Calculate gypsum tons per acre to reduce sodium percentage in soil based on soil Na%, target Na%, CEC, and treatment depth.
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 calculator uses the standard gypsum requirement equation that accounts for cation exchange capacity and treatment depth. Because field conditions vary, the calculated rate serves as a starting point — monitor soil sodium levels after application and adjust as needed. Whether you are a beginner or experienced professional, this free online tool provides instant, reliable results without manual computation.
Sodic soils are among the most challenging agricultural soils to manage. Without reclamation, they produce poor yields or may be unsuitable for farming. Gypsum is the most widely used amendment for sodium reduction because it is effective, relatively inexpensive, and does not change soil pH. Having a precise figure at your fingertips empowers better planning and more confident decisions.
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)
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.
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.
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.
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.
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.