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CEC and your soil

There are glacial soils in Canada that are recognized to be some of the most fertile soils in the world, simply because they are relatively young, contain good organic matter levels and have developed high cation exchange capacity [CEC] clays. These soils have not been impacted by the inevitable decline that comes from millions of years of rainfall. The freezing and thawing cycles, along with wind erosion, biological activity and rain all eventually break down the parent material of our soils into smaller and finer particles. The fine clay particles are worked on by the weather patterns and plant roots, which eventually degrade the clays capacity to exchange the cations. A low CEC clay is the result.

But what exactly is a cation exchange capacity? CEC is a measurement of how strongly soil particles can hold onto and exchange their nutrients. It is largely the clay and humus [organic matter] colloids in our soils that contribute to soil CEC. Clay soils tend to have very high levels of negatively charged mineral components that are able to hold onto and exchange cations. These cations are nutrients that have a positive charge, such as calcium, magnesium, sodium and potassium. They are able to ‘bind’ with the negatively charged sites that can generally be found on both the clay and humus particles. It is the amount of humus and the types of clay that determine the cation exchange capacity that a particular soil has.

Just as the amount of rain washing through the soil has an impact over the long term on leaching cation nutrients from their exchange sites, so do plant rootlets and microbes impact these nutrients, albeit in a much more immediate manner. The electrical charge that binds the positive cations to the negatively charged sites are weak, and there is a somewhat continuous oscillation of these cations on the surface of the particles as they are pushed around by the other charged particles in the soil solution around them. The plant rootlets and the various soil microbes are actually able to exude hydrogen ions [positively charged particles] that are able to push the cation ion off its negative charge, freeing it from the clay or humus particle and making it available to be taken up by the plant or microorganism.

When CEC levels are low, mineral and nutrient levels are negatively impacted and reduced. This causes plants to expend considerable energy in their efforts to scrounge for the nutrients that are necessary to develop roots, flowers and seeds.

High organic matter is found in soils that are able to support high levels of vegetation development. This organic matter is eventually broken down due to the moisture, oxygen and temperature levels. As this degradation of the organic matter progresses, the particles become smaller and simpler. But due to the continuation of this process, as well as the impact of soil microorganisms, fungi, earthworms and other soil biology, this process reverses and very complex carbon structures called humus or colloids are created. Providing that the soil has adequate sulphur, and a reasonable balance of minerals, humus increase in the soil. Given that humus can contain greater CEC than clays, it is very important in proper soil management.

Sandy soils, which by definition are lacking in clay minerals, have very low CEC. Not only is the ability of cation holding clay particles missing, but often organic matter does not have the chance to be transformed into humus in these soils. Instead, organic matter is often decomposed completely, which results in its being off-gassed as Co2 and ammonia.

But all soils age, and with this CEC levels can decline. Weather alone can eventually erode the capacity of clay particles, reducing the ability of these soils to hold onto and maintain these vital nutrient holding capacities. Humus can also be eroded. This is very easily done when salt based fertilizers are relied upon and when organic matter is not returned to the soils, which reduces the levels of soil biology. This creates a viscous cascading cycle which sees very little humus developments in soils, which further reduces the nutrient content in soils, which results in the increased reliance on salt based fertilizers.

And don’t get caught up in the think that an abundance of organic matter in our soils is an effective strategy in building up the humus levels. This approach works poorly. Far better to build humus in the compost pile. There is a far greater amount of biology in a well built and maintained compost pile that is mandatory for proper humus development. In fact, the quality range of compost is much broader than most people think. You are not just looking for broken down organic matter, earthworms and a nice smell to your compost. You want to take the whole process through the cycle of organic degradation into very simple carbon and sugars, back up again into quite complex carbon chains. It is only at this point that humus is formed and with it, the powerful capacity of nutrient maintenance. But this is a subject for another day!


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