EC, ppm, and the 500 and 700 scales

What EC measures

Fertilizer salts dissolve into charged ions such as nitrate, potassium, calcium and sulfate. Charged ions carry current, so the more fertilizer in the water, the higher its electrical conductivity (EC). Pure water carries almost none.

EC says how much is dissolved, not what. A solution of table salt and a balanced nutrient solution can read the same. Urea carries no charge until soil bacteria convert it, so a urea-based feed reads lower than the nitrogen in it would suggest.

One solution, written five ways. Every row is the same reading.

The ppm scales

Growers like ppm because it sounds like a quantity of fertilizer. A meter cannot measure ppm directly, though. It measures EC and multiplies by a conversion factor, and the factor depends on which salt the maker calibrated against:

• 500 scale (also called NaCl or TDS): calibrated to sodium chloride. 1.0 mS/cm reads 500 ppm. Common on US pens.
• 700 scale (also called KCl): calibrated to potassium chloride. 1.0 mS/cm reads 700 ppm. Common on European and Australian meters.
• 640 scale: a general factor for natural waters, used by some meters and water labs.

None of the three is the real ppm of a nutrient solution. Each is EC multiplied by a constant. The useful habit is to think in EC and treat ppm as a translation.

Try it

One solution, three meters

EC of the solution at 25 °C

2.00 mS/cm

Solution temperature

25 °C / 77 °F

500 scale

1,000

640 scale

1,280

700 scale

1,400

A meter without temperature compensation would show

2.00 mS/cm

.

Same as the corrected reading at 25 °C.

Try it

Convert a ppm figure to another scale

Reading Written on 500 scale640 scale700 scale My meter 500 scale640 scale700 scale

EC

1.50 mS/cm

My meter reads

750 ppm

Temperature

Ions move faster in warm water, so conductivity rises about 2% per °C. A solution reading 2.0 mS/cm at 25 °C reads about 1.6 at 15 °C. Most meters correct for this automatically (ATC, automatic temperature compensation) and report the reading as if the solution were at 25 °C. A few use 20 °C as the reference instead, which is worth checking in the manual when two meters disagree by a few percent.

• Let the probe settle. A cold probe in a warm reservoir reads wrong until the probe and its temperature sensor catch up, usually under a minute.
• Calibrate with a standard solution. 1.413 mS/cm is the common standard. Calibration solution goes off once opened, so pour a little out rather than dipping the probe in the bottle.
• Rinse between readings with distilled or RO water, and store the probe the way the maker says. A fouled probe reads low.

Using EC

Start with the source water. Tap and well water already carry dissolved salts, often 0.2 to 0.8 mS/cm and sometimes more. A feed target of 1.8 mS/cm on water that starts at 0.5 means adding 1.3 from fertilizer. Recipes written for RO water assume a starting point near zero. The irrigation water test guide covers what is in that starting number.

Seedlings, cuttings 0.5 to 1.2

Lettuce, leafy greens 0.8 to 1.6

Strawberries 1.0 to 1.8

Tomatoes, peppers, cucumbers 2.0 to 3.5

Scale 01234 mS/cm

Watch which way a reservoir moves. In a recirculating system, plants take up water and nutrients at different rates. If the EC climbs between top-offs, they are drinking water faster than they are using fertilizer: top off with plain water. If it falls, they are using fertilizer faster: top off with a weak feed. The Stock Tank Top-Off Calculator works out the mix.

Measure what comes out of the pot. In soilless mixes, the EC of runoff or a PourThru sample shows whether salts are building up in the root zone. The soilless mix guide covers how to take and read one.