YielixGypsum Requirement Calculator: Sodic Soil Reclamation Rates from ESP or SAR
Calculate the gypsum application rate needed to reclaim sodic soils. This calculator converts between ESP and SAR, factors in soil CEC and depth, adjusts for gypsum purity, and outputs rates in both tons/acre and tonnes/ha for effective sodium displacement.
Inputs Explained
- Sodium Input (ESP or SAR)
- Choose whether to enter ESP (Exchangeable Sodium Percentage) directly from a soil test, or SAR (Sodium Adsorption Ratio) from a saturated paste extract. SAR is automatically converted to ESP using the USSL formula.
- Target ESP
- The desired ESP after reclamation. A target of 5% is standard for most crops. Some salt-sensitive crops may benefit from an even lower target.
- CEC (Cation Exchange Capacity)
- Soil CEC in meq/100g from your soil test. Higher CEC soils hold more sodium and require more gypsum to displace it.
- Soil Depth
- The depth of soil to be reclaimed, typically 6-12 inches (15-30 cm). Deeper reclamation requires proportionally more gypsum.
- Gypsum Purity
- The purity of your gypsum product as a percentage. Agricultural-grade gypsum is typically 85%. Mined gypsum varies from 70-95%.
How This Calculator Works
Worked Example
Soil test shows ESP of 25%, CEC of 20 meq/100g, treating 20 cm (about 7.9 inches) of soil at a bulk density of 1.3 g/cm³ with 85% purity gypsum.
- 1. Determine sodium reduction needed
ESP reduction needed = 25% - 5% (target) = 20 percentage points of exchangeable sodium to remove.
- 2. Apply the reclamation formula
Pure gypsum rate (tonnes/ha) = 0.086 × depth (20 cm) × bulk density (1.3 g/cm³) × CEC (20 meq/100g) × ESP reduction (20) ÷ 100 = 8.94 tonnes/ha.
- 3. Adjust for gypsum purity
Adjusted rate = 8.94 tonnes/ha pure gypsum ÷ (85% purity / 100) = 10.52 tonnes/ha of 85%-purity agricultural gypsum.
- 4. Convert to tons/acre
Tons/acre = 10.52 tonnes/ha ÷ 2.242 (tonnes/ha per ton/acre) = 4.69 tons/acre.
- 5. Confirm the final reclamation rate
Final application rate = 10.52 tonnes/ha (4.69 tons/acre), applied uniformly and followed by irrigation or rainfall to leach the displaced sodium below the root zone.
Apply 10.52 tonnes/ha (4.69 tons/acre) of 85% gypsum to reduce ESP from 25% to 5% in the top 20 cm (7.9 in) of soil.
How to Interpret Your Results
| Condition | What It Means |
|---|---|
| ESP < 5% | Non-sodic soil — gypsum for sodium reclamation is not needed. |
| ESP 5-15% | Mildly sodic — some crops may show reduced performance. Moderate gypsum application recommended. |
| ESP 15-30% | Sodic soil — significant structural degradation likely. Gypsum amendment is strongly recommended. |
| ESP > 30% | Severely sodic — major reclamation effort needed. Consider professional soil management consultation. |
Common Mistakes to Avoid
Applying gypsum without adequate drainage
Gypsum displaces sodium from clay particles, but the sodium must be leached out with irrigation or rainfall. Without drainage, sodium remains in the root zone.
Confusing gypsum (for sodic soils) with lime (for acidic soils)
Gypsum supplies calcium to displace sodium but does not significantly change soil pH. Lime raises pH. Use gypsum for high-sodium soils and lime for low-pH soils.
Using SAR values without converting to ESP
SAR and ESP are related but not identical. The calculator converts SAR to ESP automatically using the USSL formula before computing the gypsum rate.
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