The likelihood that sodium present in irrigation water will cause permeability problems can be evaluated by computing a parameter known as the sodium adsorption ratio, or SAR. In short, the SAR is a ratio of the concentration of sodium ions to the concentration of calcium plus magnesium ions, as follows:
SAR = [ Na ] / SQRT { ( [Ca] + [Mg] ) / 2 }
Note that in this expression, the concentrations for Na, Ca, and Mg are expressed in milliequivalents per liter (not as milligrams per liter or as ppm), and SQRT designates taking the square root of the operand in parentheses.
A commercial laboratory typically will compute and report SAR as part of a standard irrigation water analysis. If such is not provided, SAR can be computed manually by using the aforementioned equation.
As a rule, water that has an SAR below 3 is safe for irrigating turf and other ornamental landscape plants. Water that has an SAR greater than 9, on the other hand, can cause severe permeability problems when applied to fine-textured soils (for example, a silty clay loam) and should be avoided.
Coarse-textured (sandy) soils typically are less susceptible to permeability problems. For those types of soils, the irrigation water's SAR can be a bit higher than for fine-textured soils.
If an irrigation water contains relatively high amounts of bicarbonate ion, the bicarbonate can affect the calcium and magnesium concentrations in a soil to which the water is applied. For this reason, some laboratories compute and report an "adjusted SAR" that takes into account the water's bicarbonate and total salinity as well as its calcium, sodium, and magnesium content. Indeed, some labs also use another method that accounts for the precipitation and dissolution of solid calcium carbonate, reporting the result as "adjusted RNa." Fortunately, the unadjusted SAR usually suffices, as it is accurate enough for most situations.
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