Residual Sodium Carbonate - Irrigation Water

Residual Sodium Carbonate, also known as RSC, is a parameter that estimates the capacity of a water to alkalinise the medium it irrigates. This over-alkalinisation of the water triggers a series of chemical reactions that cause a large number of very important disadvantages for crop development.

The imbalance of Residual Sodium Carbonate in the water can be critical for crop development.

The equation determining Residual Sodium Carbonate sum the carbonates (CO₃^–) y bicarbonates (HCO₃^–) y subtract the calcium ions (Ca²⁺) y magnesium (Mg⁺²). The total result indicates the excess or deficiency of alkalising power. The desired values of Residual Sodium Carbonate are always less than 1.5 meq/lvalues above this level are considered dangerous.

CSR=(CO₃^–+HCO₃^–)-(Ca²⁺+Mg⁺²)

As you can see in the equation the calcium and magnesium ions subtract the value of carbonate and bicarbonate, that means that if they are in equilibrium the value will be close to zero or negative. If they are not, there will be a lot of free carbonate and bicarbonate ready to react.

In excess of carbonates and bicarbonates, calcium and magnesium precipitate in the soil in the form of calcium carbonate and magnesium carbonate. This effect causes the Sodium Absorption Ratio (SAR) increases as can be deduced from the following equation. As the divisor is reduced within a square root the result grows exponentially.

High SAR and PSI clog the exchange bases, cause soil dispersion, and can both sequester other elements for plant uptake and end up producing phytotoxicity due to excess sodium.

One of the risks of the whole process between soil and water is due to the tendency of clay particles to absorb sodium and release magnesium and calcium ions. Resulting in a poorly permeable soil due to this ion exchange. Weakening the soil structure and causing serious drainage problems.

In the case of waters with high CSR, action must be immediate to avoid severe problems.

Another very important problem of carbonate waters and soils with low calcium and magnesium is the formation of HO ions.^–. The hydroxyl ion increases the Soil pH in which it is found due to the following hydrolysis reactions.

There are solutions for high residual sodium carbonate.

You can hardly ever choose the water to irrigate with, but instead there are methods to combat these problems:

• The injection of acids such as sulphuric acid allows the separation of bicarbonate ions (pH around 6.2) by releasing carbon as CO2. This way helps the balance and allows calcium and magnesium to remain in a more balanced relationship with the sodium in the soil.
•  Gypsum amendments (CaSO₄-2H₂O) are a perfect solution in well-drained soils, for drained soils.
•  The calcium sulphate amendment CaSo₄ also with draining soils.

The amendment should always be as fine as possible and its effectiveness is higher in draining soils, which makes it easier for water to distribute the material. If the soil is not draining at all, it may be necessary to physically mix the amendment with the soil so that the coagulating effect is faster and a calcic horizon is not generated.