How do you calculate exchangeable sodium percentage?

ESP = exchangeable sodium divided by CEC, multiplied by 100, with both values in meq/100g. Sodium at 1.8 meq/100g in a soil with a CEC of 18 gives an ESP of 10%. If the report lists sodium in mg/kg, divide by 229.9 first to reach meq/100g.

What ESP value is considered sodic?

An ESP of 6% or more is treated as sodic in current guidance, the point where clay begins to disperse and surface crusting appears. An ESP at or above 15% is the US Salinity Laboratory definition of a strongly sodic, or alkali, soil where most crops fail.

What is the difference between ESP and SAR?

ESP measures the sodium already held on a soil's exchange complex, while SAR measures the sodicity hazard of irrigation water using sodium divided by the square root of (Ca + Mg) over 2 in meq/L. SAR predicts whether ESP will rise over time; it does not replace the soil measurement.

How do you lower a high ESP?

Replace exchangeable sodium with calcium using gypsum, or lime where free carbonate is present and pH is low, then leach the displaced sodium below the root zone with adequate drainage. Re-test afterwards, and rebuild structure with organic matter and controlled traffic while the soil recovers.

US Salinity Lab thresholds · ESP ≥ 6% sodic, ≥ 15% strongly sodic

Exchangeable Sodium Percentage (ESP) Calculator: Exchangeable Na from CEC

Turn exchangeable sodium and cation exchange capacity from a lab report into ESP, sodium base saturation, and a dispersion verdict. Enter sodium as meq/100g, as ppm (mg/kg), or as a percentage of CEC, and read the classification with the amendments that follow from it.

Live ESPpercent of CEC –

Lab report entry

Exchangeable sodium is given as

Exchangeable sodium

meq/100g (cmol_c/kg)

Cation exchange capacity (CEC)

meq/100g (cmol_c/kg), pH 7 or as reported

ESP = exchangeable sodium ÷ CEC × 100. Both values must be on the same unit basis.

Result

Result · ESP

10.00%

Sodic

0%6%: sodic line15%: strongly sodic line100%

Exchangeable Na, meq/100g	1.80
Exchangeable Na, mg/kg (ppm)	413.8
CEC used, meq/100g	18.00
Na as a fraction of CEC	10.00%
Exchangeable Na, lb/acre (6-inch slice)	927

Both inputs read as meq/100g, so ESP is a straight ratio: 1.80 ÷ 18.00 × 100.

Sodic (6 to 15%): sodium is high enough to deflocculate clay and block pores. Gypsum plus a leaching fraction usually restores structure: size the rate with the gypsum requirement calculator.

Exchangeable sodium percentage: the formula, the thresholds and the fix

ESP is the simplest sodicity number on a soil test: the share of the exchange complex occupied by sodium. It matters because sodium does not flocculate clay. Where calcium holds particles in crumbs, sodium pushes them apart, the crumbs disperse, and the same soil that drained last season now crusts, ponds and sets hard. Everything below is one ratio plus the consequences of that ratio.

1. The calculation, worked through

ESP (%) = (exchangeable Na⁺ ÷ CEC) × 100
exchangeable Na (meq/100g) = exchangeable Na (mg/kg) ÷ 229.9
exchangeable Na (mg/kg) = exchangeable Na (meq/100g) × 229.9

Worked example. A report lists exchangeable sodium at 1.8 meq/100g and CEC at 18 meq/100g. ESP = 1.8 ÷ 18 × 100 = 10.00%. Sodium occupies a tenth of the exchange sites, which lands in the sodic band: dispersion risk is real, but the structure is still recoverable with gypsum and drainage. The same report written as ppm would show 414 mg/kg, and 414 ÷ 229.9 returns the same 1.80 meq/100g.

2. Why 6% and 15% are the numbers people quote

ESP	Classification	What is happening in the soil
0 – 6%	Non-sodic	Sodium is dispersed harmlessly; structure and infiltration are governed by something else.
6 – 15%	Sodic	Clay begins to disperse on wetting; surface crusting, slower infiltration, seal-prone seedbeds.
> 15%	Strongly sodic	The classic US Salinity Laboratory definition of an alkali soil: hard setting, poor aeration, most crops fail.

The 15% line comes from the US Salinity Laboratory's Diagnosis and Improvement of Saline and Alkali Soils (Handbook 60), where "alkali soil" was defined by ESP at or above 15. Field experience moved the practical threshold down: dispersion of many clay soils begins near 6%, which is why modern guidance treats 6% as the action line and 15% as the severity line. The calculator flags both.

3. ESP, SAR and sodium saturation are three views of the same problem

Index	Sample	How it is measured	When to use it
ESP	Solid soil	Exchangeable Na ÷ CEC × 100	The sodicity status of the field itself
Na saturation	Solid soil	Same ratio, expressed as base saturation of Na	Base saturation work alongside Ca, Mg and K
SAR	Irrigation water	Na ÷ √((Ca + Mg) / 2) in meq/L	Predicting whether the water will cause sodicity

SAR describes the water you are applying; ESP describes the soil you already have. They track each other closely in the field, but they are not interchangeable, and a sodic soil under good water still needs gypsum. Calculate the incoming hazard with the SAR and CROSS calculator, and read the exchange complex as a whole in the base saturation calculator.

4. Unit conversions you will meet on a lab report

meq/100g (cmol_c/kg) to ppm (mg/kg): multiply by 229.9, the equivalent weight of sodium (22.99 mg/meq) scaled by the 100 g basis.
ppm to meq/100g: divide by 229.9. Sodium at 230 mg/kg is 1.00 meq/100g.
Percent of CEC is already ESP. If your report lists "Na base saturation %", you do not need CEC at all.
lb/acre in the results assumes a 6-inch (15 cm) acre-slice at a bulk density near 1.3 g/cm³, about 2.24 million lb of soil per acre.

Units are the most common source of a tenfold sodicity error, so the calculator keeps the conversion visible rather than hiding it: switch the entry mode above and watch which rows recompute.

5. What to do once the number is above 6%

Confirm the diagnosis. Re-run the test, check the CEC method (pH 7 versus measured), and rule out a saline soil where the problem is total salt rather than exchangeable sodium: ESP can sit low while ECe is high.
Replace sodium with calcium. Gypsum (calcium sulphate) is the standard amendment where free lime is absent; in calcareous soils the lime already present is often enough once pH and drainage are right. Rate it from CEC and exchangeable sodium with the gypsum requirement calculator.
Leach. Displaced sodium has to leave the profile. Apply the amendment, then enough water to move the displaced Na below the root zone, with a drainage plan if the water table is high.
Rebuild structure. Organic matter, crop residues and traffic control at wet consistencies keep the recovered aggregates from re-dispersing. Then re-test at the same depth and season.

The full walkthrough, including how ESP appears alongside CEC and base saturation on a real report, is in exchangeable sodium percentage (ESP): what it is, how to calculate it and how to fix it.

References

Richards, L.A. (1954). Diagnosis and Improvement of Saline and Alkali Soils, USDA Agriculture Handbook No. 60 — source of the ESP ≥ 15% alkali-soil definition and the exchangeable sodium methods.
US Salinity Laboratory Staff and Brady, N.C. and Weil, R.R., The Nature and Properties of Soils — sodicity thresholds, dispersion and reclamation practice.
Rengasamy, P. (2006). "World salinity and sodicity problems", in Salinization of Land and Water Resources — the case for the 6% dispersion threshold.
Sumner, M.E. and Naidu, R. (1998). Sodic Soils: Distribution, Properties, Management, and Environmental Consequences, Oxford University Press.

Agronomic notice and disclaimer: The ESP calculator reports the ratio you enter. It does not diagnose the cause of poor structure, does not account for salinity (ECe), carbonate, or water table effects, and does not replace a full soil test with a local agronomist's interpretation. Amendment rates must be confirmed against material purity, application method and leaching capacity.

Sources and method: the publications this tool cites are listed in the calculator sources register, one table per calculator, with a link to each source that still answers and the formula this page prints kept as the primary record.

Lime Rate

Turn buffering capacity (CEC, buffer pH or OM%) into tons of ag lime per acre, adjusted for ECCE and ENV.

pH Down

Elemental sulfur, iron or aluminum rates to hit a target pH — with acidifier comparison and rebound warnings.

CEC

Convert lab ppm to meq/100g and read total CEC, base saturation and cation balance from one soil test.

Base Saturation

Balance Ca, Mg, K and Na against Albrecht targets with a saturation donut, what-if simulator and plan.

Gypsum

Sodic soil reclamation: exchangeable sodium percentage into tons of gypsum per acre by source purity.

SAR / CROSS

Irrigation water quality — sodium adsorption ratio, CROSS and adjusted SAR with dispersion diagnostics.

pH Converter

Any one of pH, [H+], pOH or [OH-] into the whole equilibrium, with soil target ranges for that value.