It's important to keep track of nutrient levels in hydroponics growing solutions. The most convenient way to check is by measuring the solution's electrical conductivity (EC), and using this to estimate total dissolved solids (TDS). EC measures how easily electric current flows through a substance, and TDS measures the quantity of solid particles dissolved. The largest component of TDS in hydroponics is typically nutrients, so this is a good way to gauge how much you've added, and if you need to add more. It can also be used to diagnose problems.
The ideal amount of nutrients varies based on plant species, strain, and growth phase, and is usually in the range of 700 - 2,000 \(\frac{μS}{cm}\). You can find guides online, and we briefly discuss this later in this article, but most info you'll find will be loose recommendations.
Pure water (H₂O) is a poor conductor of electricity: If you were to measure the EC of distilled water, the reading would be close to 0. (Distilled water has been boiled to the point of evaporation, then condensed back to liquid form, to remove impurities.) The water you drink, bathe in, and swim in conducts electricity due to the impurities it contains. These usually include calcium, iron, potassium, and magnesium.
A small amount of impurity raise conductivity a lot! This is due to water providing an easy medium for charged ions to move through. Drinking water has a conductivity of 50 - 500 \(\frac{μS}{cm}\), and sea water has a conductivity of about 50,000 \(\frac{μS}{cm}\) (50\(\frac{mS}{cm}\)). This big increase is due to the Na+ and Cl- ions in saltwater, which let electric charges flow through it.
Units for conductivity are expressed as conductance divided by distance. Conductance is usually measured in ~Siemens, and distance in meters. Some examples:
TDS is measured as a portion of impurities, using mass, volume, or both. Examples: - Parts per Million (ppm) - Milligrams (or solids) per Liter (\(\frac{mg}{L}\))
We can think of EC as a primary measurement, and TDS as a secondary one: We measure EC directly by testing the flow of electricity. Then using some assumptions, calculate TDS from it. The relationship may be approximated as linear: To find TDS, multiply EC by a constant. For example:
$$ TDS (mg/L) = \frac{1}{1000} \times EC(\frac{μS}{cm})$$
The constant depends on the units you're using, and what type of solids you suspect are dissolved. For hydroponics nutrients, this is usually 0.5 to 0.7. So, to convert \(\frac{μS}{cm}\) to ppm, you might multiply EC by \(\frac{1}{2}\). Example: EC = 1,000\(\frac{μS}{cm}\) = 1\(\frac{mS}{cm}\) = 500ppm TDS.
Keep in mind that TDS implies a measure of all dissolved solids, while EC only measures conductive ones. Nonconductive solids like sulfur, silicon, and organic molecules won't be detected by EC measurements. If you'd like to verify this, try measuring ec of tap water, then dividing it in 2 portions. Add sugar to one, and an equal amount of salt to the other. Measure again. You've added the same amount of solids, but the saltwater EC will have raised greatly, while the sugar water will have remained steady.
Whether you use EC or TDS in your measurements, and which units you use is up to you! You can treat them as equivalent, as long as you're consistent.
Example recommendations for mature plants in a growth phase:
You can find recommendations for many plant types on this page from hydroponic.co.za.
Seedlings may do best with no nutrients at all, plants going through vegetative growth need a normal amount, and flowering or plants need more still.
Digital EC and TDS meters are available in a range of styles and price points. For example, you can buy small, inexpensive handheld pen-type meters for less than $30 USD, or lab-grade benchtop meters that use a separate probe for hundreds or thousands of dollars. Benchtop meters often connect to probes using coaxial connections, and can accept probes suited to different conductivity ranges. For hydroponics, look for probes labeled K = 1.0 - these are suitable in the range of 0 to 2,000 \(\frac{μS}{cm}\), which is what you'll find in hydroponics.
Scientists prefer to measure TDS using gravimetric analysis. This involves evaporating the liquid, and measuring mass of the remaining solids. This is more accurate, and takes into account non-conductive solids. This isn't practical for hydroponics, so you'll almost certainly estimate TDS using an EC measurement.
Measuring conductivity is more complicated than measuring pH and temperature, and there are a number of approaches and variations. Usually, the intent is to run an excitation current through two electrodes submerged in the solution you're measuring. You then determine how much the current drops across the electrodes - the result might be interpreted by a voltage fed into an analog-to-digital converter(ADC), or by measuring the frequency of a timer circuit.
It's important that the excitation current alternate polarity, like in AC: Otherwise, the electrodes will corrode.
Here's an example of a DIY circuit using a 555 timer. This more robust reference design by Analog Devices uses a microcontroller and a number of components to provide an accurate measurement, while limiting power consumption.
Some sensors dynamically change the excitation current's voltage, or select different resistors in order to handle a large range of conductivity. Simpler ones are cheaper, but may have a limited range. Auto-ranging sensors can measure accurate in the μS, mS, and S ranges.
The electronics determine conductance. To convert to conductivity, the measured conductance is divided by the distance between the probe's electrodes. If the electrodes are 1cm apart, the conductivity in S corresponds to \(\frac{S}{cm}\).
(Coming soon)
When measuring EC and TDS for hydroponics, you're interested in the amount of nutrients in solution. If you start or refill your solution with tap water and add nutrients, the nutrient will be the biggest contribution to conductivity, but not the only one! One easy thing to control for is impurities in your tap water. This may be around 200\(\frac{μS}{cm}\), and will vary depending on your water supply. To compensate:
If you're able to use distilled, or reverse-osmosis-filtered water for your solution, you don't have to worry about this - but this may not be practical.
Other things that might contribute to EC:
EC levels have use beyond gauging how much nutrients you've added. For example, a rising level might indicate the plant are nutrient blocked, from built-up salts. This could cause your plant to show signs of insufficient nutrients, even if you keep adding more.
As time passes after replacing water and nutrients, plant will use nutrients, and convert them to waste products. During this time, your EC may remain stable, but the nutrients are depleting. EC and TDS measurements are only good indicators of nutrient levels right after adding. It's tough to estimate how fast nutrient are depleting, but the more often you change water and nutrients, the more consistent your system will be. This is because you're resetting it to a known condition you control, while it's tough to gauge nutrient depletion (both in overall levels, and ratio), and the buildup up byproducts (minerals and organic molecules from the plant's biochemistry). Once every 2 weeks is a good rule of thumb.
It's possible to measure the amount of nutrient remaining by using ion-selective electrodes. These are similar to pH sensors and use the same circuits, but are very expensive. Each one may cost hundreds of dollars, and you generally need 3 to effectively measure nutrients: One for Nitrogen, one of Phosphorus, and one for Potassium. These setups are uncommon due to their high price, but can measure nutrient ratio, and depletion over time.
