---
title: "Soil SAR Calculator: Diagnose Sodic Soil and Calculate Precise Gypsum Rates to Reverse Clay Dispersion"
canonical: "https://theyieldgrid.com/soil-sar-calculator/"
model_id: "tyg-638"
model_version: "1.0.0"
last_reviewed: "2026-04-27T18:36:39"
reviewed_by: "Umer Hayiat"
---

# Soil SAR Calculator: Diagnose Sodic Soil and Calculate Precise Gypsum Rates to Reverse Clay Dispersion

> Canonical calculator: [https://theyieldgrid.com/soil-sar-calculator/](https://theyieldgrid.com/soil-sar-calculator/)

## What this calculator does

Home - Free Gardening Calculators & Tools - Soil SAR Calculator: Diagnose Sodic Soil and Calculate Precise Gypsum Rates to Reverse Clay Dispersion Sodium in irrigation water or native soil can quietly displace calcium and magnesium on clay exchange sites. When the ratio tips too far, clay platelets lose their natural grouping and the entire soil structure collapses. The soil SAR calculator instantly quantifies this risk using only four lab values and returns the exact gypsum tonnage needed to restore flocculation before drainage fails completely.

## Inputs

| Input | ID | Type | Unit | Range or choices | Required |
|---|---|---|---|---|---|
| Sodium (Na⁺) | `sargypsum_na` | number |  | 0.01 to 200 | Yes |
| Calcium (Ca²⁺) | `sargypsum_ca` | number |  | 0.01 to 500 | Yes |
| Magnesium (Mg²⁺) | `sargypsum_mg` | number |  | 0.01 to 200 | Yes |
| Cation Exchange Capacity (CEC) | `sargypsum_cec` | number |  | 1 to 200 | Yes |

## Outputs

| Output ID | Default state |
|---|---|
| `sargypsum_na_err` |  |
| `sargypsum_ca_err` |  |
| `sargypsum_mg_err` |  |
| `sargypsum_cec_err` |  |
| `sargypsum_results` | — SAR Value SAR Risk Level Gauge (0–30+) 0 (Safe) SAR 6 SAR 13 ⚠ SAR 18 30+ (Severe) ⚠ Warnings & Standards Gypsum Required — lbs / acre Gypsum (Metric) — kg / hectare ESP (Initial) — % Ca + Mg Combined — mEq/L SAR Classification Reference Table SAR Range Classification ESP (approx) Gypsum Need Risk Recommended Products for Sodic Soil Remediation 🏅 Bulk Pelletized Agricultural Gypsum 📡 Professional Soil Salinity / EC Meter 🚜 Heavy-Duty Broadcast Spreader 🌿 Granular Humic Acid Soil Conditioner |
| `sargypsum_out_primary` | — |
| `sargypsum_warnings_box` | ⚠ Warnings & Standards |
| `sargypsum_out_gypsum` | — |
| `sargypsum_out_gypsum_kg` | — |
| `sargypsum_out_esp` | — |
| `sargypsum_out_camg` | — |

## Formula and method

The tool calculates the exact gypsum required to lower Exchangeable Sodium Percentage to the 10% agronomic safety threshold. Show the calculation steps The sodium adsorption ratio is computed as SAR = Na⁺ / √((Ca²⁺ + Mg²⁺) / 2). All four inputs are used in milliequivalents per liter or meq/100 g exactly as reported by the laboratory. Exchangeable sodium percentage is calculated as ESP (%) = (100 × Na⁺) / CEC. If ESP exceeds the 10 % management target the difference is converted to the quantity of sodium that must be replaced. Gypsum requirement is then derived as ΔNa (meq/100 g) × CEC × 86 = lbs gypsum per acre (6-inch depth). The metric equivalent multiplies the English value by 1.121 to give kg per hectare. Results are rounded to two decimal places for SAR and whole numbers for gypsum tonnage. Assumptions & Limits All concentrations must be in mEq/L; ppm values must be converted before entry. CEC is assumed to be in standard meq/100 g units from a routine soil test. Gypsum math assumes a 6-inch tillage depth and average soil bulk density of 1.3 g/cm³. The 10 % ESP target is the default safe level for most field crops. The tool uses USSL classification thresholds for SAR interpretation. Results are estimates only; the calculator cannot account for irrigation water SAR, existing gypsum in the soil, or long-term salt accumulation. If lab reports use different extraction methods the numbers may need adjustment before entry. The sodium adsorption ratio formula is SAR = Na⁺ / √((Ca²⁺ + Mg²⁺) / 2). All ions are expressed in mEq/L. The result is a unitless number that predicts the likelihood of clay dispersion when sodium dominates the soil solution.

## Verified worked examples

### Worked Examples (Real Numbers)

Example 1: Healthy soil profile Sodium (Na⁺) = 2.0 mEq/L Calcium (Ca²⁺) = 22.0 mEq/L Magnesium (Mg²⁺) = 9.0 mEq/L CEC = 28 meq/100 g Result: SAR = 0.51, ESP = 7.1 %, Gypsum required = 0 lbs/acre (0 kg/ha). The soil shows no sodium hazard; structure remains stable and no amendment is needed at this time. Example 2: Moderate sodium hazard Sodium (Na⁺) = 9.0 mEq/L Calcium (Ca²⁺) = 14.0 mEq/L Magnesium (Mg²⁺) = 6.0 mEq/L CEC = 20 meq/100 g Result: SAR = 2.85, ESP = 45 %, Gypsum required = 12040 lbs/acre (13497 kg/ha). Sodium is beginning to occupy exchange sites; a preventive gypsum application will keep ESP safely below the 10 % target. Example 3: Severe sodic condition Sodium (Na⁺) = 25.0 mEq/L Calcium (Ca²⁺) = 5.0 mEq/L Magnesium (Mg²⁺) = 2.0 mEq/L CEC = 18 meq/100 g Result: SAR = 13.36, ESP = 138.9 %, Gypsum required = 35913 lbs/acre (40259 kg/ha). The soil has crossed the sodic threshold; immediate heavy gypsum application plus leaching irrigation is required to restore permeability.

## Assumptions

The tool calculates the exact gypsum required to lower Exchangeable Sodium Percentage to the 10% agronomic safety threshold. Show the calculation steps The sodium adsorption ratio is computed as SAR = Na⁺ / √((Ca²⁺ + Mg²⁺) / 2). All four inputs are used in milliequivalents per liter or meq/100 g exactly as reported by the laboratory. Exchangeable sodium percentage is calculated as ESP (%) = (100 × Na⁺) / CEC. If ESP exceeds the 10 % management target the difference is converted to the quantity of sodium that must be replaced. Gypsum requirement is then derived as ΔNa (meq/100 g) × CEC × 86 = lbs gypsum per acre (6-inch depth). The metric equivalent multiplies the English value by 1.121 to give kg per hectare. Results are rounded to two decimal places for SAR and whole numbers for gypsum tonnage. Assumptions & Limits All concentrations must be in mEq/L; ppm values must be converted before entry. CEC is assumed to be in standard meq/100 g units from a routine soil test. Gypsum math assumes a 6-inch tillage depth and average soil bulk density of 1.3 g/cm³. The 10 % ESP target is the default safe level for most field crops. The tool uses USSL classification thresholds for SAR interpretation. Results are estimates only; the calculator cannot account for irrigation water SAR, existing gypsum in the soil, or long-term salt accumulation. If lab reports use different extraction methods the numbers may need adjustment before entry. All concentrations must be in mEq/L; ppm values must be converted before entry. CEC is assumed to be in standard meq/100 g units from a routine soil test. Gypsum math assumes a 6-inch tillage depth and average soil bulk density of 1.3 g/cm³. The 10 % ESP target is the default safe level for most field crops. The tool uses USSL classification thresholds for SAR interpretation. Results are estimates only; the calculator cannot account for irrigation water SAR, existing gypsum in the soil, or long-term salt accumulation. If lab reports use different extraction methods the numbers may need adjustment before entry. The calculator applies two hard-coded safety thresholds derived directly from the underlying chemistry. SAR greater than 13 triggers the sodic soil alert and displays the mandatory gypsum rate. ESP greater than 15 % is flagged as exceeding the classic sodic threshold used by the U.S. Salinity Laboratory. When sodium dominates the soil solution the calculator automatically shows the clay dispersion warning and recommends gypsum to restore structure. Gypsum rates are never shown as zero when ESP exceeds the 10 % target. Minimum Standards Gypsum must be agricultural-grade calcium sulfate to supply the exact calcium needed for cation exchange. Application should be followed by sufficient irrigation to move displaced sodium below the root zone. Retest SAR and ESP four to six weeks after treatment to confirm improvement. The most common competitor trap is telling growers that poor drainage is always a physical problem solved by deep tillage. The real issue is often chemical: excess sodium has already dispersed the clay into an impermeable concrete-like layer. Tillage alone cannot fix dispersed clay; only the correct gypsum rate can. For related salinity management see the soil leaching requirement calculator . The formula is calibrated for a 6-inch depth. Deeper incorporation requires proportional scaling. Fix: adjust tonnage upward if your tillage plan exceeds 6 inches.

## Limitations and safety

All concentrations must be in mEq/L; ppm values must be converted before entry. CEC is assumed to be in standard meq/100 g units from a routine soil test. Gypsum math assumes a 6-inch tillage depth and average soil bulk density of 1.3 g/cm³. The 10 % ESP target is the default safe level for most field crops. The tool uses USSL classification thresholds for SAR interpretation. Results are estimates only; the calculator cannot account for irrigation water SAR, existing gypsum in the soil, or long-term salt accumulation. If lab reports use different extraction methods the numbers may need adjustment before entry. The calculator applies two hard-coded safety thresholds derived directly from the underlying chemistry. SAR greater than 13 triggers the sodic soil alert and displays the mandatory gypsum rate. ESP greater than 15 % is flagged as exceeding the classic sodic threshold used by the U.S. Salinity Laboratory. When sodium dominates the soil solution the calculator automatically shows the clay dispersion warning and recommends gypsum to restore structure. Gypsum rates are never shown as zero when ESP exceeds the 10 % target. Minimum Standards Gypsum must be agricultural-grade calcium sulfate to supply the exact calcium needed for cation exchange. Application should be followed by sufficient irrigation to move displaced sodium below the root zone. Retest SAR and ESP four to six weeks after treatment to confirm improvement. The most common competitor trap is telling growers that poor drainage is always a physical problem solved by deep tillage. The real issue is often chemical: excess sodium has already dispersed the clay into an impermeable concrete-like layer. Tillage alone cannot fix dispersed clay; only the correct gypsum rate can. For related salinity management see the soil leaching requirement calculator .

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## Provenance

- Model ID: `tyg-638`
- Model version: `1.0.0`
- Reviewed by: Umer Hayiat
- Page modified: 2026-04-27T18:36:39
- Runtime SHA-256: `461c294186b017e263e59244ed92aed4d8775a561759afabafe1ad9461733f5b`

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