Is my water source good for hydroponics?

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Before starting your hydroponic project it is important to know whether your local water source can actually be used to water plants. Not all water sources are compatible with plants and some require special adaptations to the nutrient solution in order to become viable. In this post I will talk about the things that can make a water source unsuitable for hydroponics and the sort of modifications that would be required to make these water sources work with plants. The main points in the post are summarized in the diagram below.

Diagram to figure out if your water can or cannot be used in hydroponics

Tap and well water sources can contain different substances characteristic of the natural environment where the water originated. Water that goes through rocky formations containing a lot of limestone will contain high amounts of calcium and carbonates, while water that goes through dolomitic rock will contain significant amounts of magnesium as well. Water that contains high amounts of Ca or Mg is not necessarily problematic and can be dealt with by adapting the nutrient solution to account for these ions, you can read more about hard water and its use in hydroponics by reading my previous post on the subject. These water sources usually need a significant amount of acid to reach the 5.5-6.5 range, so accounting for the nutrient contribution of the acid in the nutrient formulation is also fundamental.

The most problematic water sources will contain high amounts of either sodium or chlorides, two ions that we cannot deal with easily in hydroponics and that can be specially bad for plants. You can read more about sodium in hydroponics here, and chlorides and hydroponics here. Sodium concentrations below 200 ppm can be manageable, but any higher concentrations will invariably lead to issues in hydroponics. Chlorides are even more harmful with the threshold for problems at just 50 ppm. Iron can be similarly problematic as sources that contain high amounts of Fe can be incompatible with plants and the Fe can be difficult to remove. This is why the first step in analyzing a water source should always be an analysis including Na, Cl and Fe. If the values are too high then this water source will require reverse osmosis to be usable.

If Na, Cl and Fe are within limits then we can ask the question of whether this water source is approved for human consumption. If it is then we know that the amounts of heavy metals within it should be low, as well as the amount of other ions, such as nitrate and ammonium. If the water has not been approved by a utility company for human consumption then we need to do heavy metal and nitrate/ammonium analysis to figure out if this is actually safe to use. In some cases well water sources can be perfectly fine to grow plants but the products might be contaminated with heavy metals that make them unsuitable for human consumption.

If a water source is within limits for all of the above then we should take into consideration whether we need custom formulations or whether we can get away with using commercially available hydroponic products “as is”. For sources that have relatively low amounts of Fe, Ca and Mg this is usually a possibility but for sources that have quantities of Fe above 2.5 ppm, Ca above 10 ppm or Mg above 5 ppm, it is advisable to go with a custom formulation that can account for the amount of minerals already present within the water. This can still mean using commercially prepared fertilizers only that the mixing ratios and schedules need to be adapted to manage what is already present in the water, so significant deviations from the manufacturer suggestions are to be expected.

Another important point is that none of the above accounts for potential biological activity within the water, which can be a big source of problems in plant culture. For this reason always make sure to run the water through carbon filtration and have in-line UV filters to ensure that no bacteria, viruses or fungal spores get to your plants through your water source.

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3 Comments

  • Bill
    November 12, 2022 @ 11:12 am

    I have been doing hobby hydroponics for 2 years now, starting with Kratky and have moved on to small Dutch Buckets and NFT systems. In all systems, where the plants are over 90 days old, the plants suffer from what looks like calcium deficiency. I know our well water (700 feet deep) is hard but I have not yet sent a sample in to be tested.

    My question is, can test strips like for aquariums be used to estimate Ca, Mg and Fe levels?
    The blog post “Do you really need to be using RO water?” seems to indicate that Ca = ~28% of CaCO3 hardness and Mg = ~7.3% of CaCO3 hardness. Carbonate hardness and Fe levels are tested with these aquarium strips but are they accurate enough for a decent estimate?

    I used a 9-in-1 aquarium test strip with the following results:
    Iron (Fe) ppm = 3
    Total hardness (GH) = 20
    Total Alkalinity (TA) ppm = 60
    Carbonate (KH) ppm = 180
    pH = 7.5 – 8

    I estimated Ca @ 50 ppm and Mg @ 13 ppm and I’m hoping this will get me closer than doing no measurements at all. After a lab test, it would also be good to know if these test strips could be used to see if the water is changing over time…

    Thank you Danial for the work you do and your fantastic videos and blog!

    • Bill
      January 16, 2023 @ 11:55 am

      After much reading in aquarium forums, test strips seem to have a reputation for being inaccurate while liquid titration test kits tend to produce more accurate results. A challenge with aquarium test kits is that most of them are geared towards saltwater aquariums where the salt concentrations are much higher than would be found in a hydroponic solution. Freshwater aquarium enthousiasts, in particular those that grow plants in freshwater aquariums, have found ways to use these saltwater test kits to their advantage. While reading the forum on plantedtank.net, several members show ways to increase the resolution of the test kits for freshwater use. This is basically done by increasing the water sample size. Going from 5ml to 50ml water sample has the effect of increasing the titration resolution by 10. For example, if 1 drop of reagent in 5ml water = 20ppm, 1 drop in 50ml would now be 2ppm.

      One company that has been producing water test kits for the aquarium industry is API and their kits are considered some of the most affordable and readily available. Of particular interest to me was finding out Ca and Mg concentrations in my well water. Several freshwater aquarium users measure Ca2+ with the API Ca2+ test kit and measure General Hardness GH with the API GH & KH test kit. Magnesium is then found mathmatically with the results of these two tests. The calcium test kit produces ppm Ca2+ ion concentration. The general hardness test kit produces GH expressed as ppm CaCO3. To find Mg we need to convert Ca2+ ion concentration to Ca expressed as ppm CaCO3.

      Ca expressed as ppm CaCO3 = Ca2+ ppm * 2.5
      Mg ppm = ((GH expressed as CaCO3) – (Ca expressed as CaCO3))/ 4.1

      I have sent a water sample to the Oregon State University to have ICP-OES elemental analysis done but it can take up to 6-weeks to get results back. When I get the report, I will compare the ICP-OES numbers to the API titration numbers to see how well they line up.

      • Bill
        April 6, 2023 @ 8:20 am

        The Oregon State University ICP-OES test results came back and indicated higher Magnesium than Calcium whereas the API Ca2+ test kit indicated the opposite. I’m not sure what to think about the error in Calcium readings between the two tests. I am currently using the the levels as measured by the university in my nutrient formulations. The following concentrations are expressed in ppm. Elements that were below quantifiable limits have been omitted.
        ___________________ ICP-OES __ API test kit
        Nitrogen (NO3-)_____ 0.39
        Potassium (K) ______ 1.55
        Magnesium (Mg) ___ 5.78 _______ 0.5
        Calcium (Ca) _______ 1.18 _______ 19
        Sulpher (SO4) ______ 0.40
        Iron (Fe) ____________ 6.04
        Boron (B) ___________ 0.07
        Silicone (Si) _______ 18.87
        Sodium (Na) ________ 4.06

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