Alternative Method for Calculating Sodium Hazard As a Result of Bicarbonate.

As pressure increases on potable water supplies, turfgrass managers continue to switch to non-potable water sources to irrigate turfgrass, and face the agronomic challenges that can result from using these water sources. Sodium adsorption ratio (SAR) is a useful measure for predicting potential soil sodicity problems that may arise from using a water source, and is calculated as:

SAR = Na⁺ / [(Ca²⁺ + Mg²⁺) / 2]^1/2

Water high in bicarbonate and carbonate is thought to be undesirable for irrigation because these anions have the potential to react with dissolved calcium or magnesium to form calcite or dolomite in the soil, thereby increasing the potential for sodicity problems. As a result, golf course superintendents often purchase expensive acid injection equipment in order to acidify their irrigation water. The residual sodium carbonate (RSC) equation attempts to predict the risk of sodium hazard as a result of bicarbonate, and is calculated as:

 RSC = (CO₃^2- + HCO₃^-) – (Ca²⁺ + Mg²⁺)

This method is flawed, however, as it does not account for differential precipitation potentials of calcium and magnesium, acidity inherent in the soil or added through fertilizers, or sodium levels in the soil and water. Another common method to address sodium hazard as a result of high bicarbonate is through the adjusted sodium adsorption ratio (SAR_adj), put forth by Bower in 1968:

 SAR_adj­ = SAR * [1+(8.4-pH_c)]

This calculation attempts to adjust actual SAR to account for bicarbonate levels in the water, but tends to overestimate the potential for hazard. The calculation uses Langelier's saturation index for determining calcite precipitation potential, and assumes a constant soil pH of 8.4. Suarez improved upon Bower's method in 1981 by accounting for variations in soil pH and using HCO­₃^-/Ca²⁺ ratio and ionic strength to predict calcite precipitation. This methodology can be improved upon and simplified with the use of computer technology. Water samples were sent to testing labs, and reported SAR­_adj values were compared. Water test reports were then analyzed in-house, calculating SAR_adj using Bower's method, Suarez's method, and a chemical simulation method using the open source chemical equilibrium software program Visual Minteq. The water test analysis showed that both Bower's and Suarez's methods tend to give higher estimates of SAR_adj compared to the Visual Minteq simulation, which predicts calcite formation based on activity coefficients and accounts for soil pH. Increased accuracy in the measurement of SAR_adj could lead to savings for golf course superintendents by minimizing unnecessary water treatment recommended by traditional irrigation water assessment methods.