What ESP level requires gypsum treatment?

Soils with ESP above 15% are classified as sodic and typically benefit from gypsum amendment. Some crops show reduced performance at ESP as low as 5-10%, especially on fine-textured soils. A target ESP of 5% is standard for most reclamation goals.

How long does gypsum take to work?

Gypsum dissolution depends on particle size, moisture, and temperature. Fine-ground gypsum can begin improving soil within weeks, but full reclamation of severely sodic soils may take 1-3 years. Adequate drainage is essential for leaching displaced sodium.

Can I use gypsum on non-sodic soils?

Gypsum can benefit non-sodic soils as a calcium and sulfur source, and to improve structure in high-clay soils with poor aggregation. However, this calculator specifically addresses sodic soil reclamation based on ESP reduction.

Gypsum Requirement Calculator: Sodic Soil Reclamation Rates from ESP or SAR

Calculate the gypsum application rate needed to reclaim sodic soils. This calculator converts between ESP and SAR, factors in soil CEC and depth, adjusts for gypsum purity, and outputs rates in both tons/acre and tonnes/ha for effective sodium displacement.

Inputs Explained

Sodium Input (ESP or SAR): Choose whether to enter ESP (Exchangeable Sodium Percentage) directly from a soil test, or SAR (Sodium Adsorption Ratio) from a saturated paste extract. SAR is automatically converted to ESP using the USSL formula.
Target ESP: The desired ESP after reclamation. A target of 5% is standard for most crops. Some salt-sensitive crops may benefit from an even lower target.
CEC (Cation Exchange Capacity): Soil CEC in meq/100g from your soil test. Higher CEC soils hold more sodium and require more gypsum to displace it.
Soil Depth: The depth of soil to be reclaimed, typically 6-12 inches (15-30 cm). Deeper reclamation requires proportionally more gypsum.
Gypsum Purity: The purity of your gypsum product as a percentage. Agricultural-grade gypsum is typically 85%. Mined gypsum varies from 70-95%.

How This Calculator Works

Based on: USSL sodic soil reclamation equation with ESP-SAR conversion (ESP = 1.475 × SAR / (1 + 0.0147 × SAR))

Best for: Calculating gypsum amendment rates for sodic soil reclamation where excess exchangeable sodium degrades soil structure

Check locally: Confirm ESP/SAR values with a certified soil lab and verify adequate drainage exists for leaching displaced sodium

Units supported: Metric (tonnes/ha), Imperial (tons/acre)

Worked Example

Soil test shows ESP of 25%, CEC of 20 meq/100g, treating 8 inches of soil with 85% purity gypsum.

1. Determine sodium to displace
ESP reduction needed = 25% - 5% (target) = 20% of CEC. Sodium to displace = 20/100 × 20 meq/100g = 4 meq Na/100g soil.
2. Calculate pure gypsum needed
Each meq Na/100g in 8 inches of soil requires approximately 0.86 tons gypsum/acre. Base rate = 4 × 0.86 = 3.44 tons/acre.
3. Adjust for purity
Adjusted rate = 3.44 / (85/100) = 4.05 tons/acre of 85% purity gypsum.

Apply 4.05 tons/acre of 85% gypsum to reduce ESP from 25% to 5% in the top 8 inches.

How to Interpret Your Results

Condition	What It Means
ESP < 5%	Non-sodic soil — gypsum for sodium reclamation is not needed.
ESP 5-15%	Mildly sodic — some crops may show reduced performance. Moderate gypsum application recommended.
ESP 15-30%	Sodic soil — significant structural degradation likely. Gypsum amendment is strongly recommended.
ESP > 30%	Severely sodic — major reclamation effort needed. Consider professional soil management consultation.

Common Mistakes to Avoid

Applying gypsum without adequate drainage

Gypsum displaces sodium from clay particles, but the sodium must be leached out with irrigation or rainfall. Without drainage, sodium remains in the root zone.

Confusing gypsum (for sodic soils) with lime (for acidic soils)

Gypsum supplies calcium to displace sodium but does not significantly change soil pH. Lime raises pH. Use gypsum for high-sodium soils and lime for low-pH soils.

Using SAR values without converting to ESP

SAR and ESP are related but not identical. The calculator converts SAR to ESP automatically using the USSL formula before computing the gypsum rate.

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