Diagnosing nutrient deficiencies or toxicities in a hydroponic system just by examining the physical state of the plants is difficult. Taking a closer look at the quality of your nutrient solution is often required and usually involves measuring your solution’s EC and TDS levels using a hand-held meter. Here’s how EC and TDS are related, and where exactly PPMs fit into the equation.

The terms EC, TDS and ppm are often used by growers in relation to their feeding regimens. The following is an exploration of what these terms mean and what their relationship is to each other.

Electrical Conductivity

Electrical conductivity (EC) measures water’s ability to conduct an electrical charge. Perfectly pure water (H2O with nothing dissolved in it) does not conduct electricity and is therefore known to be a good electrical insulator—a material whose internal electric charges do not flow freely. Water quits being a good insulator once it starts dissolving substances around it.

Many of the molecules dissolved in water are salts. In chemical terms, salts are ionic compounds composed of cations (positively charged ions) and anions (negatively charged ions). Salts can be easily identified since they usually consist of cations from a metal and anions from a non-metal. In solution, these ions essentially cancel each other out so the solution is electrically neutral (without a net charge). Even a small amount of ions in a water solution make it capable of conducting electricity. The higher the solution’s salt concentration, the better it conducts electricity.

While a solution’s EC is a good indicator of its total salinity, it still doesn’t provide any information about the ion composition in the water. For instance, we don’t know if there is a lot of potassium or table salt in the solution. To further complicate the EC measurement, not all elements that are considered fertilizers will add to the EC of a solution. For example, urea is probably the most important nutrient that does not affect the EC.

Growers use EC meters to measure the EC levels of their nutrient solutions. The meters measure EC either in mS/cm (millisiemens/cm) or μS/cm (microsiemens/cm). The conversion of μS/cm (microsiemens/cm) to mS/cm (millisiemens/cm) is 1,000 μS/cm = 1 mS/cm.

Total Dissolved Solids

Total dissolved solids (TDS) is a measure of the combined content of all inorganic and organic substances contained in a liquid in molecular, ionized or micro-granular suspended form. To be considered dissolved, the solids must be small enough to fit through a 2-micrometer filter.

The two main methods of measuring total dissolved solids are gravimetry and conductivity. Gravimetric methods are the most accurate and involve evaporating the liquid solvent and measuring the weight of the residue remaining. If most of the TDS comes from inorganic salts, gravimetric methods give an accurate result, but if organic material is in the makeup of the TDS, then much of it may be destroyed during the heating process.

The way to measure the TDS of water in a laboratory setting is to boil the water until there is none remaining, and then weigh the remaining material. This isn’t practical for hobby growers, as accurate scales and special equipment is required to achieve such measurements. A TDS meter is the next best way to achieve an estimated TDS reading.

What is the Connection Between EC and TDS in a Hydroponics System?

TDS meters are really EC meters in disguise. A TDS meter reads the EC of the dissolved salts in the solution and uses a conversion factor to provide a ppm reading. Let’s say the EC is 2,200 microsiemens.

Some dissolved salts with an EC of 2,200 microsiemens will have a ppm of 1,540 (I used a conversion factor of 0.7), but this does not mean the solution you measured has that 1,540 ppm.

A conversion factor is a known value used as a multiplier for converting a quantity expressed in one set of units into an equivalent value expressed in another set of units. The conversion factor depends on what the salt solution is; it could be the correct conversion factor, but that is unlikely.

The relationship of TDS and EC can be approximated by the following equation:

TDS = keEC

In this equation, TDS is expressed in milligrams or liters and EC in microsiemens per centimeter at 77°F. The correction factor ke usually varies between 0.55 and 0.8, and is specific to each salt. When you have a mixture of salts, such as in a feeding solution, a correction factor that will emulate that mixture should be used.

EC meters that read as TDS (ppm) often use a correction factor of 0.5 or 0.7, depending on the meter being used. Because it uses a pre-set conversion factor instead of one specific to the solution, the reading is less accurate than it could be.

When comparing ppm readings with another grower, check to make sure you are both using meters with the same conversion factor. Also mind your temperatures because the temperature of a nutrient solution will cause the EC reading to change. A higher temperature will generally give a higher reading.

In a growing situation, the temperature differences and the effect on the EC is usually not significant enough to matter, and most meters these days are temperature-compensating so the grower does not have to manually make adjustments. As the temperature changes, the meter makes an adjustment in the reading to compensate. The meter manual will state what temperature is used as the base; often 77°F is used, such as with the above example.

Parts Per Million

Parts per million (ppm) is a way of expressing very dilute concentrations of substances. Just as per cent means out of 100, parts per million means out of one million. The measurement is commonly used to describe the concentration of something in water or soil.

TDS meters give readings as ppm (parts per million). In the metric system, ppm is easy to figure out because the readings are equal to 1 gram in 1,000 liters of water, or 1 milligram in 1 liter of water. In soil, it is 1 milligram in 1 kilogram of soil. Parts per million is a weight-per-weight measurement, which is not always apparent when water is involved because 1 liter of water is equal to 1 kilogram of water. To put ppm in perspective, a 1% solution is the same as a solution containing 10,000 ppm.

The Next Steps

So, you have taken an EC reading (or a TDS reading) of your nutrient mixture. What do you now know from the reading? Not much, really. An EC meter only tells you what the electro conductivity is and nothing more. If you know you want your feed to have an EC of 2.5 mS/cm, you can achieve this by mixing fertilizers together, or you could mix sea water with tap water to the right proportions that will give an EC of 2.5 mS/cm.

If you want to know what is actually contributing to your EC reading and how much is in your feed, you need to have a chemical lab analysis done. This will tell you how much nitrogen is in the solution, how much of the nitrogen is nitrate nitrogen, how much is ammonia, and provide information on any other element you have requested.

But if you don’t ask for the analysis to include a particular element, it won’t tell you. Fortunately, labs usually have a standard nutrient solutions test that will include the most common fertilizer elements so you don’t have to list everything. However, if you are feeding your plants something unusual, you will have to ask for that element to be included in the analysis.

An EC meter is a good tool for checking up on yourself. If you test your feed solution and it has the EC you expected, then you have used the right proportions. If it is too high, you have used too much, and if it is too low, you have not used enough. If you test your drain water and the EC level is too high, you are probably not watering enough, and if it is too low, you are likely watering too much.

I don’t understand why growers use a TDS meter and read in ppm, since it is really just an EC meter using a corrections factor. However, both meters are only providing basic information, which is how much salt is in the solution.

With this in mind, meters can help growers replicate a successful feed solution as close to previous times as possible. In other words, a meter allows growers to check and make sure their end result is close to previous, successful results. Meters must be calibrated periodically to remain reliable.

Monitoring your TDS or EC readings is important. Doing so helps you understand if you’re feeding your plants too much or not enough. If you’re new to growing, using a well-balanced feed is probably a good place to start.

Plants benefit from a well-balanced nutrient mixture the same way humans benefit from eating a well-balanced diet—their bodies become healthier, and their production rates go way up.