What are Cation Exchange and Cation Exchange Capacity?
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What are Cation Exchange and Cation Exchange Capacity?
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Soil clay minerals and organic matter tend to be negatively charged, thus attracting positively charged ions

(cations) on their surfaces by electrostatic forces. As a result, the cations remain within the soil root zone and

are not easily lost through leaching. The adsorbed cations may easily exchange with other cations in the soil

solution, hence the term “cation exchange.” The adsorbed cations replenish the ions in the soil solution when

concentrations decrease due to uptake by plant roots.

Cation exchange capacity (CEC) is a measure of the total negative charges within the soil that adsorb plant

nutrient cations such as calcium (Ca2+), magnesium (Mg2+) and potassium (K+

). As such, the CEC is a property of

a soil that describes its capacity to supply nutrient cations to the soil solution for plant uptake. Figure 1 illustrates

cations retained on soil clay minerals that can exchange with those in the soil solution. Plant roots can remove

nutrients from the soil solution, which results in nutrients moving away from the clay particles. Addition of

fertilizer to soil causes an initial increase in

nutrient concentration in the soil solution,

which results in nutrients moving toward

clay particles.

The nutrient cations plants use in the largest

amounts are potassium (K+

), calcium (Ca2+)

and magnesium (Mg2+). Other cations

adsorbed on exchange sites are ammonium

(NH4

+

), sodium (Na+

), hydrogen (H+

),

aluminum (Al3+), iron (Fe2+ or Fe3+),

manganese (Mn2+), copper (Cu2+) and zinc

(Zn2+). Micronutrient cations such as zinc,

copper, iron and manganese are typically

present at very low concentrations in

soils. Ammonium concentrations are also

typically very low because microorganisms

convert ammonium to nitrate in a process

called nitrification.

1. Schematic diagram showing exchange of cations

between the soil surfaces and the soil solution, and the movement

of these cations from soil solution to roots (rhizosphere) for uptake.

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