Calculating SARw

In order to calculate SARw, you must insert the values provided in the water quality report into the following formula:

If the values provided are expressed in ppm (parts per million) or mg/L (milligrams per liter), you must first convert them to their respective equivalent weights. These can be obtained by dividing the molecular weights of each ion by their valences, as shown in the following table:

Ion	Molecular Wt. (mg/mmole)	Valence (#)	Equivalent Wt. (mg/meq)
Sodium (Na)	23	1	23
Calcium (Ca)	40	2	20
Magnesium (Mg)	24	2	12

Thus, if the concentrations of sodium, calcium, and magnesium were 56, 62, and 29 mg/L, respectively, the equivalent weights of these ions would be as follows:

Sodium: 56mg/L / 23mg/meq = 2.43meq/L

Calcium: 62mg/L / 20 mg/meq = 3.10 meq/L

Magnesium: 29mg/L / 12 mg/meq = 2.42 meq/L

 

Plugging these values into the formula yields the following values:

Since the SARw is less than 10, the water is considered to have a low sodium harard and would be suitable for use in irrigating turfgrass with no restrictions.