CEC

Cation-Exchange Capacity Indicates soil’s Ability to Retain Nutrients.

Cation-exchange capacity (CEC) is a measure of how many cations can be retained on soil particle surfaces.

CEC affects many aspects of soil chemistry, and is used as a measure of soil fertility, as it indicates the capacity of the soil to retain several nutrients (e.g. K+, NH4+, Ca2+) in plant-available form.

Cations are held by negatively charged particles of clay and humus called colloids. Colloids consist of thin, flat plates, and for their size have a large surface area. For this reason they are capable of holding huge quantities of cations. They act as a storehouse of nutrients for plant roots.

You can improve CEC by applying lime and increasing the pH. However, increasing organic matter is the most efficient way of improving cation-exchange capacity.

Cation-Exchange Capacity of Different Soil Types

A cation is a positively charged ion, with fewer electrons than protons, while an anion is negatively charged, with more electrons than protons. Because of their opposite electric charges, cations and anions attract each other and readily form ionic compounds.

The cations used by plants in the largest amounts are calcium, magnesium, and potassium.

Common soil anions include: chlorine, nitrate, sulfate and phosphate.

Sand

Sandy soil carry less negative charge. It has a lower CEC and less binding sites for cations and water.

Clay

Clay colloids carry negative charges and therefore attract clay particles. Clay soil is built from smaller particles than sandy soil. Clay soil has higher CEC and more binding sites for cations and water.

Organic matter

Organic matter can have a 4 to 50 times higher CEC per given weight than clay.

The source of negative charge in organic matter is different from that of clay minerals; the dissociation (separation into smaller units) of organic acids causes a net negative charge in soil organic matter, and again this negative charge is balanced by cations in the soil.

Organic matter contains 58 % organic carbon. High soil organic carbon (SoC) levels are associated with high CEC.

CEC Improves With Higher pH Levels

Soil pH is essential for CEC because as pH increases and soil becomes less acid, the number of negative charges on the colloids increase, hence increasing CEC.

Aluminum (Al+++) and sodium (Na++) cations cannot be used by plants for nutrition. Aluminum is not available as a cation when soil pH is over 5 because it is precipitated out of the soil. It is only at pH levels below 5 that it may become available as a cation, and under 4.5 may become available in toxic levels. This is one reason why it is important to maintain pH levels at 5.0 or more.

If exchangeable sodium (Na++) is present in quantities greater than about 5% of the CEC, it makes the clay particles unstable in rainwater, which can be observed as dispersion or cloudiness in water. Dispersive soils have poor water entry and drainage and set hard on drying.