄^–), sulfate (SO₄^-), borate (BO₃^-), and molybdate (MoO₄^–). Phosphates are unique among the negatively charged anions, in that they are not mobile in the soil. This is because they are highly reactive, and nearly all of them will combine with other elements or compounds in the soil, other than clay and organic matter. The resulting compounds are not soluble, thus they precipitate out of soil solution. In this state, they are unavailable to plants, and form the phosphorus “reserve” in the soil.

Larger CEC values indicate that a soil has a greater capacity to hold cations. Therefore, it requires higher rates of fertilizer or lime to change a high CEC soil. When a high CEC soil has good test levels, it offers a large nutrient reserve. However, when it is poor, it can take a large amount of fertilizer or lime to correct that soil test. A high CEC soil requires a higher soil cation level, or soil test, to provide adequate crop nutrition. Low CEC soils hold fewer nutrients, and will likely be subject to leaching of mobile “anion” nutrients. These soils may benefit from split applications of several nutrients. The particular CEC of a soil is neither good nor bad, but knowing it is a valuable management tool.

The following, is data on how CEC is calculated at Spectrum Analytic.

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