Soil Gypsum Requirement Calculator
Enter soil-test sodium, CEC, treatment depth, and the gypsum product details. The calculator converts the Oster-Shainberg-Abrol requirement into field rate, garden rate, product pounds, bags, and optional cost.
As-purchased product required151.87
- Effective gypsum purity
- 80%
- Gypsum requirement
- 7.04
- Application rate
- 3.14
- Garden-scale application rate
- 14.43
- Dry product required
- 144.28
- Whole bags to purchase
- 4
Rate conversions
| Measure | Value | Unit |
|---|---|---|
| Gypsum requirement | 7.04 | Mg/ha |
| Field application rate | 3.14 | short tons/acre |
| Garden application rate | 14.43 | lb/100 ft2 |
| Dry product for entered area | 144.28 | lb |
| As-purchased product for entered area | 151.87 | lb |
Assumes the treated soil layer is reasonably uniform and that gypsum can contact that layer.
Successful sodic-soil reclamation also requires drainage and enough suitable water to leach displaced sodium.
How to use this calculator
- Choose whether the lab reported ESP or SAR, then enter the initial and target values.
- Enter CEC, treatment depth, bulk density, and the reaction factor appropriate for the target.
- Enter product purity directly or calculate it from the sulfur percentage on the analysis.
- Add the treated area, moisture, bag weight, and optional bag price to estimate what to purchase.
How the gypsum requirement is calculated
This calculator uses the Oster-Shainberg-Abrol gypsum-requirement equation for sodic-soil reclamation. It estimates how much gypsum is needed to replace exchangeable sodium in the treated soil layer, then adjusts the result for product purity and converts it into practical application and purchasing units.
The working formula is:
GR = 8.6 x F x D x rho_b x CEC x (Xi - Xf) / p
- GR is gypsum requirement in Mg/ha.
- F is the reaction-efficiency factor.
- D is treatment depth in meters, converted from inches with
D = inches x 0.0254. - rho_b is bulk density in Mg/m3, numerically equal to g/cm3.
- CEC is cation exchange capacity in cmolc/kg.
- Xi - Xf is the reduction in ESP or SAR.
- p is gypsum-equivalent purity as a percent.
If product purity is entered from a label, the calculator uses that percentage directly. If sulfur is used, purity is estimated as sulfur percent / 0.186 because pure gypsum contains about 18.6% sulfur. A sulfur analysis that implies more than 100% gypsum is treated as a data-entry or product-analysis problem.
How the rate is converted to product
The requirement in Mg/ha is converted with 1 Mg/ha = 0.44609 short ton/acre and 1 Mg/ha = 2.04816 lb/100 ft2. Dry product pounds equal the garden-scale rate times the entered area divided by 100. As-purchased pounds divide that dry amount by 1 - moisture/100. Whole bags are rounded up from as-purchased pounds divided by bag weight, and cost is shown only when a positive bag price is entered.
What moves the result most
The result changes directly with the sodium reduction, CEC, treatment depth, and bulk density. A soil with high CEC and a large ESP reduction can need several times more gypsum than a sandy soil with a modest sodium problem. Product purity also matters: an 80% gypsum product requires one quarter more product than a pure gypsum equivalent.
What this calculator leaves out
The calculation does not diagnose sodicity from pH alone, does not test salinity, and does not account for carbonate chemistry, irrigation-water quality, crop salt tolerance, dispersion tests, or how much leaching water is available. It also assumes the sampled layer matches the layer being treated and that the gypsum can be incorporated or otherwise brought into contact with that soil. Use the result as a planning estimate and verify unusually high rates with a soil laboratory, extension specialist, or qualified agronomist.
Worked example
Suppose a soil report shows initial ESP of 20, the target ESP is 5, CEC is 15 cmolc/kg, treatment depth is 6 inches, bulk density is 1.47 g/cm3, the reaction factor is 1.3, and the gypsum product is 80% pure. The depth is 6 x 0.0254, or 0.1524 m. The equation gives 8.6 x 1.3 x 0.1524 x 1.47 x 15 x 15 / 80, or 7.04 Mg/ha.
That is 3.14 short tons per acre, or 14.43 lb per 100 ft2. For a 1,000 ft2 area, the dry product requirement is 144.28 lb. With 5% product moisture, the as-purchased amount is 151.87 lb, so a 40 lb bag size means buying 4 whole bags. If each bag costs $18, the estimated material cost is $72.
Common questions
What are ESP, SAR, and CEC on a soil test?
ESP is exchangeable sodium percentage, the share of the soil exchange complex occupied by sodium. SAR is sodium adsorption ratio, a related measure often reported for soil extracts or water. CEC is cation exchange capacity, which describes how much exchangeable nutrient and sodium charge the soil can hold.
How is gypsum purity calculated from sulfur percentage?
Pure gypsum contains about 18.6% sulfur by weight. The calculator divides the entered sulfur percentage by 0.186 to estimate gypsum-equivalent purity as a percent. For example, 14.88% sulfur implies about 80% gypsum purity.
What target ESP should I use?
A common reclamation target is near ESP 5 for sensitive uses, while less sensitive plants or soils may tolerate a higher target such as 10 or 15. The right target depends on crop or landscape tolerance, soil texture, salinity, and local guidance. When a lab or extension recommendation gives a target, use that value.
Does gypsum lower soil pH?
Gypsum is calcium sulfate, not an acidifying amendment in the way elemental sulfur can be. Its main role in sodic soil is to supply calcium that can replace exchangeable sodium. Soil pH may change as reclamation proceeds, but pH alone is not enough to calculate a gypsum requirement.
Why are drainage and leaching water still needed after applying gypsum?
Gypsum can help move sodium off exchange sites, but that sodium must still be carried out of the root zone in drainage water. Without adequate drainage and suitable leaching water, sodium salts can remain in the treated layer or move back upward as the soil dries.
Should gypsum be incorporated or surface-applied?
Incorporation improves contact when the sodic layer is shallow and the soil can be worked without damage. Surface application may be used for established turf or deeper problems, but movement depends on water infiltration and time. Match the treatment depth in the calculator to the layer the gypsum can realistically affect.