Calculator
Pick an approach, enter your details, then calculate. Results will appear above this form after submission.
Formula used
This tool supports two calculation paths. The area-based method multiplies your area by the chosen surface rate and then adjusts the total for product purity and moisture.
In soil-test mode, gypsum requirement is estimated from soil depth, bulk density, CEC, and the change in ESP (or SAR). Purity is applied inside the equation.
How to use this calculator
- Choose Area-based rate for routine gardening or lawn planning.
- Choose Soil-test when you have CEC and ESP/SAR values from a lab.
- Enter the area, then set purity, bag size, and optional price.
- For area-based mode, pick a preset or enter a custom rate and basis.
- For soil-test mode, enter depth, bulk density, CEC, initial value, target value, and factor F.
- Press Calculate to view results above the form, then export CSV or PDF.
When gypsum helps
Gypsum is most useful when soil structure is tight, sodium is elevated, or water infiltration is slow. Calcium from gypsum can displace sodium on exchange sites, letting salts leach with irrigation. In sandy soils with good drainage, repeated gypsum often delivers smaller gains, so a soil test should guide long-term use. Unlike lime, gypsum barely changes pH, so you can add calcium without pushing soil toward alkalinity.
Selecting a realistic rate
Area-based rates are practical for routine gardening. Many lawns use light maintenance applications, while clay-heavy or traffic-compacted areas may need higher, split doses. Soil-test mode estimates requirement from depth, bulk density, CEC, and the planned reduction in ESP or SAR, producing a mass per hectare that scales to your plot. Use the surface-rate references to match spreader charts and keep calibration consistent across units. If the calculated rate is high, split the application, water between passes, and avoid stacking product where seedlings emerge during early spring planting windows.
Interpreting purity and moisture
Product labels vary: agricultural gypsum, pelletized gypsum, and by‑product sources can differ in calcium sulfate content. The calculator converts your target dry requirement into an adjusted product weight using purity and moisture, helping you avoid under-application. Moisture does not change chemistry, but it changes how many bags you must purchase and carry.
Timing, watering, and incorporation
For lawns, apply to a dry canopy, water in, and avoid windy days to reduce drift. For beds, incorporate lightly into the top layer when possible, then irrigate to move dissolved calcium downward. Gypsum works best with drainage; if water cannot percolate, address compaction, grading, or organic matter management alongside amendments.
Recording results and budgeting
Keep a simple log of area, rate, and dates. Re-test problem soils after a season to verify that ESP or SAR is trending down. The export buttons produce a clean record for contractors or farm notes, and the bag-count and cost fields help compare products by delivered calcium rather than sticker price alone.
FAQs
Should I use gypsum every year?
If a soil test shows sodium issues or poor infiltration, yearly light applications may help. Otherwise, apply only when needed, then re-test after a season to confirm measurable improvement.
Can gypsum replace compost or organic matter?
No. Gypsum supplies calcium and sulfate and can improve sodic soil structure, but it does not add carbon. Combine gypsum with organic matter management for long-term tilth and biology.
Will gypsum raise my soil pH?
Gypsum has little effect on pH compared with lime. It is often chosen when you want calcium without increasing alkalinity, especially around acid-loving plants.
What if my calculated rate is very high?
Split the total into two or more passes, irrigate between applications, and confirm lab targets are realistic. Very high requirements can indicate severe sodicity that also needs drainage and leaching.
Do pelletized products change the calculation?
The chemistry is similar, but pelletized forms spread differently. Use the same purity adjustment, and rely on the surface-rate references to match your spreader settings and coverage pattern.
Why does purity matter so much?
Purity represents how much effective calcium sulfate is in the bag. Lower purity means fewer active pounds per bag, so you must apply more product to deliver the same amendment effect.
Example data table
Sample scenarios to illustrate typical inputs and outputs.
| Scenario | Area | Method | Rate / Target | Purity | Approx total product |
|---|---|---|---|---|---|
| Lawn maintenance | 1,000 sq ft | Area-based | 10 lb per 100 sq ft | 90% | ~111 lb (50 lb bags: ~2.3) |
| Garden beds | 100 m² | Area-based | 3 kg per 100 m² | 85% | ~3.5 kg |
| Sodic patch | 0.25 acre | Soil-test | ESP 15 → 5, D 0.15 m | 90% | Varies with CEC and density |