Welcome to use the NDSU gypsum requirement (GR) calculator. The calculations are based on the equation of Oster et al. (1999). To determine GR (in Mg/ha; e.g. 1 Mg/ha = 0.44609 tons/acre) the equation requires knowledge of the

- depth of the soil (unit: m)

the depth to which the gypsum will be applied

- bulk density of the soil (unit: Mg/m³)

default value in the calculator is 1.47 Mg/m³

- cation exchange capacity (unit: cmol_c/kg)

e.g. 10cmol_c/kg = 10meq/100g soil

- reaction efficiency factor (unitless)

e.g. choose 1.3 if the final ESP is 5 and 1.1 if the final ESP is 15 or greater

- initial ESP (unit: %)

the initial ESP determined by a qualified laboratory. ESP is a % of the Na ions on the soil's exchange sites

- target or final ESP (unit: %)

the desired ESP

- purity of the gypsum being used (unit: %)

the purity of the gypsum is calculated by dividing Sulphur % by 0.186

You will need to adjust the final mass of gypsum based on the moisture content of the product you are using. For example, if the moisture content is 20% then take GR x 1.2 to get the mass of moist gypsum needed to supply GR.

You can substitute the sodium adsorption ratio (SAR) determined from a saturated paste extract for the ESP if the ESP is less than 50.

The cost of applying gypsum will be much greater than the costs for determining the soil metrics outlined above so have your soils analyzed by your regional soil testing laboratory. Depth of your soil sample should be great enough to understand the depth that gypsum should be incorporated. Gypsum should be placed as near to the depth of sodic behavior as possible to maximize its effectiveness.

[Reference]
Oster, J.D., I. Shainberg, and I.P. Abrol. 1999. Reclamation of salt-affected soils. In 806 Agricultural Drainage. Agronomy Monograph no. 38. American Society of Agronomy, 807 Crop Science Society of America, Soil Science Society of America, 677 S. Segoe Rd. 808 Madison, WI 53711. USA. p.659-691.