RSC & Gypsum & Treat Sodic Irrigation Water

RSC is a measure of how much carbonate and bicarbonate in irrigation water exceeds its calcium and magnesium: RSC = (CO₃ + HCO₃) − (Ca + Mg), all in meq/L. When carbonates win, they precipitate calcium and magnesium out of the water and soil, leaving sodium to dominate — which makes the soil sodic over time.

Add the carbonate and bicarbonate of the water in meq/L, then subtract the calcium and magnesium in meq/L. A positive result means surplus carbonates and sodic hazard; zero or negative means the water has enough calcium and magnesium to stay safe. The calculator does the meq arithmetic and classifies the result for you.

Below about 1.25 meq/L the water is safe; 1.25–2.5 is marginal and needs care; above 2.5 is unsafe and will make soil sodic unless treated. The tool flags your water as safe, marginal or unsafe so you know whether it can be used as-is, used with management, or must be amended first.

Gypsum (calcium sulphate) adds calcium to the water, which reacts with the surplus bicarbonate so it no longer precipitates the soil's own calcium and magnesium. Roughly, every meq/L of RSC to be neutralised needs about 86 mg/L of gypsum in the applied water; the calculator scales that to your RSC and water volume.

As bicarbonate-rich water dries in the soil, it precipitates calcium and magnesium as carbonates, removing them from the exchange sites. Sodium then takes those sites, the clay disperses, and the soil seals up — poor infiltration, crusting and waterlogging. Neutralising RSC keeps calcium available so sodium can't take over.

The tool returns the gypsum needed for your water volume from the RSC. You can apply finely ground agricultural gypsum to the soil before irrigation, or dose it into the irrigation stream (gypsum beds or solution tanks). The water-volume output lets you size the dose to a tank, a pond or a season's irrigation.

From a water test report — a lab measures carbonate, bicarbonate, calcium and magnesium in your irrigation water, usually reported in meq/L (or mg/L, which converts). Borewell and canal water can be tested cheaply at soil-and-water testing labs; retest periodically as groundwater chemistry shifts through the season.

Both rate sodium hazard, but differently: SAR compares sodium against calcium and magnesium directly, while RSC focuses on the carbonate excess that strips calcium and magnesium out. High-bicarbonate water can pass on SAR yet fail on RSC. Use the Irrigation Water SAR calculator alongside this one for the full picture.

They are sound planning figures using standard RSC and gypsum-equivalence formulas. Real gypsum needs depend on gypsum purity and fineness, soil buffering, and how the water is applied. Treat the output as a starting dose, then confirm with a follow-up water test and adjust.