Just how important is CEC?

Monday, February 28, 2011

A hazy understanding of cation exchange capacity can lead to management decisions that are not necessarily good economics or agronomics

by KEITH REID

Cation exchange capacity, more commonly referred to as CEC, is one of those basic soil properties that is poorly understood by most non-soil scientists. This may mean that CEC is ignored as a factor in soil management. Too often, this lack of knowledge leads to some very strange and expensive decisions regarding fertilizer applications.

What is CEC? Many of the important nutrients for plants, like potassium, calcium and magnesium, are positively charged ions, or cations ("Cat-eye-ons"). Since opposite electrical charges attract, these are held in the soil by attraction to the negative charges on clay minerals and organic matter. This is not a strong chemical bond, so the cations held on the soil can swap places with cations in the soil solution.

While we think of soil chemistry as being rather static, this exchange is actually happening continuously, so there is equilibrium between concentrations in the soil solution and held on the soil. CEC is simply the measure of the amount of negative charge in the soil that can hold on to the cations.

Plants absorb nutrients from the soil solution but, as they take in one ion, they release another with the same charge to stay electrically neutral. When a root takes up a potassium ion, it releases a hydrogen ion, so the concentration of potassium in the soil solution decreases and the concentration of hydrogen increases. The hydrogen then exchanges with potassium on the soil surfaces, replenishing the supply available for uptake in the soil solution.

CEC varies considerably between different soil types, depending on the amount and type of clay in the soil, and the amount of organic matter. As a rough rule of thumb, each per cent of clay in the soil will add about half a unit of CEC, and each per cent of organic matter will add about two units. Soils with low CEC will tend to have low native fertility, since they don't have the capacity to hold on to a lot of nutrients, and will also become acid more quickly than soils with high CEC. This is because there are few basic cations to buffer the addition of hydrogen ions from natural processes.

Measuring CEC. Measurement of CEC involves flooding the soil with a single cation (often barium), then washing out any excess and extracting what remains with a different cation so the amount held on the soil can be measured. While most labs will offer this test, it is more complex and time-consuming than a regular soil test, and it is not what is reported on most soil test reports.

More commonly, labs will estimate the CEC by adding up the cations extracted during the regular soil analysis, including a factor for the amount of hydrogen ions from the buffer pH. This estimate is relatively close to the measured CEC on acid soils, but will give inflated values on most alkaline soils. These hold a lot of calcium and magnesium in carbonate minerals, which are dissolved in the soil extraction and generate much higher numbers for these minerals than that held on the soil.

Changing CEC. Since CEC is an inherent soil property, it is not easily changed unless you are adding either clay or organic matter to the soil. For some very sandy soils, increasing organic matter is the only practical way to increase the capacity of that soil to hold nutrients. Part of the CEC from organic matter is dependent on pH, since hydrogen will bind tightly to some of the organic acids in an acid soil but becomes exchangeable when the soil is alkaline. In this case, liming the soil will actually increase the CEC.

More commonly, when a grower is asking about increasing CEC, he is asking about adding a calcium or magnesium source to increase the calculated number used in the lab estimate. While it is possible to change the numbers in this way, the actual change in the chemistry of the soil is pretty minimal. This is an example of how a hazy understanding of CEC can lead to management decisions that are not necessarily good economics or agronomics. BF

Keith Reid is soil fertility specialist with the Ontario Ministry of Agriculture Food and Rural Affairs based in Stratford. Email: keith.reid@ontario.ca