When a mineral fertiliser, such as potassium nitrate, is dissolved in water it splits into two changed entities called ions. One of these has a positive charge, called a cation, in this case potassium ion, symbol K+. The other has a negative charge, called an anion, in this case nitrate ion, symbol NO3-. Similarly, other fertilisers in solution also split into anions and cations.
The electrical strength of ionic fertiliser solutions can be detected by an electrical conductivity (EC) meter. The higher the ionic concentration, the higher the level of EC, hence EC can be used as an indicator of solution strength.
Terminology & Fundamentals of EC in Hydroponics
Within most of the international hydroponic community, the standard unit of electrical conductivity or EC is milliSiemens per centimeter, symbol mS/cm. A typical value for a hydroponic nutrient solution would be around 2.0 mS/cm. This is the unit we use in this magazine. Sometimes used is the unit microSiemens/cm, symbol µS/cm, which is one thousand times greater, that is 2000 µS/cm.
The scientists use the same term, but use the units deciSiemens/metre (dS/m), which has the same value as mS/cm. That is, the same solution would be 2.0 dS/m.
There is an alternative term called Conductivity Factor (CF) that factors the value of EC by 10 times for convenience, which is widely used in some countries. It has no units hence the 2.0 dS/m solution would have a CF of 20.
EC Measurement & Testing Standards
At first glance it would make sense to measure the strength of a nutrient solution as total dissolved solids, probably expressed as parts per million. In theory, this is absolutely correct, however, there are major practical difficulties. To analyse directly for TDS is difficult and prohibitively expensive, consequently, an indirect method is used.
This is to use a meter which indicates TDS. Apparently very simple, but in practice it is not. The meter used is actually an EC meter and there is an internal correction factor which converts the EC to the TDS readout. Unfortunately, this is where it comes unstuck. Different meters have different correction factors, usually dependant upon the industry in which they are principally used.
For example, for salt water a factor of 500 ppm per mS/cm is used. Other meters use 700 or 750 ppm per mS/cm. Some textbooks quote a conversion factor of 654 for hydroponic solutions, but this only applies to a specific balance of nutrients. Change the nutrient balance and the factor changes.
Consequently, I strongly recommend that growers use an EC meter and avoid using TDS meters.
Limitations of EC value
It is important to recognise that while EC gives a good indication of the strength of a nutrient solution, it has its limitations. The first is that the EC gives absolutely no indication of the nutrient balance of that solution. The second is that it does not measure any non-ionic components in the solution. This means that when using organic fertilisers, the solution strength will be higher than indicated by its EC, because most carbon-based compounds are not ionic and won’t register on an EC meter.
EC changes within a Hydroponics system
When a nutrient solution is used in a hydroponic system growing plants, whether recirculating or not, its EC will change with time. This is because there is almost always a difference in the uptake rate of water and nutrients by the plants. Typically, if using an initial solution strength of say 2.0 mS/cm, especially in warm weather, the plants will usually take up more water than nutrients. This extra water is evaporated by the plant to keep itself cool, a process is known as transpiration. The end result is that any solution remaining will get stronger and hence have a higher EC.
Controlling & reducing EC in a Hydroponics system
The most fundamental aspect of managing a hydroponic system is to manage the solution around the root zone of the plants.
Many growers, especially beginners, tend to concentrate exclusively on their feed solution, but this is only important in terms of controlling the root zone solution. The root zone solution will always have a different nutrient balance to the feed and usually a different EC and pH.
‘Closed’ (recirculating) systems
For recirculating type systems most commercial growers would have automated control of EC. Typical are recirculating channel and ‘flood & drain’ systems. If you have automatic EC and pH control, then these are obviously controlled. What is not controlled is the nutrient balance of the recirculating solution, especially if there is significant acid addition.
Without a controller, the EC of the recirculating solution will usually rise with time as mentioned earlier. How quickly this happens depends upon the size of your plants, the climatic conditions, and especially the volume of solution you have in the system – the smaller the volume, the quicker the change. If the EC gets too high, the plants will suffer and eventually die.
To prevent this happening and especially if you don’t have an EC meter, it is much safer to have water makeup in your system. In this case, as fertiliser is taken up by the plants the EC will fall. A low EC will give soft plants, but they will survive.
If you are a hobby grower and can’t afford an EC meter, I strongly recommend that you install automatic water make-up in your system. This would take the form of a cistern or float valve adding water to maintain a set level in your tank. The result would be that the EC never gets higher, but will fall with time until the next nutrient addition. While weaker nutrient solutions can lead to weaker plants, they will survive, whereas too high an EC can kill plants.
‘Open’ (free drainage) systems
For free drainage systems, having and using an EC meter is very important. The best indication of the root zone solution is the solution running off. This needs to be checked regularly and managed to avoid extremes. For the hobby grower who can’t afford an EC meter, the safest way to operate is to use a feed solution that is no higher than EC 1.5mS/cm. Provided that there is a modest amount run-off its EC won’t rise to dangerous levels. If using a solid hydroponic fertilizer, this is a solution containing no more that 1 gram per litre. Setting this level with liquid fertilizers will be more difficult because they can have different concentrations and recommended dosages. I suggest that you make up a recommended solution and take a sample to your hydroponic store to test its EC, and then adjust your dosage to suit.
These are dripper fed, media-based systems, where a small proportion of the feed is run off from each container. It is critical to measure the pH and EC of this run-off solution, because this is what you need to control. As with closed systems, the EC will usually rise through the system, and how far it changes depends upon the same factors, that is, size of plants, etc. Here also, if the EC gets too high the plants will die.
Read more about best growing media in hydroponics here
The controls that you have are the EC of the feed solution and the proportion of run-off. That is, if the run-off EC is too high, you can lower the EC of the feed and/or increase the proportion of run-off by increasing the volume and/or frequency of irrigation.
Factors Influencing EC in Hydroponics System
Water uptake by osmosis
The water uptake by the roots is driven by osmosis. Osmosis is the process of water transferring through a semi-permeable membrane (such as a root cell wall) and the rate of transfer is driven by the difference in concentration across the membrane.
The lower the EC in the root zone solution the lower its concentration, hence the concentration difference to the strong solutions within the plant will be larger. Consequently, the driving force for water uptake will be higher. This then gives the grower a tool to help plants uptake more water during periods of high water demand on the plant, by reducing the EC. Water demand is increased by high radiation, high air temperature, low relative humidity and high wind speeds.
Climate influence on EC
The most important effect of climate on EC is a result of a big difference in radiation between summer and winter. Radiation is the main driver of transpiration and hence water uptake. Therefore in summer it is usual to run a lower EC to help with water uptake. In contrast, the relatively lower radiation level, and hence transpiration, in winter means that the EC is normally run slightly higher than in summer.
Impact of EC upon yield
At low EC the yield is depressed by nutrient starvation and deficiencies. The yield then levels out over a range of EC, that is, the yield doesn’t have a peak, but rather a plateau. At higher EC, the yield falls off and eventually plants would die because of nutrient toxicities. The height of the plateau and its start and end points will differ for different species, and will also be influenced by other factors, such as the growing environment.