Gypsum Application Estimator

Gypsum (calcium sulfate) is a soil amendment used to improve soil structure, especially in sodic (high sodium) soils. It supplies calcium, which replaces sodium on soil particles, helping to flocculate clay and improve water infiltration, drainage, and aeration. It also provides sulfur and can reduce aluminum toxicity without significantly altering soil pH.

This calculator uses the widely accepted Oster et al. (1999) equation to determine gypsum requirements. It considers your soil’s depth, bulk density, cation exchange capacity (CEC), initial and target exchangeable sodium percentage (ESP), gypsum purity, and a reaction efficiency factor to provide an accurate estimate of the gypsum needed per hectare or acre.

Exchangeable Sodium Percentage (ESP) is a measure of the proportion of sodium ions held on the soil’s cation exchange sites, expressed as a percentage of the total cation exchange capacity (CEC). High ESP values (typically >15%) indicate sodic soil conditions, which can lead to poor soil structure.

Cation Exchange Capacity (CEC) is a measure of a soil’s ability to hold positively charged ions (cations), such as calcium, magnesium, potassium, and sodium. It indicates the soil’s nutrient-holding capacity and its potential to store and release nutrients to plants. Higher CEC generally means better nutrient retention.

The frequency of gypsum application depends on the severity of the soil sodicity, the initial application rate, and ongoing management practices. Regular soil testing is recommended to monitor ESP levels and determine if subsequent applications are necessary. For severe sodicity, multiple applications over several years may be required.