Cation Exchange Capacity (CEC) is the soil’s ability to adsorb positively charged nutrient ions on negatively charged clay and organic matter surfaces.

Definitions:

Cation:
Element or molecular compound with a positive charge, such as Ca2+, Mg2+, K+, NH4+, etc.
Exchange:
To trade places.
Capacity:
The maximum amount something can contain.
Cation Exchange:
Occurs when one cation desorbs from a negatively charged clay or organic matter surface and is replaced by another cation, or multiple cations, of equal charge. For instance, if Ca2+ desorbs, either one Mg2+ or two K+ can replace it.
Cation exchange is one of the main soil processes determining plant nutrient availability. Positively charged essential nutrients including ammonium, calcium, potassium, and others are held on negatively charged exchange sites until they are replaced with other ions to maintain balance between elements in soil solution. For instance, if calcium concentration in soil water drops too low, calcium ions on exchange sites will detach to replace those that were lost.
Negatively charged nutrients, such as nitrate (NO3), cannot adsorb to clay surfaces, so they move freely with soil water until they are taken up by plants or until they leach from the root zone.

Soils with high CEC can store large nutrient reserves, benefiting crops by slowly releasing cations into soil solution to replenish those absorbed by plants (see diagram). Soil pH, mineralogy, texture, and percent organic matter determine the soil’s CEC. Soils high in clay and organic matter have high CEC and large nutrient reserves, while sandy, low organic matter soils do not retain large quantities of nutrients. Knowing the soil’s CEC helps Tri-Tech advisors determine appropriate fertilization rates for each field.


salt accumulation chart

pH

pH, defined as the negative logarithm of the hydrogen ion activity, measures the acidity or alkalinity of an aqueous solution. pH is on a logarithmic scale, so a one unit increase corresponds with a 10 fold increase in H+ activity, usually estimated by H+ concentration. pH 7 indicates a neutral solution. Above pH 7 the solution is basic, and below 7 the solution is acidic.

pH = -log [H+]

Soil scientists consider pH a “master variable” driving almost all soil processes. pH influences crop production through its effects on nutrient availability, biological activity, cation exchange capacity, and more. pH affects nutrient availability by governing mineral solubility in soil water. Plant roots absorb the minerals dissolved in soil solution, but if pH is too high or too low, essential plant nutrients precipitate out of solution, preventing root uptake. In Ventura County, high pH soils may cause phosphorus, iron, and zinc deficiencies. Above pH 7.2 calcium and magnesium availability also decreases as the cations precipitate as calcium and magnesium carbonates (CaCO3, MgCO3). Soluble iron ions Fe3+ and Fe2+ decrease a thousandfold and a hundredfold, respectively, for every one unit increase in pH (Tisdale et al. 8th edition). By maintaining a pH near 6.5, growers can maximize solubility for most of the essential elements.


To learn how pH affects CEC and nutrient availability read this blog post.

Blog Link:

How pH Affects Nutrient Availability

pH affects all aspects of soil health and fertility, but one important impact is the tradeoff between cation exchange capacity, nutrient retention, and mineral solubility. When managing pH, growers want to optimize cation exchange capacity […]

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