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Epsom Salts and Electrical Conductivity

In the article on Magnesium Supplementation, we addressed the use of Epsom Salts, and concluded that 3/4- to 1 teaspoon per gallon was a decent level for monthly addition to your normal fertigation regimen if you water and fertilizer don’t provide enough. As growers get more sophisticated though, they tend to use electrical conductivity (EC) of solutions to control the concentrations to be applied.

When measuring solution EC, we must keep in mind that each component of the solution – the water, the fertilizer and any additives – each contribute to the total.

The EC of your water should be measured. If you are using distilled water or that produced by a well-maintained reverse osmosis system, you can count on that being quite low.

Fertilizer manufacturers often publish the conductivity versus nitrogen concentration data for their formulas, and I urge you to use their data, as it can vary a bit, but here are some general, representative values:

Electrical Conductivity (µS/cm2) by Fertilizer Concentration

Formula

50 ppm N

100 ppm N

150 ppm N

30-10-10

70

140

210

21-7-7

280

560

840

21-5-20

290

580

930

20-20-20

200

400

600

20-10-20

330

660

990

19-4-23-2Ca

340

680

1020

17-5-17-3Ca-1Mg

320

640

960

15-5-15-5Ca-3Mg

390

780

1170

15-3-20-3Ca-1Mg

350

700

1050

14-4-14-5Ca-2Mg

350

700

1050

13-3-15-8Ca-2Mg

400

800

1200

12-1-1-8Ca-3Mg

350

710

1060

13-2-13-6Ca-3Mg

340

680

1020

10-30-20

480

950

1420

9-45-15

600

1200

1800

So let’s say we have pretty good RO water, with a conductivity of about 15 µS/cm2, and measure our fertilizer solution to be 350 µS/cm2. Subtracting the water’s conductivity from that of the solution and referring to the table above for our specific formula (we’ll use 20-10-20 for this example), we see that the fertilizer contributed 350-15=335 µS/cm2, meaning we’re a fraction over 50 ppm N. So now we want to look at adding Epsom Salts.

Epsom Salts (MgSO4-7H2O) contributes approximately 7 µS/cm2 per ppm Mg. A decent dosing rate is in the neighborhood of 30%-60% of the nitrogen level, so with our example solution above, at about 50 ppm N, we should add somewhere between 15 and 30 ppm Mg, so let’s just use 25 ppm Mg. Our calculation, therefore, is as follows:

Component

Concentration

EC Contribution (µS/cm2)

Water

15

Fertilizer

~50 ppm N

335

Epsom Salts

~25 ppm Mg
(25 ppm x 7 µS/ppm)

175

Total Solution Conductivity

525