As the liquid fertilizer gets used up how can I maintain the right levels of nutrients in it?


I am just starting to grow vegetables hydroponically in my small outdoor greenhouse. As the liquid fertilizer gets used up how can I maintain the right levels of nutrients in it? How do I know how much fertilizer to add in my continual mixes? And what is “EC level” that you keep referring to? I’m a beginner.

Thank you,


Thank you for your question. Maintaining the optimal nutrient concentration in a hydroponic system will allow your plants to continue to perform to their full potential. Each nutrient manufacturer has its own suggested nutrient concentration for hydroponic applications. This is usually expressed in either parts per million (PPM), total dissolved solids (TDS), or electrical conductivity (EC).

Most hydroponic growers use a monitoring device that expresses the nutrient concentration in the form of PPM or TDS. Parts per million is a form of measurement used to determine the concentration of diluted substances. One PPM is equal to one milligram of substance per liter of water. In hydroponic gardening, PPM or TDS is used to express the amount of nutrients in the nutrient solution.

In fact, PPM and TDS are just different terms that actually express the same thing. Even meters that express PPM/TDS measurements rely on electrical conductivity that is automatically converted into a PPM/TDS format. Some of the older meters do not automatically convert the EC values into a PPM or TDS value. If a grower is using an EC meter that expresses the nutrient concentration in EC form, he or she may need to convert the value to a PPM or TDS value in order to compare it to the manufacturer’s PPM or TDS recommendation.

As the plants grow and use the individual elements in the nutrient solution, most hydroponic growers simply replace the used solution with a premixed nutrient solution based on the manufacturer’s recommendations. This is totally acceptable for the short term. However, because nutrient meters express the total nutrient concentrations and not individual elements, the actual ratio of nutrients to one another may get out of whack over time. If an individual nutrient concentration becomes too low, the plants will not receive all that they need to sustain accelerated growth rates. If a particular individual nutrient gets over-concentrated, it may cause serious burning or lock out issues, which can also lead to nutrient deficiencies. Perhaps the best safeguard against a nutrient solution with an undesirable ratio of individual elements is to do a complete reservoir change every two weeks or so. A periodic, complete reservoir change removes the unbalanced nutrient solution and replaces it with a fresh, balanced nutrient solution.

In summary, hydroponic gardeners should invest in a decent nutrient meter to compare PPM or TDS values to the manufacturer’s recommendations. To safeguard against an unbalanced ratio of nutrients that naturally occurs as the plants feed, a hydroponic grower should do a periodic reservoir change. I hope this answers your questions.

Keep on Growing,
Eric Hopper

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Written by Eric Hopper
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Eric Hopper’s past experiences within the indoor gardening industry include being a hydroponic retail store manager and owner. Currently, he works as a writer, consultant and product tester for various indoor horticulture companies. His inquisitive nature keeps him busy seeking new technologies and methods that could help maximize a garden’s performance.

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