Gypsum Requirement Calculator

Work out gypsum requirement instantly with clear inputs, formula shown and shareable results.

About Gypsum Requirement Calculator

Reclaiming a sodic soil means replacing exchangeable sodium with calcium. The sodium to displace is CEC x (current ESP - target ESP)/100 in cmol/kg, and each cmol/kg needs 0.086 tonnes of pure gypsum per 100 tonnes of soil. Soil mass follows from the reclamation depth and a bulk density of 1.4 t/m3.

Formulas

Gypsum requirement

Exchangeable Na to replace (cmol/kg) = CEC x (ESP_current - ESP_target)/100

Mass basis

Gypsum (t/ha) = 0.086 x cmol/kg x soil mass (t/ha) / 1000; soil mass = depth (m) x 10,000 x 1.4

Worked example

With the default values below, Gypsum Requirement Calculator returns Gypsum Requirement of 0.54. Change any input above and every figure here is recalculated in your browser.

Inputs used

Current Exchangeable Sodium Percentageaccepts 1–100 %: 25 %
Target ESPaccepts 1–100 %: 10 %
Cation Exchange Capacityminimum 0.5 cmol/kg: 20 cmol/kg
Depth To Reclaimminimum 1 cm: 15 cm

Results

Gypsum Requirement: 0.54
Exchangeable Sodium To Replace: 3.000
ESP Reduction Targeted: 15.00
Mass Of Soil Treated: 2,100

Step by step

1Values used
2Gypsum requirement
3Mass basis
4Gypsum Requirement= 0.54
5Exchangeable Sodium To Replace= 3.000
6ESP Reduction Targeted= 15.00
7Mass Of Soil Treated= 2,100

Assumptions and limits

The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
Sodic soil reclamation requires adequate drainage; leaching displaced sodium without it can salinise adjacent land and shallow groundwater. Have a soil chemistry report interpreted by a specialist before large-scale application.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Sodic soil reclamation requires adequate drainage; leaching displaced sodium without it can salinise adjacent land and shallow groundwater. Have a soil chemistry report interpreted by a specialist before large-scale application.

Frequently Asked Questions

Does gypsum lower soil pH?

Only slightly. Gypsum swaps sodium for calcium and improves structure without acidifying. For a sodic soil that is also alkaline with free carbonate, elemental sulphur or acidified amendments are needed to move pH.

Is leaching needed after applying gypsum?

Yes. The displaced sodium must be flushed below the root zone with good-quality water and adequate drainage, otherwise it re-adsorbs and the reclamation fails.