Available Water Capacity Calculator
Calculate available water capacity from field capacity, wilting point, root depth, and bulk density. Plan irrigation amounts and frequency.
Gypsum Application Rate Calculator
Calculate gypsum tons per acre to reduce sodium percentage in soil based on soil Na%, target Na%, CEC, and treatment depth.
%
%
inches
%
$/ton
ac
Na to Replace⧉
3.750 meq/100g
Exchangeable sodium to displace from CEC sites
Pure Gypsum⧉
0.58 tons/ac
100% pure product requirement
Actual Product Needed⧉
0.65 tons/ac
Adjusted for 90% purity = 1,290 lbs/ac
Per Split Application⧉
0.32 tons/ac
2 application(s) recommended
ESP Reduction⧉
20% -> 5%
0.75% relative reduction in exchangeable sodium
Cost per Acre⧉
$29.00
$45.00/ton x 0.65 tons
Total Cost⧉
$2,322.00
For 80 acres
ESP Reduction
Current: 20%
→Target: 5%
Requirement by Treatment Depth
| Depth (in) | Product (tons/ac) | Product (lbs/ac) | Cost/Acre |
|---|---|---|---|
| 6 | 0.32 | 645 | $15.00 |
| 8 | 0.43 | 860 | $19.00 |
| 12 * | 0.65 | 1,290 | $29.00 |
| 18 | 0.97 | 1,935 | $44.00 |
| 24 | 1.29 | 2,580 | $58.00 |
ESP Classification Reference
| ESP Range | Classification | Management |
|---|---|---|
| < 5% | Non-sodic | Normal management |
| 5 - 15% | Slightly sodic | Monitor and light amendment |
| 15 - 25% | Moderately sodic | Gypsum amendment required |
| > 25% | Severely sodic | Heavy amendment + drainage |
Planning notes, formulas, and examples
About the Gypsum Application Rate Calculator
The Gypsum Rate Calculator determines how many tons of gypsum (calcium sulfate) to apply per acre to reduce the exchangeable sodium percentage (ESP) in sodic soils. Sodic soils — those with ESP above 15% — suffer from poor structure, reduced infiltration, and crusting that severely limits crop growth.
Gypsum works by supplying calcium ions that replace sodium on the soil exchange complex. The displaced sodium is then leached below the root zone with irrigation or rainfall. The amount of gypsum needed depends on the current ESP, target ESP, soil CEC, and the depth of soil to be treated.
This page converts ESP, CEC, and treatment depth into a starting gypsum tonnage that can then be checked against field response and follow-up tests.
When This Page Helps
Sodic-soil reclamation usually fails when the rate is guessed or when leaching needs are ignored. This page gives a defensible starting rate.
How to Use the Inputs
- Enter your current exchangeable sodium percentage (ESP) from a soil test.
- Enter your target ESP (typically 5% or below).
- Enter your soil’s CEC from the soil test.
- Enter the depth of soil to be treated (typically 6 or 12 inches).
- Review the gypsum requirement in tons per acre.
- Plan for adequate water to leach displaced sodium through the profile.
Formula used
Gypsum tons/ac = (Current ESP% − Target ESP%) / 100 × CEC × Depth factor × 0.0086
Where:
ESP = Exchangeable Sodium Percentage
CEC = Cation Exchange Capacity (meq/100g)
Depth factor = inches of soil × soil weight per inch (≈ 300,000 lbs per acre-inch)
0.0086 = Conversion factor (tons gypsum per meq Na)Example Calculation
Result: 7.7 tons gypsum/ac
Na to replace = (20% − 5%) / 100 × 20 meq/100g = 3.0 meq Na/100g. For 12 inches: 3.0 × 12 × 300,000 / 100 / 2000 × 0.086 = approximately 7.7 tons gypsum per acre.
Tips & Best Practices
- Apply gypsum to the surface and incorporate into the top 6 inches for fastest results.
- Adequate irrigation or rainfall is essential to leach displaced sodium out of the root zone.
- For very high ESP (>30%), consider split applications over multiple years.
- Mined gypsum and FGD (flue gas desulfurization) gypsum are both effective sources.
- Monitor electrical conductivity (EC) during reclamation — initial salt flush may temporarily raise EC.
- Combine gypsum with organic matter additions to improve soil structure during reclamation.
Identifying Sodic Soils
Sodic soils are identified by ESP greater than 15%, poor water infiltration, surface crusting when dry, and a slick or greasy feel when wet. Lab analysis of a saturated paste extract provides ESP, EC, and SAR (Sodium Adsorption Ratio) values. Soils with SAR above 13 are generally classified as sodic.
Gypsum Sources and Quality
Mined gypsum is quarried from natural deposits and typically has 85–98% purity. FGD gypsum is a byproduct of coal-fired power plant scrubbers and is equally effective with purity often above 90%. Both sources are approved for agricultural use. Phosphogypsum from fertilizer manufacturing is restricted in some areas due to radioactivity concerns.
Leaching Requirement
Gypsum application alone doesn’t solve the problem — water is needed to move displaced sodium out of the root zone. The leaching requirement depends on the amount of sodium to remove and the soil’s infiltration rate. Plan to apply 6–12 inches of water per treatment cycle. In arid regions, coordinate gypsum application with irrigation scheduling.
Sources & Methodology
Last updated:
Frequently Asked Questions
Exchangeable Sodium Percentage is the fraction of the CEC occupied by sodium. ESP above 15% causes soil dispersion, poor water infiltration, and surface crusting. Reducing ESP restores soil structure.
Gypsum dissolves to release calcium ions, which replace sodium on soil exchange sites. The freed sodium moves into the soil solution and is leached downward by water. Calcium is preferred by clays, so the exchange is thermodynamically favorable.
Lime (CaCO₃) supplies calcium but raises pH. For sodic soils that are already alkaline (pH >8.5), lime is less effective because it has low solubility at high pH. Gypsum is more soluble and does not change pH.
Time depends on water availability for leaching. With adequate irrigation, measurable improvement occurs in one season. Full reclamation of deeply sodic soils may take 2–4 years with repeated applications and leaching.
Yes. Gypsum is a neutral salt (pH 6.5–7.0) and supplies calcium and sulfur, both essential nutrients. It will not harm crops at recommended rates.
Sulfuric acid is effective for calcareous sodic soils because it dissolves native calcium carbonate, releasing calcium to replace sodium. It is faster-acting but requires specialized application equipment and is hazardous to handle.
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