Gypsum Requirement Calculator & Reclaim Sodic Soil
Heals sodic soil
Work out the gypsum to reclaim a sodic (alkali) soil — from your CEC and target ESP get the dose in tonnes per hectare, the total tonnes, 50 kg bags and the cost.
Enter your soil & target
ESP = exchangeable sodium percentage. Soils with ESP ≥ 15 are sodic. GR = CEC × (current − target ESP) ÷ 100; commercial dose adjusts for product purity.
Next: broadcast, incorporate into the topsoil, then leach with good-quality water and ensure drainage so displaced sodium washes out. Re-test ESP after one season.
Gypsum supplies Ca²⁺ that replaces exchangeable Na⁺ on clay; the freed sodium must be leached below the root zone.
Gypsum & sodic soil — key facts
- Sodic soil
- ESP ≥ 15
- Formula
- CEC × (ESPi − ESPf) ÷ 100
- Gypsum
- CaSO₄·2H₂O
- Typical dose
- 2–15 t/ha
- Product purity
- ≈ 70–85%
- Reclaim depth
- usually top 15 cm
- Key step
- leach sodium after applying
- Privacy
- Runs in your browser; nothing uploaded
Bring hard, alkali land back to life
Sodic soils hold too much sodium on their clay, which collapses soil structure — leaving land that's hard when dry, sticky when wet, slow to drain and hostile to roots. The fix is calcium: gypsum supplies Ca²⁺ that swaps out the sodium, which is then leached away. How much gypsum depends on the soil's cation exchange capacity and how far you need the exchangeable sodium percentage (ESP) to fall, scaled to the depth and bulk density of soil being treated.
This calculator runs the standard gypsum-requirement equation and adjusts for your product's purity, so you get both the agronomic rate (t/ha) and the practical buy quantity (total tonnes and 50 kg bags) with an optional cost. Remember reclamation needs drainage and leaching — broadcast, incorporate, then irrigate so the freed sodium washes below the root zone. Pair with the Lime Requirement tool for acidic soils, and re-test after each season.
Size the dose
Get the exact gypsum rate from your soil test, not a guess.
Buy the right amount
Purity-adjusted total tonnes and bags so you don't over- or under-buy.
Budget the job
Add a price per tonne to see the reclamation cost up front.
Track progress
Re-test ESP each season and recompute the remaining requirement.
Frequently Asked Questions
What is gypsum requirement?+
Gypsum requirement (GR) is the amount of gypsum needed to replace the excess exchangeable sodium on a sodic soil's clay with calcium, lowering the ESP to a safe level. It's calculated as GR (meq/100g) = CEC × (current ESP − target ESP) ÷ 100, then converted to tonnes per hectare for your soil depth and bulk density.
What is a sodic (alkali) soil?+
A sodic soil has too much exchangeable sodium — an ESP (exchangeable sodium percentage) of 15 or more. Sodium disperses clay, destroying structure, so the soil becomes hard, poorly drained and crusty when dry. Gypsum supplies calcium that displaces the sodium so it can be leached away.
How does gypsum reclaim sodic soil?+
Gypsum (calcium sulphate, CaSO₄·2H₂O) dissolves and releases Ca²⁺ ions that swap places with the Na⁺ held on clay particles. The freed sodium goes into solution as sodium sulphate and is then washed below the root zone by irrigation or rain — so good drainage is essential for reclamation to work.
How much gypsum do I need per acre?+
It depends on the soil's CEC and how far the ESP must drop. Typical doses range from 2 to 15 tonnes per hectare (about 1 to 6 t/acre). This tool computes the exact pure and commercial gypsum rate from your CEC, ESP change, depth, bulk density and product purity.
Why adjust for gypsum purity?+
Agricultural gypsum is rarely 100% pure — mined and by-product gypsum is often 70–85% CaSO₄·2H₂O, the rest being clay, lime and impurities. The calculator divides the pure requirement by your product's purity so you buy and apply the correct commercial quantity.
What depth should I reclaim?+
Usually the top 15 cm (one plough layer) is reclaimed first, since that's where roots and the worst sodium build-up sit. Deeper reclamation (30 cm) needs proportionally more gypsum. The tool lets you set the depth and scales the requirement accordingly.
How do I apply gypsum?+
Broadcast finely ground gypsum evenly over the field and mix it into the topsoil by ploughing or harrowing. Then irrigate with good-quality water (or rely on rain) to dissolve it and leach the displaced sodium down. Ensure the field drains; without drainage the sodium has nowhere to go.
How long does reclamation take?+
Improvement begins within one season as structure recovers, but full reclamation of a strongly sodic soil can take one to three years and sometimes repeat gypsum applications. Re-test ESP after each season and grow salt-tolerant crops (like barley or dhaincha green manure) while the soil improves.
Can I use other amendments instead?+
Yes — on calcareous sodic soils, elemental sulphur or acids (which free native lime to release calcium) can work, and pressmud or organic matter helps. But gypsum is the cheapest, safest and most widely used amendment for most sodic soils. This tool sizes the gypsum option.
What is CEC and where do I get it?+
Cation exchange capacity (CEC) is the soil's capacity to hold exchangeable cations, in cmol(+)/kg (same as meq/100g). It comes from a soil test; clay and organic soils have high CEC (20–40), sandy soils low (5–10). Higher CEC means more sodium to replace, so more gypsum.