EC of a nutrient solution: how to measure and interpret it

Electrical conductivity (EC) estimates the total concentration of dissolved ions. It is useful for checking solution strength and salt accumulation, but it does not identify individual nutrients. Two solutions can have the same EC and very different ratios of nitrate, potassium, calcium, sodium or chloride.

Measure consistently

Calibrate the meter with the correct standard, rinse the probe and allow the reading to stabilise. Record temperature compensation, units and sampling point. The common units mS/cm and dS/m are numerically equivalent; 1 mS/cm equals 1 dS/m.

Measure source water before fertilizer, the mixed feed solution and drain or root-zone extract. Use the same time of day and sampling method so that trends are comparable.

Interpret feed and drain together

A rising drain EC usually means salts are accumulating because plants remove proportionally more water, the irrigation volume is low, or source-water salts are entering the system. A falling EC can indicate dilution, stronger nutrient uptake or a dosing problem.

As a practical trigger, treat a drain EC running more than 20–30% above feed EC as accumulation worth investigating, not yet an emergency; treat a drain EC below feed EC as a reason to check the injector and stock tanks before assuming better uptake.

Do not respond to one reading alone. Compare EC with drain percentage, substrate moisture, weather, crop stage and plant appearance. If sodium or chloride accumulates, adding more standard nutrient to restore EC can make the imbalance worse.

• High feed EC: verify recipe, injector, stock concentration and source water.
• Normal feed but high drain EC: review irrigation timing, volume and leaching strategy; a leaching fraction of roughly 15–20% of applied volume is a common target to prevent salt buildup.
• Low feed EC: check stock tanks, injector ratio, mixing and meter calibration.
• Unstable EC: inspect tank mixing, sensor placement and automatic dosing sequence.

Correct without shocking roots

If EC is too high, confirm the cause before diluting or flushing. Large, sudden changes can stress the crop. Adjust in stages and monitor drain EC and plant water status.

A practical staging rule is to change the feed EC setpoint by no more than 0.2–0.3 mS/cm per day, giving roots time to adjust osmotic pressure without wilting."

If EC is too low, restore the verified recipe rather than adding a single salt by guesswork.

Schedule periodic laboratory analysis because EC cannot show nutrient ratios. Closed systems need particular attention to ions that plants remove slowly.

Local adaptation: the United States

For U.S. farms, keep both °F and °C in the operating log, use a local laboratory for water and tissue tests, and follow the label and state rules for every crop-protection product.

Diagnostic checklist

1. Calibrate and clean the EC meter.
2. Record source, feed and drain EC in the same units.
3. Compare EC with irrigation and climate data.
4. Change setpoints gradually after confirming the cause.
5. Use laboratory analysis to check individual ions.

EC is most valuable as a trend linked to a defined sampling method. It is a control signal, not a complete nutrient analysis.

Important: Use Gros.farm to keep sensor readings, observations, photos and corrective actions in one production log. It supports decisions but does not replace local controllers, safety systems or laboratory diagnosis.

Sources and further reading

• Oklahoma State University: EC and pH Guide for Hydroponics
• UMass Extension: Soluble Salts and EC for Greenhouse Crops