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Irrigation water salinity (RAS and leaching)

Is the irrigation water suitable for the soil and the crop? This calculator answers based on the water analysis.

Pre-sizing and estimating tool: the results do not replace a design, calculation, or report signed by a qualified professional, nor verification against the applicable standards.

Preliminary estimateRevision 12/11/2026
Irrigation water analysis
Crop

Cations come from the water analysis. The crop's salinity tolerance is in reference tables and varies with species and cultivar.

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—Water class (Richards)
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Water cations

Concentration of sodium, calcium and magnesium, in meq/L.

Tap the chart to see the values.

Leaching fraction by crop tolerance

How much of the applied water must percolate to keep soil salinity within the limit.

Tap the chart to see the values.

What if the data changes?

Drag the controls to simulate data different from what you entered (from 30% less to 30% more) and watch the result change instantly.

Tap the chart to see the values.

Richards classes for irrigation water

Salinity (C) and sodicity (S) classes from the Richards diagram, with the practical reading of each. Your water is highlighted.

Step by step with your numbers

The calculation done in steps, with the values you entered.

    The calculation memo gathers this page's inputs, results, equations, assumptions, and references, with fields for project, responsible person, and review. The full report also includes the charts and interpretations, in color. In the print window, choose Save as PDF.

    How to use

    Enter the water's electrical conductivity (EC) and the sodium, calcium, and magnesium contents, in meq/L, from the lab analysis. Also enter the EC of the soil that the crop tolerates. The calculator gives the SAR, the water class, and the leaching fraction.

    Hypothetical example

    ECa of 1.2 dS/m; Na⁺ of 6 meq/L; Ca²⁺ of 4 meq/L; Mg²⁺ of 2 meq/L; crop with tolerated ECe of 2.0 dS/m.

    RAS = 6 ÷ √((4 + 2) ÷ 2) = 6 ÷ 1.73 = 3.46, low sodium (S1). EC of 1.2 dS/m: high salinity (C3). Class C3-S1.

    Leaching = 1.2 ÷ (5 × 2.0 − 1.2) = 1.2 ÷ 8.8 = 13.6% of the water applied.

    Precautions

    • Use a complete water analysis, with all cations and anions, and confirm the soil analysis.
    • Leaching only works with adequate drainage. Without drainage, salt builds up.
    • Water with high SAR requires specific correction and management; consult a technician with the soil analysis in hand.

    Method, assumptions, and references

    Equations used

    RAS = Na⁺ ÷ √((Ca²⁺ + Mg²⁺) ÷ 2), with ions in meq/L (Richards, 1954)
    Leaching fraction LR = CEa ÷ (5 × CEe − CEa) (Ayers and Westcot, 1985)
    Richards classes: C1 to C4 by EC and S1 to S4 by SAR

    Assumptions and limits

    • Complete laboratory analysis of cations; the correction of RAS for carbonate precipitation is not included.
    • ECe is the electrical conductivity of the saturated soil extract tolerated by the crop; the value varies with the species and cultivar.
    • Leaching assumes adequate drainage of the soil profile.

    Standards and technical documents

    • Richards, L. A. (ed.). Diagnosis and improvement of saline and alkali soils. USDA Agriculture Handbook 60. Washington: USDA, 1954.
    • Ayers, R. S.; Westcot, D. W. Water quality for agriculture. FAO Irrigation and Drainage Paper 29, rev. 1. Rome: FAO, 1985.

    Frequently asked questions

    What to do with water that has high SAR?

    Generally, soil correction (such as agricultural gypsum in sodic soils) and drainage management. The decision depends on the full soil and water analysis.

    Is leaching always necessary?

    Only when the water’s EC is high for the crop’s tolerance. The leaching fraction shows the extra water to apply beyond evapotranspiration.

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