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Conductivity (EC): How to control nutrient concentration

Conductivity (EC) and TDS meters are useful for monitoring the concentration of inorganic nutrient solutions.

Conductivity Units

Nutrient labels and feed charts quote conductivity values in a variety of units. The most common are ‘mS’ (short for mS/cm) and ‘ppm’ (used for TDS meters). Be very careful when dealing with ‘ppm’ as there are 3 different scales commonly used and if the meter’s scale does not match what is quoted in a recommendation, then the nutrient concentration will be grossly incorrect.

Using mS or ppm?

To utilize conductivity recommendations, the grower first needs to know what ‘units’ the meter operates in e.g. mS/cm, ppm, etc. This will be shown on either the digital display or stamped on the meter’s body. Some meter’s offer multiple options. If this is the case, choose the option that matches the recommendation e.g. if your growshop recommends 2.2mS in bloom, then switch the meter to read “mS”. If the meter does not provide the units specified in the recommendation then it will need to be manually converted (see Table 10.2) e.g. if the recommendation is 2.2mS and the grower is using a Truncheon 700ppm, the target TDS will be 1,540ppm.

Uses of EC in Hydroponics

1. When making nutrient solutions, EC meters are useful for setting the target EC. The EC requirement depends on factors such as the stage of plant growth and the type of medium. Burning of roots or foliage can occur if the EC is too high. If the EC is too low, deficiency symptoms can occur. Refer to Table 10.1 for commonly recommended EC levels for the main stages of growth. Most plant species will generally tolerate being above or below the recommended value by at least 10%.

2. In recirculating systems, EC meters are useful for monitoring and maintaining the target EC as nutrient and water are consumed. If the EC is too low, concentrated nutrient will need to be added to prevent the risk of the solution becoming depleted in any one nutrient element. Be aware that the EC reading is a “gross” figure. It will not warn of an imbalance of nutrient salts, or the accumulation of toxic or nuisance salts (e.g. sodium chloride) which are often present in the top-up water. Therefore, as a precaution, the recirculating nutrient must be frequently discarded and replaced.


How to check and adjust EC

Step 1. Stir the nutrient reservoir thoroughly before sampling. Ensure the sampling container is clean.
NOTE: For recirculating systems, it is easier to interpret EC readings by always ensuring the reservoir is filled to the same volume before taking the sample i.e. if the reservoir level has dropped to half, then the EC reading will be twice as high as what it would be when full (assuming the top-up water contains nil salts).

Step 2. Rinse the electrode in distilled water then immerse it into the sample and record the EC reading. For some meter’s, it may be necessary to wait up to a minute or longer before the reading stabilizes, especially if the sample’s temperature is significantly different from ambient.

Step 3. Adjusting the EC:

a) If the EC is below target, add nutrient until the target is reached (do not use additives for this purpose).
b) If the EC is well above target, dilute with raw water.

Step 4. When finished, rinse the electrode with distilled water. Always store the electrode in distilled water when not in use.

© Andrew M Taylor (FloraMax)