Cation Exchange Capacity (CEC)

A ‘Cation’ is a positively (+) charged ion; an ‘Anion’ is a negatively (-) charged ion.  The cation exchange capacity, (CEC), is major measure of potential for the soil’s inherent fertility.  The CEC indicates the soil’s ability to store cations, which include some of the major plant nutrients:  (K⁺), for example.

The CEC depends mostly on a soil’s colloidal content.  Colloids in soil are gelatinous substances made of clay particles, raw organic matter, and humus.  They have a large surface area relative to their weight, and many negatively charged anion sites.  The positively charged cations are held in place at these exchange sites, (opposite charges attract one another), safe from leaching but available to the plant roots.

Cation exchange capacities are expressed in milliequivalents, (mEq), (how many thousandths of a gram of hydrogen, or it’s equivalent charge, can be held by 100 grams of dry soil).  The higher the soil’s CEC, the greater the quantity of mineral nutrients needed to fill its reserves, but the longer they will be held available in the soil.  CEC’s in western soils generally range between 10 - 30 mEq, whereas a predominantly sandy soil will have a CEC of below 4 mEq.  A soil with a low CEC can be significantly improved by adding more humus.  Pure humus has a CEC value of about 100 mEq per 100 grams.

Base Saturation Ratio (Cation Balance)

The major cation (+) nutrients are Calcium, Magnesium, Potassium and Sodium.  To assure adequate plant nutrition they must be present in certain minimum amounts, which must also be balanced.  The excess of any one mineral, for example Magnesium, may interfere with the availability of another such as Potassium, even if the lab tests show adequate Potassium levels.

Base saturation refers to the percentage of a soil’s CEC occupied by the base elements, (cations other than hydrogen or aluminum).

Some efforts have been made to find the “ideal” cation balance or base saturation ratio; they are:  potassium 3 to 5%, magnesium 10 to 15%, calcium 65 to 75%.

In most cases it is not necessary to completely saturate the exchange complex, (exchangeable sites), with these exchangeable base elements.  Usually an 80% to 90% saturation of the CEC with a balanced ratio of exchangeable bases will be adequate for high yields of most crops.