Measuring the total dissolved solids, or TDS, gauges the strength of a hydroponic nutrient solution and is an unparalleled method of determining water quality. When growing hydro make sure to take all the necessary considerations so you know how to interpret the results of this powerful tool.
To understand how TDS meters work and what they measure you have to go back to make sure you understand some basic chemistry. When you take table salt, which we all know is sodium chloride, or NaCl, and put a teaspoon of it into water and stir, it eventually dissolves. Sodium chloride is an ionic substance, meaning its constituent atoms, sodium and chlorine, are ions; sodium has a strong positive charge and chlorine has a strong negative charge. When NaCl dissolves in water, these ions break apart and float around. Water with some salt in it will conduct electricity better than absolutely pure H2O because the dissolved charges can move in solution, allowing current to flow. In general, the more salt dissolved in water the higher the electrical conductivity is. On the other hand, if you dissolve a teaspoon of sugar in pure H2O you will not see any significant increase in electrical conductivity. This is because sugar does not form ions in solution, so dissolved sugar doesn’t help move electrical charge through water.
You may have seen some TDS meters with the label TDS/EC. EC, or electrical conductivity, and TDS are actually two ways of looking at the same thing, more or less. While TDS actually should refer to all substances dissolved in water ionic or not, a measurement of electrical conductivity actually gives a good estimate for the measurement of TDS, with an error of about 15%.
Most TDS meters are actually TDS/EC meters, and they will likely give you the option to switch between EC and TDS. Meters give EC measurements in milli Siemens per centimeter (mS/cm), and TDS measurements in part per million (ppm). The meter uses a conversion factor to go back and forth between these two different units. Since most people use units of ppm instead of mS/cm because it’s more tangible, we’ll stick with that. Parts per million is equivalent to milligrams per liter of water; a 300 ppm solution has 300 mg/L of dissolved minerals.
Nutrient solutions for hydroponically grown cannabis should be around 500-600 ppm for young clones and seedlings, 800-900 ppm for vegetating plants and 1000-1100 ppm for flowering. When flushing, TDS should be around 400-500 ppm. The TDS of a nutrient solution will steadily go down as your plants absorb minerals from the water. It’s safe to “re-up” a reservoir one or two times with fertilizer before you completely change the water, so keep an eye on TDS measurements to track how often you need to add fertilizer or completely change your reservoir.
It should be simple enough to add the correct amount of fertilizer to water and have it reach the desired TDS for your plant’s stage of growth, but things get tricky when you start to consider the TDS your water starts at coming out of the tap. Ideally, you should be starting with pure water at 0-10 ppm, but not everyone can afford to have a powerful reverse osmosis filtration system. Activated charcoal filters are an acceptable means of reducing levels of chloramine and heavy metals, but they will not significantly reduce the mineral content of the water, so TDS/EC measurements will be about the same.
Water quality can vary a lot across the United States and the world. Water with a TDS of 1000 ppm or higher is unfit to drink, and over 500 ppm you start to encounter the problems of water hardness (scaling pipes, soap not forming suds and bubbles, “heavy” taste). Unless you know that you have hard water in your area, the TDS is probably under 100 ppm.
If you don’t start with pure (0-10 ppm) water from a reverse osmosis filter, making nutrient solutions involves a little bit of guesswork. You need to juggle between two conflicting factors: if your TDS is too high the roots won’t absorb nutrients at the correct rate, but if you don’t add enough fertilizer in order to keep the TDS low, you run the risk of getting a nutrient deficiency. It’s generally safer to air on the side of using less fertilizer, but the main objective is the health of your plants, so keep a close eye on them to see how they respond.
Organic nutrients are a whole different ball game. Adding the recommended amount of a nutrient solutions that uses chelates to a reservoir will never give you a TDS reading as high as adding the recommended amount of a conventional hydroponic nutrient solution that uses inorganic salts. This does not mean that nutrient solutions with chelates aren’t as good, they just have different chemistries. Many chelated minerals don’t behave as ions in solution, so an electrical conductivity reading will not pick them up. Keep this in mind when preparing a nutrient solution that is labeled as organic or advertises the use of chelated minerals. Even though TDS readings will be less, you could still run into problems with the starting amount of TDS in water, and the guesswork get slightly more
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