RSC & Gypsum & Treat Sodic Irrigation Water
Checks borewell water
Enter carbonate, bicarbonate, calcium and magnesium to get the Residual Sodium Carbonate, its hazard class and the gypsum needed — so high-bicarbonate water doesn't turn soil sodic.
Irrigation water analysis
Next: treat the water with 258 kg gypsum (or use an acid/SO₂ generator) before irrigating — your RSC of 3 meq/L is unsafe.
RSC above ~2.5 meq/L makes water unsuitable without amendment; gypsum supplies Ca to precipitate excess carbonates as harmless calcite.
RSC & gypsum — key facts
- RSC
- (CO₃ + HCO₃) − (Ca + Mg)
- Units
- meq/L
- Safe
- below 1.25 meq/L
- Marginal
- 1.25–2.5 meq/L
- Unsafe
- above 2.5 meq/L
- Gypsum
- ≈ 86 mg/L per meq RSC
- Effect
- high RSC → sodic soil
- Privacy
- Runs in your browser; nothing uploaded
When the water itself turns the soil sodic
Not all irrigation water is created equal. Water high in carbonate and bicarbonate carries a hidden hazard: as it dries in the field it precipitates the soil's calcium and magnesium, leaving sodium to take over the exchange sites. The clay disperses, the surface seals, infiltration collapses. Residual Sodium Carbonate puts a number on that risk — and gypsum, added to the water, neutralises it before the damage is done.
This tool gives the RSC, the safe/marginal/unsafe hazard class and the gypsum needed from your water analysis and volume. Use it to judge a borewell or canal source, size a gypsum dose for a tank or season, and protect soil structure. Pair it with the Irrigation Water SAR, Gypsum Requirement and Soil Salinity tools for full water-and-soil management.
Judge your water
Safe, marginal or unsafe by RSC level.
Dose the gypsum
Right amount to neutralise the carbonate excess.
Protect soil structure
Keep calcium available, stop sodium taking over.
Size to any volume
Tank, pond or a whole season of irrigation.
Frequently Asked Questions
What is Residual Sodium Carbonate (RSC)?+
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.
How is RSC calculated?+
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.
What RSC level is safe?+
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.
How does gypsum neutralise RSC?+
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.
Why does high-bicarbonate water make soil sodic?+
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.
How much gypsum do I add, and where?+
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.
Where do I get the water analysis numbers?+
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.
How does RSC relate to SAR?+
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.
Are the figures precise?+
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.