Gypsum Dose & Reclaim the Sodic Soil
Displaces sodium
A sodic soil holds too much exchangeable sodium, and gypsum supplies the calcium that displaces it. Enter the ESP, target, CEC, depth and grade to get the gypsum tonnes per hectare, leaching water and a staged ESP timeline.
Describe your sodic soil
ESP reclamation timeline
Next: spread 10.2 t/ha (20.64 t over 5 ac), incorporate to 15 cm, then pond-leach with about 150 mm (1,500 m³/ha) of low-sodium water and grow a salt-tolerant crop (rice) the first season.
Handbook 60 gypsum-requirement equation: GR(pure) = (CEC × ΔESP/100) × 0.086 × bulk density × depth, then divided by gypsum purity × fineness efficiency. Field gypsum response is gradual — split across seasons and re-test ESP.
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Gypsum reclamation — key facts
- Na to displace
- CEC × ΔESP ÷ 100 (meq/100g)
- Pure gypsum
- Na × 0.086 × ρb × depth(cm) t/ha
- Applied dose
- pure ÷ (purity × fineness)
- Sodic threshold
- ESP ≥ 15%
- Equivalent weight
- gypsum 86 g/eq
- Leaching water
- depth × leaching fraction
- 1 mm over 1 ha
- = 10 m³ water
- Privacy
- Runs in your browser; nothing uploaded
Gypsum grades, sodicity classes and texture references
The dose adjusts for the gypsum grade you can source; sodicity classes set how aggressive the reclamation must be; texture gives a CEC and bulk density when you have no soil test.
| Gypsum grade | Purity | Fineness eff. | Effective | Note |
|---|---|---|---|---|
| Mineral gypsum, agri grade (70%) | 70% | 0.80 | 56% | Common field-applied agricultural gypsum, coarse. |
| Mineral gypsum (75%) | 75% | 0.85 | 64% | Standard ICAR reclamation grade. |
| Mineral gypsum, fine (85%) | 85% | 0.92 | 78% | Finely ground, < 2 mm; reacts fast. |
| Phosphogypsum (80%) | 80% | 0.90 | 72% | By-product gypsum, fine, slightly acidic. |
| Pure gypsum (100%, reference) | 100% | 1.00 | 100% | Laboratory reference, fully reactive. |
| ESP up to | Class | Severity | Note |
|---|---|---|---|
| 5% | Non-sodic | none | No reclamation needed; ESP below 5%. |
| 10% | Slightly sodic | slight | Mild structural risk; light gypsum may help. |
| 15% | Moderately sodic | moderate | Sodic threshold (ESP 15) reached; reclaim. |
| 25% | Strongly sodic | strong | Crusting, poor infiltration; full gypsum + leaching. |
| 40% | Very strongly sodic | extreme | Severe alkali; staged reclamation over seasons. |
| over 40% | Extremely sodic (alkali) | extreme | Black alkali; multi-year reclamation programme. |
| Texture | CEC (cmol/kg) | Bulk density |
|---|---|---|
| Sandy | 5 | 1.6 g/cm³ |
| Sandy loam | 10 | 1.55 g/cm³ |
| Loam | 15 | 1.45 g/cm³ |
| Silt loam | 18 | 1.4 g/cm³ |
| Clay loam | 25 | 1.35 g/cm³ |
| Clay | 40 | 1.3 g/cm³ |
| Stage | Cumulative drop | Note |
|---|---|---|
| Season 1 (after 1st leaching) | 50% | Apply, incorporate, ponded leaching; ~half the ESP drop. |
| Season 2 | 80% | Salt-tolerant crop (rice), continued leaching. |
| Season 3+ | 100% | ESP reaches target; rotate to normal crops. |
Sources: USDA Agriculture Handbook 60 (U.S. Salinity Laboratory, 1954); ICAR-CSSRI sodic-soil reclamation guidelines.
What gypsum actually does to a sodic soil
A sodic soil carries too much exchangeable sodium on its clay exchange sites. Sodium makes clay disperse, so the soil crusts, seals and drains badly — water ponds, seedlings fail, and roots cannot breathe. The cure is calcium: gypsum (calcium sulphate) dissolves and its calcium ions swap onto the exchange sites in place of sodium, which then leaves the field dissolved in the drainage as sodium sulphate. The amount of gypsum is set by how much sodium has to be displaced, which is the cation-exchange capacity times the drop in ESP you want, scaled by the depth and density of soil being treated.
This tool turns that USDA Handbook 60 equation into a field plan: the gypsum tonnes per hectare and per acre (adjusted for the real purity and fineness of your material), the total tonnes for your area, the leaching water to flush the salts, the cost, and a staged ESP timeline so you can spread the dose over seasons. Pair it with the Residual Sodium Carbonate and CEC & Base Saturation tools for a full salt-affected-soil program.
Order the right tonnage
Get gypsum t/ha and total tonnes adjusted for the grade you can buy.
Plan the leaching
See the water depth and volume needed to flush displaced salts.
Stage over seasons
Follow the CSSRI trajectory from current ESP down to your target.
Budget the cost
Cost per hectare and total from your gypsum price per tonne.
Frequently Asked Questions
How much gypsum do I need to reclaim a sodic soil?+
The pure-gypsum requirement is the exchangeable sodium you must displace, scaled by depth and bulk density: GR (t/ha of 100% gypsum) = (CEC × ΔESP ÷ 100) × 0.086 × bulk density × depth in cm, where ΔESP is the initial ESP minus the target ESP. For example a clay soil at CEC 25, dropping from ESP 30 to 10 over 15 cm at bulk density 1.35 needs (25 × 20 ÷ 100) × 0.086 × 1.35 × 15 ≈ 8.7 t/ha of pure gypsum, then divided by the grade's purity × fineness to get the field rate.
What is the 0.086 factor in the gypsum equation?+
It is the tonnes of pure gypsum (CaSO4·2H2O) per hectare that supply one milliequivalent of calcium per 100 g of soil over a 1 cm depth at bulk density 1.0. It comes directly from the equivalent weight of gypsum, 86 g per equivalent, in USDA Agriculture Handbook 60. Every gypsum requirement calculation is just this factor multiplied by the sodium to displace, the depth and the bulk density.
What ESP makes a soil sodic?+
A soil is classed as sodic once its exchangeable sodium percentage (ESP) reaches 15 percent of the cation-exchange capacity. Below 5 percent it is non-sodic and needs no gypsum; 5–10 percent is slightly sodic; 15–25 percent is strongly sodic with crusting and poor infiltration; above 25 percent it is very strongly sodic and usually needs staged reclamation over several seasons.
Why divide by purity and fineness?+
The formula gives the requirement as pure, fully reactive gypsum. Field gypsum is rarely 100 percent CaSO4·2H2O and coarse grinds do not fully dissolve in a season, so the calculator divides the pure requirement by the grade's purity fraction times its fineness/reaction efficiency. Agricultural mineral gypsum at 70 percent purity and 0.80 fineness, for instance, needs about 1.8 times the pure figure to deliver the same calcium.
How much leaching water does reclamation need?+
Once gypsum has displaced the sodium, water must flush the resulting sodium salts below the root zone. The tool estimates the leaching depth as the reclamation depth times your leaching fraction, then converts it to cubic metres per hectare (1 mm over 1 ha = 10 m³). Handbook 60 notes that about one pore volume removes roughly 80 percent of the soluble salts in a uniform soil.
Which gypsum grade should I choose?+
Finer, higher-purity grades react more completely in a single season, so they need a lower application rate, but mineral agricultural gypsum at 70–75 percent is the most common and cheapest. Phosphogypsum is a fine by-product grade that is slightly acidic and reacts well. Pick the grade you can actually source; the calculator adjusts the dose for whatever purity and fineness you select.
Does the gypsum have to be applied all at once?+
No. The Central Soil Salinity Research Institute (CSSRI) recommends staged reclamation: apply, incorporate and pond-leach in season one for roughly half the ESP drop, grow a salt-tolerant crop such as rice in season two, and reach the target ESP by season three. The staged timeline in the tool shows the expected ESP at each step so you can budget the dose across seasons.
What CEC should I enter if I do not have a soil test?+
Use the texture-based estimate: sandy soils sit near 5 cmol(+)/kg, loams near 15, and clays near 40. The reference table on this page lists a typical CEC and bulk density for each texture class. A measured CEC is always better because the gypsum requirement scales directly with it, but the texture value is a sound planning starting point.
Does the tool work in acres as well as hectares?+
Yes. The core calculation is metric (tonnes per hectare), but the result also reports tonnes per acre and the total tonnes for the field area you enter in acres, so you can order material in whichever unit your supplier uses. One acre equals 0.4047 hectare.
Is gypsum the same as lime for soil?+
No. Lime (calcium carbonate) raises pH and is used on acid soils. Gypsum (calcium sulphate) does not change pH appreciably; it supplies calcium to displace sodium on a sodic soil while the sulphate leaves with the sodium. Use lime to correct acidity and gypsum to correct sodicity — a sodic soil that is also alkaline still uses gypsum, not lime.
How accurate is this gypsum requirement estimate?+
It is a sound planning figure from the Handbook 60 equation, but real soils are layered and reaction is never perfect, so treat it as a starting dose. Re-test ESP after the first leaching, adjust the next application, and confirm the leaching water actually drains. The staged trajectory is designed for exactly this iterative approach.
Can too much gypsum harm the soil?+
Over-applying wastes money and can temporarily raise the soil's electrical conductivity, but gypsum is not toxic and the excess calcium and sulphate leach out. The bigger risk is under-applying, which leaves the soil sodic. The calculator's purity- and fineness-adjusted figure is the rate that actually delivers the calcium the soil needs, not an inflated one.