CEC And Buffering
CEC has been used historically to describe the buffering capacity of soil (mainly based on crops grown in field soils). Buffering means the resistance to change in pH or nutrient concentration in the soil solution. Therefore, a medium that is high in CEC would help growers maintain a stable pH or nutrient concentration over time.Media particles, such as soil or peat, have negatively charged "exchange sites" that allow the particles to loosely hold onto positively-charged "cations" (Figure 1). Cations include acid (hydrogen H+), fertilizer cations (e.g. ammonium NH4+, calcium Ca2+, magnesium Mg2+, potassium K+), and other waste cations (e.g. sodium Na+). Media with high CEC have the ability to absorb and release large amounts of cations from the soil solution, which evens out high and low concentrations of nutrients available for plant uptake.
Let's take an example of how CEC affects liming rates of acidic sphagnum peats. The exchange sites before liming are mainly loaded with acid (H+). The higher the CEC of the batch of peat, the more lime that must be added to raise pH up to around pH 6. That is why the amount of lime needed to reach a target pH may vary between sources, or why finer peats (higher bulk density with more exchange sites per unit volume) have greater lime requirements than coarser peats (lower bulk density with less exchange sites per unit volume).
CEC also helps explain how lime reacts with the growing medium. During the dissolution of limestone, some of the CO3-2 from the limestone neutralizes the H+ contained in the soil solution to form carbon dioxide and water. As the H+ is reduced in the soil solution, additional H+ moves from the exchange sites to the soil solution and is neutralized by the lime. Finally, calcium (Ca2+) and magnesium (Mg2+) from the neutralized lime moves from the soil solution onto the peat exchange sites. The net result is that the exchange sites become saturated with mostly calcium and magnesium, and the concentration of H+ in the soil solution is reduced (higher media pH).
Figure 2. Effect of root media on medium pH. The grower grade peat was
long fibered with little dust and the consumer grade peat was a
more degraded fine peats with large amounts of dust. All media
were blended with 30 percent perlite and the lime source
was hydrated lime (low residual). The crop was impatiens grown
with a water-soluble fertilizer containing 50 percent
ammoniacal nitrogen and RO purified water. Acceptable pH ranges from
5.8 to 6.2. Research by Bill Argo and John Biernbaum.
How will CEC of the peat buffer pH during crop production? Let's assume a grower uses an acid-reaction fertilizer (e.g. 20-10-20). This fertilizer adds acid (H+) into the soil solution, which lowers the pH. However, some of that H+ in the soil solution is exchanged with the Ca2+ and Mg2+ on the peat. The acidity is removed from the soil solution, and the pH goes back up. In the process, Ca2+ and Mg2+ are also added into the soil solution.
How can CEC buffer nutrient concentrations? A growing medium can exchange nutrient cations back and forth between the exchange sites and the soil solution, in the same way as it can exchange H+ acid. Therefore, the exchange sites act as a back up "pool" of nutrients to recharge the soil solution when nutrient levels are low.
Media that have high CEC (more buffered) can resist a change in pH or nutrient concentrations for long periods of time, whereas pH or nutrient concentrations can change very rapidly in media that have low CEC (less buffered).