A one-part hydroponic nutrient formulation for very hard water


What is water hardness?

There are many parameters that determine the quality of a water source. Water that has a composition closer to distilled water is considered of a higher quality, while water with many dissolved solids or high turbidity is considered low quality. Calcium carbonate, magnesium carbonate, calcium sulfate and calcium silicate are some of the most common minerals that get dissolved into water as it runs through river beds and underground aquifers. The carbonates and silicates will make water more basic, will increase the water’s buffering capacity and will also increase the amount of magnesium and calcium present in the water.

Water hardness is determined experimentally by measuring the amount of Calcium and Magnesium in solution using a colorimetric titration with EDTA. Although both Calcium hardness (specific amount of Ca) and Magnesium hardness (specific amount of Mg) are measured, total water hardness (the sum of both) is the usually reported value. The result is often expressed as mg/L of CaCO3, telling us how much CaCO3 we would require to get a solution that gave the same result in the EDTA titration.

The Calcium and Magnesium present in water sources with high hardness is fully available to plants – once the pH is reduced to the pH used in hydroponics – and it is therefore critical to take these into account when formulating nutrients using these water sources. It is a common myth that these Ca and Mg are unavailable, this is not true.

What about alkalinity?

Water alkalinity tells us the equivalent amount of calcium carbonate we would need to add to distilled water, to get water that has the same pH and buffering capacity. An alkalinity value of 100 mg/L of CaCO3 does not mean that the water has this amount of carbonate, but it means that the water behaves with some of the chemical properties of a solution containing 100mg/L of CaCO3. In this particular case, it means that the water requires the same amount of acid to be titrated as a solution that has 100mg/L of CaCO3.

Water sources with high hardness will also tend to have high alkalinity as the main salts that dissolve in the water are magnesium and calcium carbonates. Since these carbonates need to be neutralized to create a hydroponic solution suitable to plants, the anion contribution of the acid that we will use to perform the neutralization needs to be accounted for by the nutrient formulation.

An example using Valencia, Spain

Valencia, in the Mediterranean Spanish coast (my current home), has particularly bad water. Its water has both high alkalinity and high hardness, complicating its use in hydroponics. You can see some of the characteristics of the water below (taken from this analysis):

Alkalinity240mg/L of CaCO3
Typical water quality values for water in Valencia, Spain.

Hard water creates several problems. Since Calcium nitrate is one of the most common sources of Nitrogen used in hydroponics, how can we avoid using Ca nitrate? Since we have more than enough. Also, how can we neutralize the input water so that we can make effective use of all the nutrients in it without overly increasing any nutrient, like P, N or S, by using too much of some mineral acids?

Creating a one-part solution for very hard water

HydroBuddy allows us to input the characteristics of the input water into the program so that we can work around them while designing nutrient solutions. To get around the above mentioned problems – but still ensure I could easily buy all the required chemicals – I decided to use a list of commonly available fertilizers. I used Calcium Nitrate, Magnesium Nitrate, Potassium Nitrate, Phosphoric acid (85%) and a micro nutrient mix called Force Mix Eco (to simplify the mixing process). This micronutrient mix is only available to people in the EU.

HydroBuddy results to create 1 gallon of 1:100 nutrient solution for Valencia’s very hard water.

Note that we use absolutely no phosphates or sulfates, since the solution already contains more than enough sulfur (89 ppm) and we need to add all the Phosphorus as phosphoric acid to be able to lower the alkalinity. I determined the amount of P to add by setting P to zero, then using the “Adjust Alkalinity” to remove half of the alkalinity of the water using phosphoric acid. This is more than enough P to be sufficient for higher plants. The above nutrient ratios should be adequate for the growth of a large variety of plants, although they are a compromise and not ideal for any particular type of plant.

Since we are adding no sulfates and the pH of the solution is going to be very low (because of the phosphoric acid), we can add all of these chemicals to the same solution (no need to make A and B solutions). The values in the image above are for the preparation of 1 gallon of concentrated solution. This solution is then added to the water at 38mL/gal of tap water to create the final hydroponic solution.

Does it work?

I have experimentally prepared the above concentrated solution – which yields a completely transparent solution – and have created hydroponic solutions I am now using to feed my home garden plants. After adding to my tap water – initial pH of 7.6 – I end up with a solution at a pH of 5.6-5.8 with around 1.5-1.8mS/cm of electrical conductivity. The plants I’m currently growing – basil, rosemary, chives, mint, malabar spinach and spear mint – all seem to thrive with the above solution. I am yet to try it on any fruiting crops, that might be something to try next year!

Are you growing using hard water, have you prepared a similar one-part for your hard-water needs? Let us know what you think in the comments below!



  • Noah Johnston
    June 27, 2022 @ 1:36 pm

    Wow, I would not have expected to be able to omit sulfur just because of using tap water. I am also amazed that this can be stored as a one part concentrate! I might have to do this, except then I’d have to mix my own instead of buying premix.

    Many fertilizer manufacturers in my area (USA) originally offered hard water formulations but most have been discontinued.

    For the past couple years I’ve been ignoring my hard water (between 200 and 600 ppm TDS, and 90-200 alkalinity) and I’ve been using a bagged fertilizer mix meant for RO. I adjust the pH with sulfuric acid.

    I haven’t noticed any issues whatsoever so I consider it “safe” to ignore what comes along with my water and the additional sulfate that I’m adding. Does the random amount of calcium/magnesium and the additional sulfate actually pose a risk I should be concerned about?

    • admin
      June 28, 2022 @ 1:04 am

      Thanks for commenting. Your preparation practice is fine. Plants are great at ignoring additional sulfur – sulfur uptake saturates quite quickly in most plants and additional sulfur is not a problem – so using sulfuric acid to correct the pH of hard water is great. Depending on how much Mg and Ca are present in your fertilizer you might be getting significantly more than needed, but this excess is unlikely to cause you significant issues. Preparing your own mix might bring some benefits on yield or the quality of your produce – depending on what you’re growing and how deviated you are from what is optimal for it – but I would see nothing immediately concerning about your current fertilization practice.

  • Julian
    July 8, 2022 @ 7:01 am

    Hello Daniel,
    in my region, most ready-to-buy liquid fertilizers supply N as nitrate and/or ammonium and/or urea.
    As far as I know, ammonium acts as the primary partner for reducing the alkalinity in such formulations. But lately, a popular brand changed to using no ammonium, but just nitrate and urea for their formulation. I see people having problems with this, as (if I am right) the effect of ammonium resulting from the hydrolosys of urea and nitrification is delayed by the time these processes take.
    I am curious, what your thoughts are on this specific topic (and if I originally did understand this right and drew the right conclusion of it).
    Thanks! And as always, great content and clarification.

    • admin
      July 8, 2022 @ 9:10 am

      Hi Julian. Thanks for posting. Your understanding is correct in that urea hydrolysis is unpredictable in hydroponics, because the microbes that usually carry out this process are mostly absent, so it takes a really long time to happen (unless the microbes are added). Additionally, if you add a lot of urea – because it decomposes slowly – but the microbes ARE present, then you will have a massive influx of ammonium that will be very detrimental to plants. Urea should not be used in normal hydroponic crops, there is really no reason why it should be used in the nutrient solutions.

  • Andrew
    September 30, 2022 @ 4:19 pm

    Hi,my name is Andrew and I cannot get around my issues with my hard tap water and Canna coco A/B fertilizer in coco coir.
    I also states that I do high frequency fertigation and use 20-10% run off from the pots.I use to dilute my tap to 0.4 Ec and mix the ferts as their schedule advise,I obtain roughly a 1.5 Ec solution that I dilute with distilled to 1.0 Ec.
    I pH to 5.8 ,at times 5.7 always some hours before fertigation

    My water values are the following from municipal tap analysis:
    pH 7,7
    DRY RESIDUE AT 180° mg/l 287
    Hardness: 24° F
    EC µScm-1 at 20°C 464
    CALCIUM mg/l 50
    MAGNESIUM mg/l 27
    AMMONIUM mg/l NH3-N 0,13
    CHLORIDES mg/l Cl- 22
    SULPHATES mg / l SO42- 25
    POTASSIUM mg / l 2.0
    SODIUM mg / l 15
    ARSENIC µg / l As <1
    RESIDUAL CHLORINE mg / l 0.2
    FLUORIDE mg / l 0.31 1.50
    NITRATES mg / l NO3 – 9 50
    NITRITES mg / l NO2- <0.05 0.50
    MANGANESE µg / l Mn <5 50

    I use a lot of their Nitric acid as pH down(3%) and experiencing nitrogen toxicity in some plants.
    I also see issues with Phosphorus and calcium/magnesium(spotting) while flowering my plants.

    My fertilizer:
    Canna coco NPK Coco
    A NPK Formula: 4-0-1. Total N=3,8%
    (N-NO3)3,8%| (K2O)0,7%| (CaO)6,3%| (MgO)0,6%| (Fe chelated with DTPA)0,02%|

    Coco B NPK Formula: 0-4-2. Total N=0,8%;
    (N-NH4)0,1%| (N-NO3)0,7%| (P2O5)3,9%| (K2O)2,2%| (MgO)1,8%| (SO3)1,8%| (B)0,007%| (CU)0,001| (Mn)0,012%| (Mo)0,02| (Zn)0,007%

    Cannazym 0-2-1 (N)0%,(P2O5) 1.7% , (K2O) 0.8%, (S) 0.1%

    So,What is wrong with my tap water,buffered coco coir directly in bags from Canna and my nutrient ratios?
    How can I use my tap with those nutrient?Should I dilute with distilled?(which I already do to 0.4 Ec before adding their ferts as they suggest)
    I hope you can help me,I am very frustrated by this problem since I am no chemist or qualified for this

    • admin
      October 2, 2022 @ 4:56 am

      Thanks for posting. If you’re interested in my help to address your problem please book an hour of consulting through the website (http://scienceinhydroponics.com/book)

      • Andrew
        October 2, 2022 @ 2:43 pm

        I really appreciate your help and youtube channel Sir,I have to deny only because I am a simple hobby grower and cannot at the moment invest in R/O system,neither I could buy 1h of your precious time,main reason being my current life situation.Many thanks for your help and reply Sir Daniel,you are so educated and polite.
        Best wishes and stay safe

Leave a Reply

Your email address will not be published.