Cation Exchange Capacity (CEC)
Pumpkinheads:
Cation exchange capacity (CEC) is a measurement of the cation nutrients the
soil is able to store on its clay and humus particles. These tiny particles,
known as colloids, have a large number of negatively charged sites all over
their surfaces. Positively charged cations are held on these sites, largely
protected from leaching away in water but still available to plant roots.
Plants give off hydrogen ions, a waste product that is positively charged, in
exchange for needed nutrients like calcium, magnesium and potassium.
Nutrients held in colloid exchange sites may not show up in soil tests
because they are not easily dissolved in water, but they are still available
to plants through direct contact between roots and soil colloids, humus and
clay.
CEC is measured by how many thousandths of a gram of hydrogen can be held by
100 grams of dry soil. This means that if 100 grams of dry soil can hold 50
thousandths of a gram of hydrogen, that soil has a CEC of 50. Different kinds
of clays have CECs that range from as little as 10 to as much as 100, while
the CEC of pure humus can approach 200. Very sand soils have a CEC of 5 or
less.
Think of you soil's CEC as a kind of nutrient savings account. As nutrients
are withdrawn, whether by removing crops or through prolonged action of water
leaching down through the soil, it is important to replace them to maintain
your reserves. A soil with a high CEC but depleted nutrients will require
greater applications of mineral nutrients to restore its fertility than will
a similarly depleted soil with a low CEC. Knowing your soils CEC will help
you to better understand and interprete your soil test recommendations. Keep
in mind that a very sandy soil will have a low CEC while soil rich in humus
or clay will have a high CEC.
Now for my question to Chris, if you had your druthers what would you like
the CEC of your soil to be??? And how difficult is it to manipulate the CEC?
And is it realistic to even think about it?
Marv in Altoona.
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