10 Feb Some information that will help those of you with higher sodium and bicarbonate irrigation waters
I wanted to share some information that will help those of you with higher sodium and bicarbonate irrigation waters determine how bad “bad” is. See the article below…..
New Calculation for SAR on Irrigation Water Reports
The traditional sodium adsorption ratio (SAR) for irrigation waters is best used for waters that are low in bicarbonate andcarbonate at concentrations < 120 ppm and < 15 ppm, respectively. In the SAR equation, bicarbonate and carbonate concentrations are not included; therefore, it does not account for the effects of these ions. When bicarbonate and carbonate ions are present in moderate to high levels, these can react with calcium and magnesium to form lime by precipitation in the soil or sometimes in the irrigation lines. Thereby, soluble calcium and magnesium forms are depleted that are essential to displace sodium from the CEC sites on clay surfaces where the sodium causes breakdown of aggregates and dispersal of clay particles. To account for bicarbonate and carbonate ions in the irrigation water an adjusted SAR was developed (Ayers and Westcot, 1976). The formula is: adj SAR = SAR [9.4 – pHc], where the pHc value is a theoretical, calculated pH value based on the irrigation water chemistry, which integrates the influence of calcium, sodium,magnesium, bicarbonate, and carbonate concentrations. Both SAR and adj SAR are currently on our irrigation water reports. However, recent research has shown that adj SAR tends to overestimate sodium permeability hazard because it did not adequately account for changes in calcium after irrigation water addition due to the potential for dissolution or precipitation of calcium. The preferred adjustment for SAR is termed adj RNa and uses a substitute Cax value in the SAR equation in place of calcium concentration. The Cax factor comes from a table of bicarbonate/calcium versus electrical conductivity. Use of adj RNa is best for irrigation waters that are high in bicarbonate and carbonate, such as when concentrations are > 120 ppm and > 15 ppm, respectively. Given this information Brookside Labs, Inc. has decided to add adj RNa to our irrigation water reports. In published literature where SAR and adj SAR have been used in interpretation tables and figures, adj RNa can be substituted for these values. So the interpretation does not change, but the way that the permeability hazard is calculated does.
Using SAR and adj RNa. SAR is best used when bicarbonate and carbonate levels are low (< 120 ppm bicarbonate and/or < 15 ppm carbonate) and adj RNa is best used by when levels are high (> 120 ppm bicarbonate and/or > 15 ppm carbonate). So unless the irrigation water is very “clean” the new adj RNa calculation will be the one to use going forward