LAISHA Explains
Soil science

Cation exchange capacity and base saturation

Why clay and humus hold nutrients, what CEC and base saturation mean, typical values, how they are measured, and how to calculate them.

Exam essential7 min readUpdated
On this page
  1. Where the negative charge comes from
  2. Exchangeable cations
  3. Units
  4. Typical values
  5. How CEC is measured
  6. Base saturation
  7. Why CEC matters on the farm
  8. Key points
  9. What the exams ask
  10. Questions people ask
  11. Sources
  12. Related on Laisha
  13. Discussion

Cation exchange capacity (CEC) is the amount of positively charged ions a soil can hold on the negatively charged surfaces of its clay and humus. It is the soil's nutrient bank: calcium, magnesium, potassium and ammonium held there are protected from leaching yet can be released to roots.

CEC explains why sandy soils need fertiliser little and often, why clay soils need more lime, why organic matter matters so much in weathered tropical soils, and how sodium ruins soil structure. It appears in every soil chemistry syllabus, usually with a calculation.

Where the negative charge comes from

Soil colloids, the clay minerals and humus finer than about 1 µm, carry a net negative charge from two sources.

Permanent charge (isomorphous substitution). When clay minerals form, a lower-charged ion sometimes takes the place of a higher-charged one in the crystal lattice without changing its shape: Al3+ for Si4+ in the tetrahedral sheet, or Mg2+ for Al3+ in the octahedral sheet. Each substitution leaves one negative charge unbalanced. This charge does not change with pH. It is large in 2:1 clays such as smectite (montmorillonite), vermiculite and illite.

pH-dependent (variable) charge. At the broken edges of clay crystals, on iron and aluminium oxides, and above all on the carboxyl and phenolic groups of humus, hydrogen ions dissociate as pH rises, leaving negative sites. So the CEC of humus and of kaolinite and oxide-rich soils increases as pH increases, which is one reason liming a weathered soil raises its CEC.

Exchangeable cations

The negative sites attract a cloud of cations from the soil solution. These exchangeable cations can be replaced by others, one charge for one charge. That replacement is cation exchange.

A negatively charged clay particle holding exchangeable cations on its surface; a root releases hydrogen ions that exchange for potassium; the exchange reaction and the order in which cations are held Clay or humus surface negative charge holds cations against leaching clay particle −−−−−−−−−− −−−−−−−−−− Ca²⁺ Mg²⁺ K⁺ Al³⁺ Ca²⁺ H⁺ Ca²⁺ NH₄⁺ Na⁺ Mg²⁺ root hair H⁺ released K⁺ taken up soil solution Exchange is reversible and charge-for-charge Clay–Ca²⁺ + 2 K⁺ ⇌ Clay–2K⁺ + Ca²⁺ Held most strongly → least strongly: Al³⁺ > Ca²⁺ > Mg²⁺ > K⁺ ≈ NH₄⁺ > Na⁺ (H⁺ behaves like a strongly held ion) Higher charge and a smaller hydrated radius mean stronger holding.
A clay or humus particle holds a mixture of exchangeable cations. A root releases hydrogen ions, which displace potassium into the solution where the root can take it up. Exchange is reversible and balanced by charge, not by number of ions.Diagram: Laisha, original.

Three features matter in practice:

  • It is charge for charge. One Ca2+ occupies two negative sites; it takes two K+ to replace it.
  • It is reversible and fast. Adding a fertiliser salt pushes its cation onto the sites; roots pulling ions from the solution draw them off again.
  • Not all cations are held equally. Ions with more charge, and with a smaller radius once surrounded by water, are held more strongly. The usual order (the lyotropic series) is Al3+ > Ca2+ > Mg2+ > K+ ≈ NH4+ > Na+. Hydrogen behaves as a strongly held ion. Sodium, held weakest, is easiest to leach once displaced by calcium, which is the basis of reclaiming sodic soils with gypsum.

The cations are divided into two groups. Basic cations (often called "bases"): Ca2+, Mg2+, K+ and Na+. Acidic cations: H+ and Al3+ (which hydrolyses to release H+).

Units

CEC is expressed as the charge held per unit mass of soil:

Units of CEC
centimoles of charge per kilogram (cmolc/kg) = milliequivalents per 100 g (meq/100 g)

The two units are numerically identical, so older texts in meq/100 g can be read directly. To convert an exchangeable cation reported in mg/kg into cmolc/kg, divide by ten times its equivalent weight (atomic weight ÷ charge):

CationEquivalent weight (g)1 cmolc/kg equals
Ca2+40.08 ÷ 2 = 20.04200 mg/kg
Mg2+24.31 ÷ 2 = 12.16122 mg/kg
K+39.10391 mg/kg
Na+22.99230 mg/kg
NH4+18.04180 mg/kg

Typical values

MaterialTypical CEC (cmolc/kg)
Humus (soil organic matter)150 – 300, rising with pH
Vermiculite100 – 150
Smectite (montmorillonite)80 – 120
Illite (fine mica)15 – 40
Kaolinite3 – 15
Iron and aluminium oxides0 – 5
Sandy soil2 – 5
Loam10 – 20
Clay soil, 2:1 clay minerals25 – 50 or more

Two lessons follow. First, a soil's CEC depends on how much clay and humus it contains and which kind of clay: a highly weathered tropical clay dominated by kaolinite and oxides can have a lower CEC than a temperate silt loam. Second, organic matter carries several times the CEC of the same mass of most clays, so in weathered soils it supplies most of the exchange capacity, and losing it through erosion or burning shrinks the nutrient bank.

How CEC is measured

The standard method saturates every exchange site with one index cation, washes out the excess, then displaces the index cation and measures it.

  • Ammonium acetate at pH 7 (1 M NH4OAc) is the most widely used method. Because it is buffered at pH 7, it measures pH-dependent charge as if the soil were neutral, so it overestimates the CEC an acid soil actually has in the field.
  • Effective CEC (ECEC) is the sum of the exchangeable bases (extracted with ammonium acetate) plus exchangeable acidity (Al3+ and H+ extracted with unbuffered 1 M KCl). It measures the soil at its own pH and is preferred for acid, weathered soils.
  • Sum of cations methods add bases to exchangeable acidity measured at a fixed higher pH (often 8.2) and give the highest values.

Always state the method alongside the number.

Base saturation

Base saturation is the share of the CEC occupied by basic cations:

Base saturation
Base saturation (%) = (Ca + Mg + K + Na) ⁄ CEC × 100

It tracks soil acidity closely. Strongly acid soils may have base saturation below 25 per cent; neutral and calcareous soils approach 100 per cent. Many soil classifications use base saturation at 50 per cent as a dividing line between strongly and weakly leached soils. Two related measures are just as important:

  • Exchangeable sodium percentage (ESP) = Na ÷ CEC × 100. Above about 15 per cent, sodium disperses clay and the soil is classed as sodic.
  • Aluminium saturation = Al ÷ ECEC × 100. Many crops suffer when it exceeds 20–60 per cent, depending on tolerance; liming targets this figure in tropical acid soils.
Worked example

Calculating CEC, base saturation and ESP

A laboratory reports these exchangeable cations: calcium 1,600 mg/kg, magnesium 304 mg/kg, potassium 195 mg/kg, sodium 46 mg/kg, and exchangeable acidity (H + Al) 3.8 cmolc/kg.

  1. Convert: Ca 1,600 ÷ 200 = 8.0; Mg 304 ÷ 122 = 2.5; K 195 ÷ 391 = 0.5; Na 46 ÷ 230 = 0.2 cmolc/kg.
  2. Sum of bases = 8.0 + 2.5 + 0.5 + 0.2 = 11.2 cmolc/kg.
  3. CEC by summation = 11.2 + 3.8 = 15.0 cmolc/kg.
  4. Base saturation = 11.2 ÷ 15.0 × 100 = 75 per cent.
  5. ESP = 0.2 ÷ 15.0 × 100 = 1.3 per cent, far below the sodic threshold.

How much potassium is that per hectare? In the top 20 cm at a bulk density of 1.3 t/m3 there are 2.6 million kg of soil, so 195 mg/kg × 2.6 million kg ≈ 507 kg of exchangeable potassium per hectare, a reserve but not all of it available in one season.

Why CEC matters on the farm

  • Fertiliser timing. A low-CEC sandy soil cannot store much potassium, ammonium or magnesium, so these are leached more easily; split applications pay.
  • Buffering and lime. A high-CEC soil holds more exchangeable acidity, so it needs more lime to change pH, and it also resists sudden pH changes.
  • Nutrient balance. Very high potassium can induce magnesium deficiency in grazing livestock (grass tetany risk) because the cations compete on the sites and at the root.
  • Pollutant retention. Exchange sites also hold heavy-metal cations and some pesticides, slowing their movement to groundwater.
  • Building CEC. Texture is fixed, but organic matter can be increased with residues, manure, cover crops and reduced tillage, adding CEC where it is most needed.

Anion exchange capacity also exists, on positively charged oxide surfaces at low pH, and it is significant in some highly weathered tropical soils. It holds nitrate and sulphate weakly; phosphate is bound more strongly by a different process, specific adsorption.

Key points

  • CEC is the total negative charge of soil colloids available to hold exchangeable cations, in cmolc/kg (= meq/100 g).
  • Charge comes from isomorphous substitution (permanent) and from pH-dependent sites on humus, clay edges and oxides.
  • Humus 150–300, smectite 80–120, kaolinite 3–15 cmolc/kg; sandy soils have low CEC.
  • Exchange is charge for charge and reversible; Al3+ > Ca2+ > Mg2+ > K+ ≈ NH4+ > Na+ in holding strength.
  • Base saturation = bases ÷ CEC × 100; ESP above about 15% marks a sodic soil.
  • Ammonium acetate at pH 7 overestimates CEC in acid soils; ECEC measures it at the soil's own pH.

What the exams ask

  • University entrance and school-leaving papers: definition and units of CEC, which clay has the highest CEC, and the lyotropic series.
  • Degree and research-fellowship entrances: sources of negative charge, pH-dependent charge, methods of measurement and their bias, base saturation and ESP.
  • Licensing and professional examinations: converting laboratory results from mg/kg to cmolc/kg, base saturation and lime or gypsum requirement calculations.

Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.

Questions people ask

Is a high CEC always good?

Mostly, because it stores nutrients and buffers pH. But a high CEC soil also needs more lime or gypsum to correct, and a high CEC clay can be sticky and hard to work. CEC is one property among several.

Why does liming raise CEC?

In soils rich in humus, kaolinite or oxides, much of the charge is pH-dependent. Raising pH dissociates more hydrogen from those surfaces, creating more negative sites.

What is the difference between CEC and ECEC?

CEC measured with buffered ammonium acetate reports the charge the soil would have at pH 7. ECEC adds up the cations actually exchangeable at the soil's own pH. In a neutral soil they are similar; in a strongly acid soil ECEC can be much lower.

Sources

  1. Brady, N. C. and Weil, R. R. The Nature and Properties of Soils, 15th edition. Pearson, 2017. Chapter 8, Soil colloids: seat of soil chemical and physical activity.
  2. Sparks, D. L. Environmental Soil Chemistry, 2nd edition. Academic Press, 2003. Chapter 9, Ion exchange processes.
  3. Sumner, M. E. and Miller, W. P. "Cation exchange capacity and exchange coefficients." In Sparks, D. L. (ed.), Methods of Soil Analysis, Part 3: Chemical Methods. Soil Science Society of America, 1996.
  4. IUSS Working Group WRB. World Reference Base for Soil Resources, 4th edition. International Union of Soil Sciences, 2022. Diagnostic criteria using base saturation.

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The essential plant nutrients and their deficiency symptoms7 min readThe nitrogen cycle in agricultural soils8 min readSaline, sodic and saline-sodic soils, and their reclamation8 min readSoil erosion and the Universal Soil Loss Equation7 min read

Discussion

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