Gypsum vs Elemental Sulfur & Pick the Right Amendment, and the Right Tonnage
Reclaims sodic
A single-amendment gypsum tool can't tell you whether gypsum, elemental sulfur or acid is right. Enter your ESP, CEC, depth and prices to get tonnes per hectare of each, an ESP-decline timeline, a cost comparison and a pick-the-winner recommendation tied to your soil pH and free-lime status.
Soil & amendment costs
Next: apply 0.22 t/ha of Elemental sulfur (S°) (1.1 t for 5 ha), incorporate and then leach with good-quality water; re-test ESP after the ~9-month reaction window before deciding on a second pass.
Gypsum requirement (USDA AH-60): GR = (ESPᵢ − ESPₜ)/100 × CEC; tonnes scale with depth × bulk density; S and acid via 0.19 and 0.61 gypsum-equivalence.
Amendment selection — key facts
- Gypsum requirement
- (ESPᵢ − ESPₜ)/100 × CEC, meq/100g
- Tonnes/ha
- GR × eq.wt × depth × bulk density
- Sulfur equivalence
- 0.19 t per t gypsum
- Acid equivalence
- 0.61 t per t gypsum
- No free lime
- → gypsum (no Ca source for S/acid)
- Calcareous + alkaline
- → sulfur viable if cheaper
- Source
- USDA Agriculture Handbook 60
- Privacy
- Runs in your browser; nothing uploaded
The three amendments compared
How each amendment works, its gypsum-equivalence factor, typical reaction time and when it is the right choice. Equivalence and reaction data from USDA Agriculture Handbook 60 and extension sodic-reclamation guides.
| Amendment | Gypsum-equiv. | Reaction (mo) | How it works | Best when |
|---|---|---|---|---|
| Gypsum (CaSO₄·2H₂O) | 1 | 2 | Soluble calcium source; displaces sodium directly | Best where there is NO free lime (non-calcareous) |
| Elemental sulfur (S°) | 0.19 | 9 | Oxidises to acid, dissolves native lime, frees calcium | Needs free lime; for calcareous, strongly alkaline soils |
| Sulfuric acid (H₂SO₄) | 0.61 | 0.25 | Immediate acid; dissolves native lime, frees calcium | Needs free lime; fast but hazardous to handle |
The right amendment depends on your lime, not just your sodium
Reclaiming a sodic soil means displacing sodium from the exchange complex with calcium, then leaching it away. Gypsum supplies that calcium directly. Elemental sulfur and sulfuric acid supply none — they acidify the soil and dissolve native lime to release calcium, so they only work where free lime is present. Get that wrong and sulfur on a non-calcareous soil simply acidifies without fixing the sodium. That is why the single most important question is not how much sodium you have, but whether your soil is calcareous.
This calculator sizes all three amendments from the USDA Handbook 60 gypsum-requirement equation, converts them via their equivalence factors, compares cost and reaction time, and then recommends the winner from your pH and free-lime status. Use it to budget reclamation and choose the amendment, then pair it with the Gypsum Requirement, Sodic Soil Reclamation and Soil Salinity EC tools to build the full leaching plan.
How to use it — five steps
- 1Enter your current ESP and the target ESP you want to reach.
- 2Add CEC, reclamation depth, bulk density and the area.
- 3Say whether the soil has free lime, and enter its pH.
- 4Enter the price per tonne for gypsum, sulfur and acid.
- 5Compare tonnes, cost and reaction time, and follow the pick-the-winner recommendation.
Frequently Asked Questions
How is the amendment requirement calculated?+
It uses the USDA Agriculture Handbook 60 gypsum-requirement equation. The gypsum requirement in milliequivalents per 100 g of soil is (initial ESP − target ESP) ÷ 100 × the cation-exchange capacity. That is converted to tonnes of pure gypsum per hectare by multiplying by the gypsum equivalent weight, the reclamation depth and the soil bulk density. Elemental sulfur and sulfuric acid rates follow from gypsum-equivalence factors.
What are the equivalence factors between amendments?+
Relative to pure gypsum (factor 1.00), the calculator uses elemental sulfur 0.19 and sulfuric acid 0.61. Because sulfur is far more concentrated in acidity, only about 0.19 tonnes of elemental sulfur replaces one tonne of gypsum — but it must first oxidise, and it only works where free lime supplies the calcium.
When should I use gypsum rather than sulfur?+
Use gypsum when the soil lacks free lime (is non-calcareous), because elemental sulfur and acid have no calcium source to release — they acidify but cannot supply the calcium that displaces sodium. Gypsum brings its own calcium, dissolves quickly and acts within weeks. On a non-calcareous sodic soil, gypsum is the answer regardless of price.
When does elemental sulfur make sense?+
On strongly alkaline, calcareous soils that contain free lime (calcium carbonate). There, sulfur oxidises to sulfuric acid, dissolves the native lime and frees calcium to displace sodium — effectively making your own gypsum in place. It can be cheaper per unit of effect, but you must allow several months for microbial oxidation, which needs warmth and moisture.
Why does the recommendation sometimes pick gypsum even on a calcareous soil?+
Because speed and cost both matter. On a calcareous soil sulfur is viable, but if at your prices gypsum costs less or you cannot wait the months sulfur needs to oxidise, gypsum wins on cost-and-speed. The tool compares the costs you enter and only recommends sulfur when it is both viable and the cheaper route.
How long does each amendment take to act?+
Gypsum is soluble and acts within roughly 2 months; sulfuric acid is effectively immediate (about 1 weeks) but hazardous to handle; elemental sulfur needs about 9 months because it must be microbially oxidised to sulfate before it does anything. The ESP-decline timeline in the tool shows these different speeds side by side.
What is ESP and what target should I aim for?+
ESP is the exchangeable-sodium percentage — the fraction of the soil's cation-exchange sites held by sodium. A soil is generally classed as sodic above about 15% ESP. Most reclamation aims to bring ESP down to around 10% or below, which restores structure and infiltration. Enter your measured ESP and the target you want to reach.
Does the calculation account for amendment purity?+
Yes. Commercial gypsum, sulfur and acid are not 100% pure, so the tool divides the theoretical rate by each amendment's purity fraction to give the actual tonnes you must apply. Lower purity means more product per hectare, which the cost comparison then reflects.
Should I apply gypsum or sulfur given my soil pH?+
If your soil has no free lime, apply gypsum whatever the pH. If it is calcareous and strongly alkaline (pH about 8 or above), elemental sulfur becomes a genuine option and may be cheaper — provided you can wait for it to oxidise. At moderate pH with free lime, gypsum is still the faster, lower-risk choice. The tool encodes exactly this decision and names the winner.
Why does deeper reclamation need so much more amendment?+
Because the requirement scales with the mass of soil you are treating, and that mass is the product of depth, bulk density and area. Doubling the reclamation depth roughly doubles the tonnes of amendment, since there is twice as much sodium-laden soil to treat. The depth and bulk-density inputs let you size this accurately.
Is this a substitute for a soil-reclamation plan?+
No — it sizes the chemical amendment, which is one part of reclamation. You still need good-quality leaching water to flush the displaced sodium below the root zone, adequate drainage, and often organic matter and a tolerant first crop. Use the tonnage and cost here to budget and choose the amendment, then build the full leaching-and-drainage plan around it.