I will concentrate for the next few issues on covering some of the fundamentals of hydroponics. This is the second part on how to manage EC. I have added charts to this answer to illustrate some points I made in Part 1.
To reiterate and emphasise—What is the most fundamental aspect of managing EC within a hydroponic system? It is the root zone solution which needs to be managed. Don’t get hung up on only the feed solution, which is not very important in itself, but rather in how it influences the root zone solution.
EC rise in a system (recap)
In my previous column I noted that in most systems, there will usually be an EC rise through the system, because the uptake of water by the plants has been higher than their uptake of nutrient.
In free drainage (open) systems, this will be seen as the difference in EC between the feed solution dripping onto the medium, and the run-off solution leaving the medium. In a recirculating (closed) system, without water make-up, this will show up as an increase with time in the EC of the recirculating solution.
If you don’t have an EC meter, and/or don’t check the EC of the run-off or recirculating solution, the rise in EC can continue until it is high enough to damage the plants, often to the extent of the plants dieing. This is shown graphically in Figure 1, which applies to both types of systems. Any commercial grower not having and using an EC meter is guilty of bad management and potentially heading for disaster.
For recirculating type systems most commercial growers would have automated control of EC. 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. This would give the pattern shown in Figure 2. While weaker nutrient solutions can lead to weaker plants, they will survive, whereas too high an EC can kill plants.
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 fertiliser, this is a solution containing no more that 1 gram per litre. Setting this level with liquid fertilisers 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.
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.
Influence of EC upon yield
The typical pattern for the influence of EC upon yield is shown in Figure 3. 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.
There are tables published giving an indication of the appropriate ranges of EC for various crops. Put ‘hydroponic crops EC’ into your search engine. A quick look at the various tables shows that this is not precision science—the guidelines are very broad. Ω
PH&G December 2013 / Issue 138