Hydroponic EC and EC Meters Explained
By Grubbycup 12 November 2021
Electrical conductivity (EC) meters are used to estimate the amount of nutrient ions in a nutrient solution. They rely on the principle that solutions with more ions conduct electricity better than those with fewer. Although the exact specifics of how each meter performs the test may differ, the basic concept is the same. All EC meters test how well a given solution conducts electricity.
Pure water is not a particularly good conductor of electricity. To reflect this, EC meters will show a reading of zero (or at least ideally zero, some meters have trouble when there isn’t enough conduction) when placed in pure water. Water with conductive contaminates in it (for example, the ions from salts) do conduct electricity, and that difference is where EC meters come in.
Nutrient solutions are created by dissolving measured amount of nutrients (often salts of nitrogen, phosphorus, and potassium) into water (the solvent). Hydrophilic (water-loving) materials like salts tend to dissolve easily in water because water molecules have a positive charge on the side with the two hydrogen atoms, and a negative charge on the oxygen side. These are attracted and attach easily to many other molecules. Salts are often used as nutrient sources because they also have a positive and negative side but are generally held together with a weaker ionic bond. Water molecules attach themselves to either side of the salt molecule and literally split them apart. That’s why many chemical hydroponic solutions make use of nutrient salts dissolved in water.
When salts such as potassium nitrate are dissolved in water, they break into their ionic forms. In the case of potassium nitrate this breaks into ions forming the cation K+ (potassium) and anion NO3- (nitrate). Not only is this handy as a source of both potassium and nitrogen for the plants, but it allows the solution to better conduct electricity. There are many salts that can be used as fertilizers, such as urea, ammonium nitrate, mono ammonium phosphate, diammonium phosphate, and potassium chloride to name a few. The more salts that are added to the solution (to a point), the better the solution conducts electricity, and the higher the EC will read on the meter.
Raising the temperature of the solution increases the movement of its molecules, including its ions, and raises its conductivity. As one of the factors of conductivity is temperature, some EC meters include a thermometer to adjust their readings automatically, or one without can be used if care is taken to measure samples at the same particular temperature each reading or with the use of a temperature-conductivity adjustment chart.
One place where EC can be immediately useful is in determining if a solution is within expected tolerances. If the EC numbers unexpectedly double in a fresh batch of nutrient solution, it may be an indication that nutrient concentrates were added twice. Paying attention to EC levels can not only help identify issues but can be used to track changes in a nutrient schedule which can then be compared with similar historical data recorded from previous seasons.
In general, nutrient schedules start light, then build in intensity to a plateau, and then taper off as harvest approaches. Careful recording of readings during one season can set a baseline to compare subsequent seasons against, possibly alerting an attentive gardener to potential issues or unexpected changes. Note what nutrient schedule was used for the week, and the EC of the nutrient solution. This way changes in the nutrient schedule (or other factor) can be compared to prior performance. Changes that can be linked to improvements can be kept, and detrimental changes can be reversed. This allows for a continuous fine tuning of the nutrient regimen.
While EC meters do a fair job at displaying how many ions are in the solution, they don’t indicate which ions are in the solution and it will not show results for any particular nutrient. A solution high in nitrogen salts but low in phosphorus may give an EC reading of 2, but so can one low in nitrogen but high in phosphorus. A solution made with table salt may show an EC of 2 but still not have enough nutrients in it for plant growth because plants don’t need much chlorine and need even less sodium. A different solution with an EC 2 value might be well suited to growing plants if the ions are part of a balanced mixture of macronutrients and micronutrients. The easiest way to ensure the solution is a mixture of the desired elements is to add them in the correct proportions, such as by following a manufacturer’s recommended feeding chart. To correct imbalances in a recirculating system, the nutrient solution can be replaced periodically to rebalance the mix of available nutrients in the solution.
Parts per million (PPM) and total dissolved solids (TDS) meters use the same principle. In fact, they are fundamentally the same EC meter with the results displayed slightly differently. There is currently an issue with the lack of a standard conversion between EC and PPM which often leads to confusion. Depending on which of the three most popular PPM standards are used, the conversion factor is 0.5, 0.64 or 0.7. Less commonly used is total soluble salt (TSS) which is x10 the EC reading.
Hydroponic gardeners can use EC meters to check the quality of the starting water, ensure freshly mixed nutrients are within expectations, and check recirculating nutrient solutions to determine if more nutrient solution or just more water should be added. It is an important tool in a hydroponic gardener’s toolbox. On the other hand, those who rely on more traditional organic nutrients tend to rely on sources other than salts to deliver their nutrients, and as a result EC metering of organic-based nutrient solutions is less useful and often not done. Adding more kelp to a nutrient solution doesn’t make the EC change as much as a similar amount of salt would.
An EC meter is an essential tool to hydroponic gardening. Since the only nutrients available are those administered by the gardener, being able to monitor the overall levels can be not only helpful but can alert a grower to possible mistakes in solution mixing.