Since going digital, the number of subscribers to the magazine has increased greatly. We have noticed that there has also been an increasing interest in the fundamentals of hydroponics, for example, the most popular item is now my answer to ‘How do I manage acid addition and pH rise’. Consequently, I will concentrate for the next few issues on covering some of the fundamentals of hydroponics, starting with how to manage EC.
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.
Units of EC
Unfortunately, there are a number of different terms and units used.
Within most of the international hydroponic community, the standard terminology is electrical conductivity or EC. The standard units are milliSiemens per centimetre, 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 another term called Conductivity Factor (CF), which started in England, but is now mainly used in New Zealand and by some in Australia. It has no units and is 10 times the value of the EC, hence the same solution would have a CF of 20. I guess that it came from trying to avoid working with a decimal point.
Total dissolved solids (TDS)
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
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 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 nutrient. This extra water is evaporated by the plant to keep itself cool, a process known as transpiration. The end result is that any solution remaining will get stronger and hence have a higher EC.
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
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 make-up 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.
‘Open’ (free drainage) systems
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.
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. Ω
November 2013 / Issue 137