A guide to different pH down options in hydroponics

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The control of pH in hydroponic nutrient solutions is important. Plants will tend to increase the pH of solutions in most cases – as nitrate uptake tends to dominate over the uptake of other ions – so most growers will tend to use pH down much more than they use pH up. While most growers prefer to use concentrated strong acids, there are a wide variety of different choices available that can achieve different outcomes at different cost levels. In this post I want to talk about different pH down options in hydroponics, along with some of their advantages and disadvantages.

Hydrangeas change color as a response to different pH values in soil

The first group of pH down chemicals are strong acids. These are technically acids with very low pKa values, meaning they react instantly with water to generate at least one mole of hydronium for each mole of added acid. They offer the strongest ability to drop pH per unit of volume, which makes them more cost effective. However the fact that they often need to be diluted to make the pH addition process practical – because of how much the concentrated forms can change pH – can make their use more difficult than other forms of pH down. These are the most common options:

Phosphoric acid (from 20 to 85% pure): This acid doubles as a plant nutrient, meaning plants will be affected by the phosphorus added. It is commonly used in food – so food grade phosphoric acid can be bought cheaply – it also has additional deprotonations with strong buffering at a pH value of 7.2 with buffering capacity against bases getting stronger as the pH goes down all the way to 6.2. This is the most commonly used acid by hydroponic growers.

Sulfuric acid (from 20 to 98% pure): This acid is commonly used in car batteries and offers the largest pH dropping ability per unit of volume among all the strong acids. It is however important to use food grade sulfuric acid in hydroponics as normal battery acid can include some heavy metal impurities – from the fabrication process of sulfuric acid – that might negatively affect a hydroponic crop. Food grade sulfuric acid is safe to use in hydroponics. A big advantage is that plants are quite insensitive to sulfate ions – the nutrient provided by sulfuric acid – so adding sulfuric acid does not really affect the nutrient profile being fed to the plants.

Nitric acid (from 30-72% pure): This acid also provides nitrate ions to plants, so it also contributes to a solution’s nutrient profile. It is however more expensive than both phosphoric and sulfuric acids and more heavily regulated due to its potential use in the fabrication of explosives. The acid itself is also a strong oxidant, so storage and spillage problems are significantly worse than with phosphoric and sulfuric acid. Although this acid can be used in hydroponics, it is generally not used by most growers due to the above issues.

Diagram showing the dissociation of a strong vs a weak acid

The second group of pH down chemicals are weak acids. These are acids that do not generate at least one mole of hydronium ions per mole of acid when put in solution, but do provide a pH down effect as some hydronium ions are generated. This means that larger additions will be needed to cause the same effect but at the same time their handling is usually much safer than for strong acids. Here are some options that could be used as a pH down.

Common food grade organic acids (citric acid, acetic acid, etc): Organic acids are a very low cost way to lower the pH of a hydroponic solution as many of these are available off the shelf in super markets in food grade qualities. The main issue with organic acids – which anyone who has used them has probably experimented – is that the effect of the acids does not seem to hold (pH goes up quickly after the acid is added and the solution comes into contact with plants). This is actually caused by the fact that plants and microbes can actually use the conjugated bases of these ions nutritionally, causing an increase in pH when they do so. The initial addition of say, citric acid, will drop the pH – generating citrate ions in the process – these will then be absorbed by microbes and plants, increasing the pH again rapidly. The use of these acids is therefore not recommended in hydroponics.

Monopotassium phosphate (MKP): This salt contains the first conjugate base of phosphoric acid and is therefore way less acidic than it’s full on acid partner. Since it’s a solid its addition is way easier to control compared to the acid and it can also be handled safely with minimal precautions. It provides both potassium and phosphorous to a solution – both important nutrients – and therefore needs to be used carefully when used as a pH down agent (as it significantly affects the nutrient profile of the solution). Since it adds both a cation that helps counter pH increases by plants and phosphate species it provides a double buffering effect against future pH increases. It is a very common ingredients of commercial pH down solutions for this reason.

Monoammonium phosphate (MAP): Similar to the above, except for the fact that this salt adds nitrogen as ammonium, which is a nitrogen form plants are very sensitive to. Plants will uptake ammonium preferentially over any other cation, so MAP provides a very strong buffering effect against nitrate absorption, with potential problems if too much is used (although this depends on the plant species being grown). When MAP is used as a pH down its addition therefore needs to be carefully controlled in order to avoid excess usage. Due to the presence of this powerful ammonium buffer, MAP is generally very effective at preventing future increases in pH, although this might be at the expense of yields or quality depending on the crop.

Potassium bisulfate: This salt contains the first conjugate base of sulfuric acid and is therefore a powerful tool to decrease the pH of a solution. The resulting sulfate ions provide no chemical buffering effect, so the only buffering effect in terms of plant absorption comes from the addition of potassium ions, which can help mitigate nitrate absorption. This salt is also considerably expensive compared with the two above – which are commonly used fertilizers – and is therefore seldom used in hydroponics.

Which is the best pH down solution? It depends on the characteristics of the growing system. Generally a pH down solution needs to be easy to administer, cheap and provide some increase in buffering capacity overtime – to make additions less frequent – so the pH down product or combination of products that best fits this bill will depend on which of the above characteristics is more important for each particular user.

People who use drain-to-waste systems usually go for stronger acids, since they only adjust pH once before watering and then forget about the solution. This means that additional buffering capacity in the solution is probably not going to be very important and cost is likely the most important driving factor. If injectors are used then the strong acids are often diluted to the concentration that makes the most sense for them and most commonly either phosphoric or sulfuric acids are used.

For growers in recirculating systems options that adjust pH with some added buffering capacity are often preferred, because the same solution is constantly subjected to interactions with the plants. In this case it’s usually preferred to create a mixture of strong and weak buffering agents so that both quick decreases in pH and some increased protection from further increases can be given to the solution. In automated control systems using something like a concentrated MKP solution is preferable over any sort of solution containing phosphoric acid, as issues from control failures are less likely to be catastrophic.

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15 Comments

  • Francisco
    May 15, 2020 @ 4:20 am

    Hi,

    I was wondering if hydrogen peroxide could be used as a pH down agent as well? And what would be the consequences?

    Cheers,
    Francisco

    • admin
      May 15, 2020 @ 9:28 am

      Hi Francisco,

      Thanks for commenting. Hydrogen peroxide is only mildly acidic and at the concentrations you would need to drop pH in any meaningful way, your plants would die because of the oxidation stress caused by the peroxide. I hope this answers your question!

      Daniel

  • Keesje
    August 29, 2020 @ 5:24 am

    Dear Dr. Fernandez,

    You say the following “”Phosphoric acid (from 20 to 85% pure): This acid doubles as a plant nutrient, meaning plants will be affected by the phosphorus added.””
    Now it is possible to buy pH Down products that contain 41% phosphoricanhydride (P2O5).
    But one can also buy Mono nutrients from several brands and these mineral fertilizer often contain 20% P2O5.

    Is this the same stuff? (“P2O5 is P2O5”)
    But just in different concentrations?

    • admin
      August 29, 2020 @ 8:07 pm

      Thanks for posting! P2O5 is a very reactive substance, so it’s never used in hydroponics. Fertilizers that contain P never contain P as literal P2O5, it is just reported this way due to some odd heritage from analytical chemistry (see my post about this https://scienceinhydroponics.com/2020/05/why-do-npk-labels-express-p-and-k-as-oxides.html). It is not that the fertilizers contain P2O5, it’s just that the composition is reported as “if the P here was P2O5, it would be X%” but it really is not P2O5.

  • Land
    October 30, 2020 @ 4:02 pm

    Hi! Question about other salt. Can I use Ammonium Sulfate by PH down?

    • admin
      October 31, 2020 @ 11:31 am

      Thanks for your comment! You could chemically, but the amount of ammonium needed to lower the pH enough will end up being toxic to your plants. Instead you might want to try a more acidic ammonium salt, such as mono ammonium phosphate. Even then, you will want to formulate a pH down including phosphoric/sulfuric acid in addition to this salt as otherwise the amount of P and ammonium you need to add might end up being problematic.

      • land
        November 2, 2020 @ 8:44 am

        In the composition of GHE PH DOWN DRY – ammonium sulfate, citric acid, and urea phosphate. Do you think this is the optimal composition? Or did you recommend any other one.

        • admin
          November 2, 2020 @ 9:53 am

          This is not the optimal composition and GH will not tell you the actual ingredients they use, but ingredients to make it harder for you to reverse engineer their formulations. If you are interested in the formulation of an effective pH down, please feel free to contact me or book an hour of consultation through the link at the top of my website.

  • Reed
    January 23, 2021 @ 7:46 am

    Hi,

    Any problem with using non food grade phosphoric acid?

    Thanks

    • admin
      January 23, 2021 @ 6:22 pm

      Thanks for commenting! Just make sure you run an analysis to make sure you’re not adding significant amounts of heavy metals. Non-food grade phosphoric acid can contain significant amounts of Fe or As.

  • Dave
    January 17, 2022 @ 11:58 am

    Hi Dr. Fernandez,

    I’ve been experiencing issues using phosphoric acid based pH downs due to amount of P added over time which causes imbalances. I recently came across PRO pH down by General Hydroponics which bills itself as “hydrogenated water”. A cursory search reveals this is synthesized by injecting hydrogen gas into water. It’s billed as a solution for commercial/professional growers who are looking for a way to adjust the pH of their solutions without adding any additional nutrients. Just Hydrogen and water. I’ve been using it for the last week but don’t have enough longitudinal data to comment on its efficacy.

    Have you heard of this type of product and what are you thoughts? Is it a sustainable option?

    Thanks,
    David

    • admin
      January 17, 2022 @ 3:14 pm

      Thanks for commenting. This is just marketing to confuse people (as is often done by companies focusing on the cannabis nutrient market). Water chemistry is pretty clear and well understood. An acid needs to contribute H+ ions to solution, and these ions require a counter-ion, which needs to be an anion to preserve charges. There’s no such thing as “hydrogenated water” that is acidic, the law of charge preservation must be obeyed in solution. Injecting hydrogen gas into water does not create an acid in any way. This is just to mislead people and prevent reverse engineering.

      There are a bunch of acids that could be used that do not contain N, P or S, that they could be using. Given the low hazmat rating, they are either using a dilute hydrochloric acid or they are using an organic acid that is inert to plant uptake (there are several choices here). That said, you are adding some anion to your solution that might not be a plant nutrient, but it will certainly be there. What effects it might have in the long term depends on what it actually is.

      The horticultural hydroponic industry uses mainly sulfuric and nitric acid and these are extremely cheap and work great (these are nutrients that are needed anyway). You just need to formulate nutrients that account for the acid additions in the formulations. No need to reinvent the wheel in my opinion.

      I hope the above answer is useful!

  • Camille
    March 21, 2022 @ 3:43 pm

    Why won’t muriatic acid be helpful?

    • admin
      March 21, 2022 @ 3:53 pm

      Thanks for writing. It can, but it can contribute too much chloride and it is also often contaminated with a lot of Iron. It can be a good option under certain circumstances though!