How to Use EC Meters in Hydroponics and Growing in Soilless Substrates

In hydroponic and soilless cultivation systems, mineral nutrients are the only source of nutrients for plants, and their precise dosing is critical for proper metabolism and root development. Electrical conductivity (EC) represents a quick and reliable method to determine the concentration of dissolved salts in the nutrient solution and substrate. In this article we will look at the principle of measurement, common mistakes made by growers and the correct use of EC meters in practice.

Indoor cultivation is not free, so most growers try to make their plants use as much energy from the grow lights as possible. Ideally, the conditions in the grow room are set so that plants in full growth receive a whole day of light comparable to midday summer sun. For plants to convert this energy into growth and metabolite production, their nutrition must match. The only way to accurately dose highly effective mineral fertilizers used in indoor cultivation is precise measurement of the concentration of dissolved salts in the irrigation solution and growing substrate.

What EC means and why it is important for growers

EC (electrical conductivity) is a parameter that expresses a solution's electrical conductivity. Growers measure the EC of the nutrient solution to determine how many nutrients it contains in the form of mineral salts. The unit of EC is µS/cm2 (microsiemens per square centimeter) and the higher the EC value, the more concentrated the solution. Tap water contains small amounts of dissolved calcium, magnesium, sodium and potassium, which increase its EC. It is extremely important to know the EC of the water you use when preparing the irrigation solution. The more dissolved salts the tap water contains, the less fertilizer you can add to it. Too high an amount of mineral salts can burn the plants. Tap water typically has a value around EC 0.3 µS/cm2, but in some regions it can be higher. Water with an EC above 0.5 µS/cm2 is not suitable for irrigation and is better treated first to remove minerals using reverse osmosis.

Approximate EC values for different growth stages

• Seedlings and early vegetative stage – EC 1.0–1.5
• Peak vegetative stage – EC 1.5–2.0
• Peak flowering – EC 2.0–2.5
• Maturation – EC 1.5–2.0

How to measure EC

Electronic EC meters are used to measure EC; they usually take the form of a digital pen with a measuring probe at the tip. Reading the value takes only a few seconds and some devices also measure solution temperature or pH. The method and frequency of measurement will differ depending on whether you grow hydroponically or in soilless substrates.

Measuring EC in hydroponic systems

When growing hydroponically, measure the EC of the nutrient solution at least once every two days. First top up the reservoir with clean water, measure the EC and then add fertilizers to raise the EC to the desired level. A common problem for novice growers is fluctuation of EC in the nutrient solution circulating through the hydroponic system. This is usually caused by the nutrient reservoir being relatively small compared to the amount of water the plants consume. If this is the case for you and you have to top up the reservoir too frequently, consider a larger one.

Measuring EC in soilless substrates

When growing in cocoa, rockwool and other soilless substrates, EC does not fluctuate as quickly as in hydroponics and measurement is sufficient about once a week.

• Before measuring, water the plants as you normally would. A few drops of water should run out of the bottom of the pot after watering.
• Let the pots rest for an hour.
• Place a tray under each pot and add 150–350 ml of distilled water depending on the volume of the growing container.
• Wait a few minutes for leachate to drip into the tray, then measure that leachate with the EC meter.

Remember that the substrate EC will be slightly higher than the irrigation EC due to accumulation of salts in the root zone. However, if you measure a significantly higher substrate EC than the nutrient solution, the plants are at risk of over-fertilization. To reduce substrate EC, flush the pots with clean water adjusted to the proper pH until approximately 30% of the volume has run through.

OUR TIP: Want to make measuring substrate EC around the root zone easier? Try the Aqua Master Tools EC meter with a probe for substrate!

You might be interested in: Tips for measuring EC

Care for EC meters

EC meters are essentially laboratory equipment and as such require appropriate handling and care. Without regular calibration and careful hygiene, even the best EC meter will not give correct readings. After each measurement, the probe should be rinsed with clean water so that residues of salts from the nutrient solution do not dry and affect subsequent measurements. For this purpose we recommend using distilled water, which contains no trace minerals.

EC meter probes need regular calibration. Ideally the device should be calibrated before each use, although once a week is sufficient. Follow the manual during calibration, which will also tell you which type of calibration solution to use. For each calibration use fresh calibration solution, pouring some into a cup so that the remainder in the bottle is not contaminated.

It is true that even the most expensive EC meter will not last forever and will eventually need to be replaced. The disadvantage of cheaper devices is that the probe cannot be replaced separately. If you are serious about cultivation, it is worth looking for an EC meter with a replaceable probe, which will last much longer with proper care.

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