Cation Exchange Capacity (CEC) and Total Exchange Capacity (TEC) are terms used to describe the mineral/nutrient storage capacity of a soil. It is based on the assumption that very small soil particles (colloids*) hold onto the bulk of the soil's nutrients.

Soil colloids tend to be negatively charged and like a magnet, they attract positively charged elements (cations) onto their surface. But soil minerals are not static, rather they are continually swapping places - or in other words 'exchanging' places. 

Colloids have enormous surface areas relative to their volume, and the more colloids present in a soil, the higher the potential nutrient storage capacity of the soil. Although this is a rather simplistic view of soil, and it tends to ignore the additional role of humates/carbon in the soil profile, it does explain some of the likely interactions that do occur. 

Clay soils have more colloids, hence a high capacity to store cations, whereas sand has fewer colloids, hence a lower capacity to store cations. The CEC/TEC can therefore be used as a guide in predicting soil type/texture. Often there is surprise when the person analysing a soil test remotely 'guesses' your soil type accurately!

Texture            CEC (meq/100gm)

Sand                  <3

Loam Sand           3

Sandy Loam        10

Loam                10-15

Clay Loam        10-30

Clay                  >30 

Humus                250

Humic Acid         450

Fulvic Acid         1400 

Is it better to have high or low CEC/TEC soils?

There are pros and cons with both. High CEC soils require more inputs to 'fill' them up, but are more forgiving soils because their nutrient supply lasts a long time before problems show up. Low CEC soils respond quickly to even small amounts of inputs, but often require regular top ups.

Cations are stripped off soil colloids using extraction solvents, and then measured (meq/100gm**). Different laboratories use different solvents and techniques which can result in very different CEC/TEC figures. Brookside/Albrecht soil tests use solvents capable of stripping off very high quantities of cations, particularly in clay based high magnesium soils, with some tests coming back with CEC/TEC readings greater than 130!

It is important to note though that laboratories determine the CEC/TEC by calculating the total quantity of cations measured in a sample, and then the assumption is that all of these cations are derived from soil colloids. But soils can contain cations that are not part of the soil colloid complex. For example, excess calcium or magnesium can be found in soils after recent lime or dolomite applications to give false high CEC/TEC readings.

What is the difference between CEC and TEC?

The CEC is simply the figure given when all the measured cation meqs are added up together. This establishes the current or existing cation storage capacity of the soil.  The TEC takes into account the effect of hydrogen, to supposedly give a better reflection of the potential cation storage capacity of soil. In acid soils, a lot of the colloidal storage space is taken up by hydrogen instead of other cations, and because hydrogen is not considered a 'nutrient', the CEC is recalculated to determine how many more cations could attach to the colloid if hydrogen was removed. The result is a TEC figure. In acid soils, the TEC is higher than the CEC. In alkaline soils, where there is little or no excess hydrogen, the CEC and TEC are the same.

* The term 'colloid' generally refers to very small particles, but there does not seem to be a universally accepted size range to define a colloid - some state the particles should range from 10-1000 Angstrom (1 Angstrom = 10^-10m), and others allow particles up to several millimeters to be included. In all cases the surface area of the particles is enormous and can create some interesting phenomena when suspended in a medium - such as Van der Waal force and Brownian motion.

** Soil CEC is normally expressed in units of charge per weight of soil. Two different, but numerically equivalent sets of units are used: meq/100gm (milliequivalents of element per 100 gm of dry soil) or cmolc/kg (centimoles of charge per kilogram of dry soil). An equivalent is the amount of a substance that will react with a certain number of hydrogen ions. A milliequivalent is one-thousandth of an equivalent.

For more information:

• Search Google for 'CEC' and 'colloids'