Carbonates & bicarbonates hazard of irrigation water

Bicarbonate hazard of irrigation water

High carbonate (CO₃⁼) and bicarbonate (HCO₃^-) increases SAR index (around >3-4mEq/L or >180-240mg/L). Let's explain why:

Bicarbonate and carbonate ions combined with calcium or magnesium will precipitate as calcium carbonate(CaCO₃) or magnesium carbonate (MgCO₃) when the soil solution concentrates in drying conditions.

The concentration of Ca and Mg decreases relative to sodium and the SAR index will be bigger. This will cause an alkalizing effect and increase the PH. Therefore when a water analysis indicates high PH level, it may be a sign of a high content of carbonate and bicarbonates ions.

Residual Sodium Carbonate (RSC)

The RSC has the following equation:

RSC=(CO₃^-+HCO₃^-)-(Ca²⁺+Mg⁺²)

It is another alternative measure of the sodium content in relation with Mg and Ca. This value may appear in some water quality reports although it is not frequently used.

If the RSC < 1.25 the water is considered safe

If the RSC > 2.5 the water is not appropriate for irrigation.

Some practices to solve problem of carbonates and bicarbonates in irrigation water

- Injection of sulfuric acid to dissociate the bicarbonate ions (PH around 6.2) giving off carbon dioxide. It allows the calcium and magnesium to stay in solution in relation with the sodium content.

- Add gypsum when soils have low free calcium plus leaching.

- Add sulfur to soils with high lime content plus leaching