What Is Electric Conductivity (EC) And Total Dissolved Solids (TDS)

When a hydroponic cannabis plant isn't happy, it can give us some pretty unhealthy signs. However, in order to get the root of any problem, it helps to know exactly what concentration of nutrients the plant has been receiving.

To get an understanding of how this works, we need to use a digital meter to measure the EC and TDS of our nutrient solution. This article teaches you the differences between EC and TDS so you know how to accurately measure the conductivity or concentration of minerals around the roots of your cannabis plant.

Understanding EC (Electrical Conductivity)

Why is EC important in cannabis cultivation? Although it is not completely necessary to measure EC to grow cannabis successfully, it can provide some extra assistance and allow a grower full control over the feeding solution.

Knowing what's in the water your cannabis plants receive gives you the ability to create the perfect balance of nutrients, meaning you can avoid all sorts of issues such as deficiencies, toxicities, pH problems and root burn. These are all common problems associated with an incorrect EC or TDS level, so before jumping to a conclusion, it helps to check all angles so you know whether or not the change might be related to the nutrients.

Checking the EC of your tap water means you can adjust your feeding schedule accordingly so that you know the solution you're giving your plants isn't heavily concentrated. It is best to start with pure water if you can, but if that is not possible, preferably use tap water that has a TDS reading under 300PPM.

The Difference Between EC and TDS

EC and TDS are more of less two ways of checking the same thing. The difference is that EC measures the charge of a solution and TDS gives a reading of the amount of minerals dissolved in it.

Electrical Conductivity (EC)

Electrical conductivity (EC) measures the charge of the salts (sodium and chloride ions) present in a water solution. A high EC means the solution contains more positively charged sodium ions and is therefore conductive and electrical current can pass through it. Electrical conductivity is measured in microsiemens (mS/cm).

Most of the dissolved solids in water are ionic compounds, and comprise of positively or negatively charged ions, which also indicate the balance between metal (positive ions) or non-metal elements (negative ions).

Pure water such as distilled or reverse osmosis water is neutral and does not have a charge to it because all the minerals and salts have been stripped from it. There is nothing dissolved in the water to conduct through. The EC in this case would give a reading of 0.

Total Dissolved Solids (TDS)

There are some non-saline elements in fertilizers which may not show on an EC reading, which is why growers often focus on TDS (total dissolved solids), as it gives an overall reading of the mineral content of the solution. However, a TDS reading is essentially just an EC conversion with some estimation.

Total dissolved solids (TDS) indicates the total amount of minerals or salts (ionic or non-ionic) that are dissolved into the solution. Although we do not know specifically which minerals and salts are present, TDS can tell us whether the nutrients are too concentrated for our plants or not.

By checking runoff we can understand how much of minerals in the solution are being consumed by each plant.

PPM - PPM, or parts per million is the metric used for measuring the total dissolved solids in a liquid solution. It can get a bit confusing here because there are different PPM scales used by certain brands that produce EC meters. With this in mind, make sure to check the conversion for the specific brand of meter you have.

EC And TDS For Cannabis Plants

Tap Water

If your tap water has a high PPM, then you generally need less nutrients. When water reaches over 500PPM, heavy metals and salts can begin to seriously affect the EC, which makes adding and maintaining nutrients more of a challenge.

Depending on the quality of the tap water you use, you may need to install a reverse osmosis system which can bring the PPM of the water down to 0-10PPM before adding any nutrients.

There are different ways to treat your tap water at home to bring down the PPM. Reverse osmosis systems can get expensive so you may prefer to look into purifying the water through distillation techniques or using carbon filters.

There’s a Herercane coming through!!!

Tap water is different wherever you are in the world and will contain dissolved solids in some quantities. If you want to be really thorough, you can ask your local water supply company for a breakdown of the salt and mineral content in the tap water. The rule of thumb is, if you can't drink it, then your plants probably can't either.

Tip: Most tap water in Europe is under 300PPM, which isn't so high that it prevents you from adding any nutrients.

Soil

Measuring EC in soil isn't so common because there is much less of a risk of overfertilizing the plants. Depending on the quality of the soil and how it is enriched, the substrate can provide the right level of conductivity and in many cases, further nutrients may not be necessary.

If the soil is of a low quality and has not been enriched, however, then measuring the EC of the runoff water can give a grower an indication as to whether the plants need more or less feeding. This is especially important if chemical fertilizers have been given, as salts can build up around the roots, increasing TDS and pH levels.

Hydroponics

In hydroponics and other soilless methods of growing where there is no substrate, measuring EC is much more valuable. This is due to way the roots come into direct contact with the nutrients, to which the plants respond to immediately. The roots are highly sensitive and improper EC can quickly deteriorate the root and plant health.

An extremely low EC level can actually cause nutrients or water to be drained out of the roots as the plant tries to balance itself with the nutrient solution (positive and negative ions cancel each other out).

Tip: Temperature can also affect the EC of a nutrient solution, causing it to increase.

Optimum EC/TDS Levels

A lot of nutrient product ranges come with a feeding chart which indicate the optimum EC/TDS levels. It is best to check your tap water before and after adding nutrients so you can start to get an understanding of how much of each you need to add to get the right balance.

Here is a rough estimate of the correct EC/TDS levels for a cannabis plant during its cycle, measured using a Hanna 500PPM hand-held EC meter:

Tip: TDS drops as plants use the nutrients, so you can simply add more to your reservoir. It is not necessary to change the res with every feed as you might end up wasting nutrients unnecessarily.

Measuring EC/TDS

Most of the EC meters on the market also measure TDS, allowing you to easily make the conversion. The conversion is not always accurate because of many variables (such as temperature), but the EC can give us a good idea of the TDS with about a 10-15% difference.

Choose a high quality meter that has a high PPM range (up to 2000PPM is enough) and calibrate it regularly to ensure your readings are as accurate as possible.

Aerated nutrient solution in hydroponics grow by 420TouristguideDWC from GrowDiaries.

Conclusion

Do not worry too much about EC and TDS if you are just starting out, as you can still grow top quality weed without it by just knowing how your plants respond to the water and nutrients you give them, as well as proper pH. Try to get a base reading for your tap water to start, and as you gain experience, you may want to experiment to see how fine tuning EC/TDS and nutrients can improve your grow practice. Happy growing!

External References

Salinity: Electrical conductivity and total dissolved solids. Soil Science Society of America Journal. - Corwin, Dennis & Yemoto, Kevin. (2020).

Effects of Temperature on Total dissolved Solid in water. - Shoukat, Ammara & Hussain, Muhammad & Shoukat, Asra. (2020). 

Correlation between conductivity and total dissolved solid in various type of water: A review. IOP Conference Series: Earth and Environmental Science. - Rusydi, Anna. (2018).

Relationships Between Electrical Conductivity - Water Content, Water Potential and Unsaturated Hydraulic Conductivity for Three Soils. - Ruy, Stephane & Doussan, Claude. (2003). 

Features of Measuring the Electrical Conductivity of Distilled Water in Contact with Air. Measurement Techniques. - Ageev, I. & Rybin, Yu. (2020).

This article was updated December 2020.