Size a specified gypsum rate for an area or bag, or estimate a sodic-soil requirement from laboratory values and explicit model assumptions. Use local soil-testing guidance to select the actual rate.
Gypsum Application Rate Formula
As-sold rate: G = (A / 1000) * R Pure-gypsum rate: G = (A / 1000) * R * (100 / P)
- G is the total gypsum needed (lb)
- A is the area to treat (sq ft)
- R is the application rate (lb per 1,000 sq ft)
- P is the product purity (%)
For sodic soil reclamation based on a lab soil test, the calculator uses the gypsum requirement formula:
GR = 1.72 * CEC * ((ESPi - ESPf) / 100) * (D / 12) * F * (100 / P)
- GR is the estimated as-sold product requirement (US short tons per acre; 2,000 lb per ton)
- CEC is the cation exchange capacity (meq/100 g)
- ESPi is the current exchangeable sodium percentage (%)
- ESPf is the target exchangeable sodium percentage (%)
- D is the soil depth to reclaim (inches)
- F is a user-selected application multiplier (blank uses the model default 1.25); P is gypsum purity in percent (blank uses 100%)
Area mode sizes a supplied rate. Select as-sold product or pure-gypsum basis; adjust purity only for the latter. Bag count is omitted if bag mass is blank. Soil-test mode uses 1.72 US short tons for about 4 million lb of soil per acre-foot (bulk density about 1.5 g/cm³), scaled by treatment depth, exchange demand, multiplier and purity. Bag coverage uses the same rate basis. These modes do not select a field rate or schedule.
Illustrative Rate Conversions and Soil-Test Estimates
The first table illustrates conversions only, not recommendations for lawns, clay or compaction. Enter a laboratory or product-label rate. Appearance alone does not establish gypsum need.
| Situation | Rate (lb / 1,000 sq ft) | Per acre equivalent |
|---|---|---|
| Illustrative rate A | 5 – 10 | 220 – 440 lb |
| Illustrative rate B | 20 – 30 | 870 – 1,300 lb |
| Illustrative rate C | 40 – 50 | 1,750 – 2,200 lb |
| Illustrative rate D | 100+ (conversion only) | 4,350+ lb |
| Illustrative rate E | 45 – 90 | 1 – 2 tons |
The second table illustrates the stated model in US short tons per acre for 6 inches of soil, multiplier 1.25 and purity 100%. Find your soil’s CEC column and the row for how many percentage points of ESP you need to remove. Sandy soils have low CEC (5 – 15), loams are mid range (10 – 25), and clays are high (20 – 40), which is why the same ESP problem takes far more gypsum on clay.
| ESP reduction needed | CEC 10 | CEC 20 | CEC 30 |
|---|---|---|---|
| 5 points | 0.5 tons/acre | 1.1 tons/acre | 1.6 tons/acre |
| 10 points | 1.1 tons/acre | 2.2 tons/acre | 3.2 tons/acre |
| 15 points | 1.6 tons/acre | 3.2 tons/acre | 4.8 tons/acre |
| 20 points | 2.2 tons/acre | 4.3 tons/acre | 6.5 tons/acre |
Example Problems
Example 1: A supplied product rate. For 5,000 ft² at a specified as-sold rate of 45 lb per 1,000 ft², G = (5,000 / 1,000) × 45 = 225 lb. With 40 lb bags, 225 / 40 = 5.625, so purchase 6 whole bags. This is arithmetic, not a lawn-treatment recommendation.
Example 2: Sodic-soil model. Current ESP 20%, target ESP 5%, CEC 15 meq/100 g, depth 6 inches, multiplier 1.25 and purity 100% give 1.72 × 15 × 0.15 × 0.5 × 1.25 = 2.41875 US short tons/acre, about 4,838 lb/acre or 111 lb per 1,000 ft². The soil-mass assumption and actual field need require professional soil assessment.
Model: Utah State University, Utah Fertilizer Guide, chapter 9. Field assessment: Colorado State University Extension, Diagnosing Saline and Sodic Soil Problems.
FAQ
Does gypsum change soil pH like lime does? Gypsum supplies calcium and sulfur; it is not a substitute for lime to neutralize acidity. Soil testing must establish the required amendment. Poor structure alone does not establish gypsum need.
Will gypsum loosen any clay soil? Only clay soils that are dispersive or sodium-affected respond structurally to gypsum. In those soils, calcium displaces sodium on the clay particles, allowing them to flocculate into stable aggregates that improve drainage and reduce crusting. On a well-aggregated, non-sodic clay, gypsum still supplies calcium and sulfur but will not dramatically change the texture. A laboratory soil and water assessment is needed before choosing an amendment or rate.
How often should gypsum be reapplied? Retest and assess drainage and leaching-water quality with local agricultural guidance. This calculator does not prescribe a maintenance or split-dose schedule; native calcium sources may reduce external gypsum need.
