It is important to control the concentration of your nutrient solution. Burning of roots or foliage can occur if the concentration is too high. If the concentration is too low then deficiency symptoms can occur. This article explains how to successfully use conductivity as a tool for controlling nutrient concentration.
A “Conductivity Meter” is a device used to help monitor the concentration of nutrient solutions. It is also known as an EC meter, CF meter or TDS meter. ‘Conductivity’ represents the ease with which a solution conducts electricity. Solid substances known as ‘salts’ (e.g. sodium chloride, potassium nitrate), yield ‘ions’ when dissolved in water. ‘Ions’ permit the flow of electricity through the solution (Fig 11.1a). Increasing the concentration of ions improves the ease with which the solution carries an electric current and therefore causes a higher conductivity. Hence, an EC meter can be used to detect the presence of salts and their approximate concentration in water.
Not all solutions or liquids conduct electricity. Water soluble ‘organic’ substances such as sugar, vitamins and hormones produce very few ions when dissolved in water and consequently yield zero conductivity. Similarly, many liquids such as petrol, alcohol and distilled water yield zero conductivity.
Nutrient labels quote conductivity values in a variety of units. The more common units are:
These are the units for “total dissolved salts” (TDS). EC meters that provide these units have internal software that mathematically converts conductivity readings into a TDS estimate. However, this estimate is prone to many errors and therefore its use should be avoided.
To utilize conductivity recommendations, the grower first needs to know what ‘units’ the meter operates in e.g. mS/cm, cF. This will be displayed on either the digital display or stamped on the body of the meter. Some brands offer multiple options such as cF, mS and µS. If this is the case, choose the option that matches that used by the recommendation. For example, if your local grow shop recommends to maintain the nutrient solution at 2.2mS, then switch the meter to read “mS”.
If the meter does not provide the units specified in the conductivity recommendation then it will need to be manually converted (see Table 11.20). For example, if the recommendation is 2.2mS and the meter only works in cF, convert the recommendation to cF by multiplying it by 10. i.e. 2.2mS x 10 = cF 22.
Nutrient solutions are generally made up by following label dose rates. However, label dose rates usually fail to take into consideration issues that could cause the nutrient solution’s concentration to be either too high, or too low. This is where an EC meter can be a useful dosing tool:
Setting the Target EC
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 11.30 for commonly recommended EC levels for the main stages of growth. There is no need to be precise with EC levels because plants will generally only consume what they need. Most plant species will tolerate being above or below the recommended value by at least 10%.
The target EC values in Table 11.30 do not include the EC of the make-up water. The EC of the water must be added to the target value. For example, if the recommended target EC is 2.0mS/cm and the water has an EC of 0.3mS/cm, the EC should be set to 2.3mS/cm (i.e. 2.0mS/cm + 0.3mS/cm).
Maintaining the Target EC in Recirculating Systems
EC meters are useful for monitoring and maintaining the 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.
Check the meter’s calibration using a reputable conductivity standard solution.
Stir the nutrient reservoir thoroughly before sampling. Ensure the sampling container is clean by rinsing a few times with the nutrient solution. 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).
Rinse the electrode in distilled water then remove any excess water by either gently shaking, or wiping with a tissue. Immerse the electrode in the sample and wait a few minutes before switching the meter on and recording the EC (Fig 11.3). Wait longer if the sample’s temperature is significantly different from ambient.
Adjusting the conductivity:
When finished, rinse the electrode with distilled water. Store the electrode in distilled water when not in use.
Calibrating is necessary to ensure the meter’s display produces accurate readings. Solutions known as ‘conductivity standards’ are made to internationally agreed recipes and are used for this purpose. Refer to your meter’s instructions for the calibration procedure. To ensure the calibration procedure is successful, note the following:
Conductivity meter electrodes normally become coated with impurities. The degree to which this occurs will depend upon the nature of the samples being tested. It will also depend upon whether or not the electrode is rinsed with distilled water and properly stored after each use (Fig 11.4 & 11.5).
Electrode contamination causes a decrease in the effective surface area of the electrode exposed to the solution. This will lead to lower values being recorded, unless frequent calibration is performed. When correct practices are followed, quality conductivity equipment needs calibrating only a few times per year. An indicator of the need to clean is given by the extent to which the meter drifts out of calibration.
Store the electrode in distilled water when it is not in use (Fig 11.5). If contaminants are not removed by the rinsing process, they will tend to encrust on the electrode’s surface if left to dry out.
The following features are important considerations when choosing an EC meter:
Andrew Taylor is the Chief Chemist at FloraMax. You can visit their website at Floramax.com.