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When nutrients dissolve in water, they split into ions. For example, potassium nitrate dissolves into a potassium ion and a nitrate ion. Each ion carries an electrical charge, which creates the potential for electricity to move through a solution. The more ions present, the better the water can conduct electricity. Plants only absorb nutrients when they are available in an ionic form. That’s why EC is a good indication of the overall strength of your nutrient solution.  

Please note: A conductivity measurement doesn’t give you the breakdown of individual nutrients within your solution.   
 
Just as with pH, each type of plant has a preferred EC range within which it can thrive. In general, though, if your EC is very low, your plant probably doesn’t have enough food. On the other hand, if your EC is very high, you run the risk of burning your roots or creating toxic buildups. While there’s only one scale for measuring acidity – pH – conductivity isn’t as straightforward. The most common scale for scientific purposes is millisiemens per centimetre (mS/cm2). However, there are at least four other conductivity scales in use.

Always ensure that you know which measurement scale your meter is set on, as well as which scale is being used by your nutrient manufacturer.
 
• EC (Electrical Conductivity) [1 mS/cm2 = 1 EC]
• PPM (Parts per Million) [EC x 700]
• TDS (Total Dissolved Solids) or DS (Dissolved Salts) or MS (Measured Salts); otherwise known as PPM 500 [EC x 500]
• CF (Conductivity Factor) [EC x 10]

There are a few different ways to measure your EC, with the quickest and simplest method being a handheld pen or probe. Here’s where you should think about measuring conductivity and the benefits of each location.

• In the nutrient reservoir: This is what you’ll be using to feed your plants, so it’s essential that you check that your reservoir has sufficient nutrient strength.
• At the root zone: This is where your plant will be absorbing nutrients. When you measure EC here, this gives you the best indication of the amount of nutrients that are available to your plants.
• In the run-off: Measuring here can give you a useful approximation, so long as the percentage of run-off is around 20%.
• In the water supply: Unless you are using treated water (via deionised or reverse osmosis), there is a risk of contaminating ions giving an EC value. This could affect your actual EC value and prevent optimal nutrient availability.

When testing nutrient levels in soil or soilless media, keep these factors in mind:

• Nutrient uptake by plants: As your plants absorb nutrients, the number of nutrient ions decreases, lowering the EC. However, plants can also release ions, which may increase the EC.
• Growing medium: Some media types break down over time and release ions, raising the EC level. Other media may bind to nutrients, preventing them from being measured accurately.
• Ion reactions: Certain ions can react with each other, forming insoluble compounds that don't conduct electricity and are no longer available to your plants. A common example is the formation of calcium sulfate complexes.
• Water concentration changes: High evaporation or transpiration rates can increase the concentration of your nutrient solution, leading to a higher EC.

EC levels can change rapidly, which is why it's essential to monitor conductivity regularly. By doing so, you can catch and resolve potential EC issues before they affect your crop.

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