How much Phosphorous are you adding to your solution to adjust pH?

Phosphoric acid is one of the most commonly used pH down agents in hydroponics. This is because phosphoric acid is available in high purity, is easier to handle and has lower cost. However, phosphorous is a significant plant macro-nutrient as well, and substantially changing the level of available P in a nutrient solution can have negative effects on plant growth. Since many hydroponic users – especially those that use hard water sources – might be adding significant amounts of acid to correct their pH level, it is important to estimate how much phosphorous you’re contributing to your solution by adjusting pH and whether this means you also need to adjust your formulation to use less P within it.

Schematic representation of a phosphoric acid molecule.

Phosphoric acid is generally available in concentrations from 30 to 80%, most hydroponic users will use pH-down solutions that are in the 35-45% range, which are prepared to be concentrated enough to last a significant amount of time while diluted enough to allow for easier handling and to be less corrosive. You can use the equation given above to calculate the P contribution in ppm from a given addition of phosphoric acid (you can look up the density for a given concentration using this table). Adding 1mL/gal of 45% phosphoric acid will contribute around ~48 ppm of P to your nutrient solution. This is a very large amount of P considering that the normal range for flowering plants is between 30-60 ppm.

Having an excess of P can be very problematic as phosphorous can strongly antagonize certain nutrients, especially if the pH of the solution drifts up as the plants are fed. At P concentrations exceeding 120 ppm, this element can start to antagonize elements like Fe, Ca and Zn very strongly, preventing their absorption and leading to plant issues. Furthermore, excess of P can often cause problems with P absorption itself – as it can become locked up inside the plant as Fe or Ca salts – which can lead to P deficiency-like symptoms. The most tricky thing about P toxicity issues is that they do not show as certain characteristic symptoms, but mostly as deficiencies for other nutrients or even P itself. The exact symptoms will depend on the VPD and particular environmental conditions as these play an important role in Ca absorption as well.

P contribution in ppm = (Acid concentration in % / 100) * 0.3161 * (volume of addition in mL) * (density of acid in g/mL) * 1000 / (total volume of solution in liters)

Many growers will indiscriminately add P without considering how much was required to adjust pH, which is a bad idea due to the above reasons. A water source that is very hard might require almost 1mL/gal to fully adjust the solution to the pH range required in hydroponics, if a normal hydroponic solution is fed – which will contain all the necessary available P (assuming the user adds very little outside of it) – then this means that the final solution might end up with P levels that will strongly antagonize several nutrients. It’s therefore no wonder that many hydroponic growers in harder water areas suffer from consistent issues with Ca and Mg, many of these cases could be caused by the presence of excess P within nutrient solutions. While many hydroponic hard-water formulations will adjust for Ca and Mg in hard water, they will generally not adjust for P as they cannot know for certain how the user will lower the pH.

If you’re a hydroponic grower using phosphoric acid, keeping track of how much P you’re adding to your nutrient solution to adjust pH is going to be very important. If you’re adding more than 0.25 mL/gal of 45% phosphoric acid – of course adjust accordingly for higher/lower concentrations – then you should consider adjusting your hydroponic formulation to account for this expected P addition and prevent your formulation from reaching abnormally high levels of P.

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6 Comments

  • Toni
    October 26, 2020 @ 3:38 am

    This is really useful info,
    When I use HydroBuddy I always lower P by ~15 ppm in my formulations which gives me approximate room for adjusting PH by H3PO4.
    One way of easy calculation is to use a separate folder of HydroBuddy where you only calculate H3PO4 concentration by using the default 59% solution from the database or creating your own concentration. In such a way you can see the relation between PPM and various H3PO4 concentrations, in my case I usually end up with ~15 ppm of P added by additional H3PO4 needed for my PH adjustment.

  • Keesje
    November 1, 2020 @ 1:42 am

    Dear Dr. Fernandez,

    I spoke to someone you know about your article, in particular about the fact that too much P could block the absorption of Ca and Mg. He said this was incorrect – that a lot of P actually improves the absorption of Ca – but I had no scientific source to show him. Except your report, of course. Do you have a source that I can show him?

    Btw: I liked your video!

    Kind regards
    Kees

    • admin
      November 1, 2020 @ 8:52 am

      Hi Kees,

      Thanks for a lot for posting! I’m glad you liked my video :)

      About your question, you can read Mulder’s research or look at any Mulder’s chart available online to see that Ca and P have an antagonistic relationship. However, things are not as simple. Within normal concentration ranges your friend is indeed correct, higher P will tend to help with Ca absorption – to a point. There is well documented evidence that this is the case, especially under higher K conditions (https://www.tandfonline.com/doi/pdf/10.1080/09064719309410224?needAccess=true). In my experience Ca deficiency problems caused by P can show up when you’re feeding relatively low Ca and higher P (100-120 ppm of Ca and 120 ppm of P). At higher P values, Ca will greatly enhance the P problem, making the plants look P deficient (see here for a researched example https://nph.onlinelibrary.wiley.com/doi/full/10.1111/nph.15447). This is particularly evident in P sensitive plants but can also happen in more common flowering plants provided extreme enough conditions are reached.

      All things considered, plant nutrition is sadly just not that simple. Nutrients can be synergistic within certain ranges and antagonistic in others. Furthermore, things like VPD conditions can change the extent of these zones, so great care is needed when evaluating a given growing situation and nutrient solution. Yes, more P can increase Ca absorption or more P can decrease Ca absorption, depending on what the P/Ca is, what the plant you’re growing is and what the environmental conditions are like.

      The point of this article is to make growers aware of the fact that P additions to set pH are not innocuous. Adding a large amount of P to set your pH can put your Ca/P in a place that is not favorable, especially if the formulation did not consider this large P addition. Thanks for your comment!

      Best Regards,

      Daniel

      PS: Mg was indeed a mistake, I changed it to Fe, Zn which were what I intended to mention besides Ca.

  • Hydroboy
    November 7, 2020 @ 1:23 am

    Hi Dr Fernandez,
    I just bought a phosphoric acid in a bottle with label 85% and I assumed this refers to the acid concentration in %. So to use the formula I just substitute it.. however it does not indicate any density of acid how will I determine? Also, the volume of addition in ml is the amount after the ph stabilize at the desired ramge?
    Thanks btw, for the informative video I’m a subscriber.
    Regards,

  • Michael
    November 21, 2020 @ 6:06 am

    Very interesting information, I always thought the contribution from phosphoric acid would be much lower but the numbers seem to prove me wrong..Another interesting topic would be how certain raw materials (like ammonium dihydrogen phosphate) influence PH. One could simply use hydo buddy and limit itself to raw materials which create nutrient solutions in the desired PH range..

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