pH Down (Phosphoric Acid) in Hydroponics: Adjusting pH and its Impact on the Nutrient Solution Profile
The pH down used by the vast majority of hydroponic growers is phosphoric acid (H3PO4). Phosphoric acid contains phosphate (P). For example, 75% phosphoric acid is 23.7% P. It’s addition to the nutrient tank to reduce pH will, therefore, add some P to solution. This, in some instances, is not an insignificant amount. Thus, if you are a hydroponic grower using phosphoric acid as pH down, keeping track of how much plant available phosphate (plant/bioavailable for uptake as H2PO4– and HPO4-2) you are adding to your nutrient solution to adjust pH is important.
For example, many growers use potassium silicate in their growing equation. Potassium silicate (K Sil) is highly alkaline. Because of this it raises pH, requiring pH down to correct the increase in pH the addition of K Sil causes. We recently calculated that by using a powder potassium silicate product (Agsil 16H) to provide 22 ppm of Si (50 ppm SiO2) to solution when pH adjusting down, after the addition of Agsil 16H, we were adding approximately 8 ppm of plant available P to solution through the use of phosphoric acid. When considering that P requirement during the height of flower (bulking/swelling) is about 75 ppm, 8 ppm of elemental P being added to solution through the use of phosphoric acid makes up 10% of the plant’s P requirement.
Calculating Application Rate of Phosphoric Acid and P Contribution to Solution
Phosphoric acid is generally available in concentrations from 40 to 80%. Most hydroponic store sold pH down products are in the 25-45% range.
Use the following steps to calculate the P contribution in ppm from a given addition of phosphoric acid.
Step 1. Establishing Elemental P
First, you need to know the purity of your phosphoric acid pH down product.
Phosphoric acid is generally listed as %w/w (percentage weight by weight) phosphoric acid (H3PO4). In rare cases, a pH down product might also have an additional listing as P2O5%w/w or this may be its only listing. E.g. 60 %w/w P2O5. In some cases, with hydroponic store sold products, there may be no listing or the listing on an SDS etc may be e.g. 60 – 80%. In this case, if your retailer or the supplier of the product cannot tell you precisely what percentage of H3PO4 or P2O5 is in the product (what is it to be, 60 or 80%?) exercise your right to pass and instead purchase another product. It’s imperative that growers know what they are adding to the nutrient solution and this begins with manufacturers/suppliers listing their products accurately.
As a tip, beyond the hype, given the usage rates, all phosphoric acid as pH down is equal and while some manufacturers may claim their phosphoric acid is special, they’d be telling fibs. Thus, what it comes down to is purchasing a concentrated brand, with an accurate purity or percentage listing, at the right price. That is, a concentrated brand that accurately lists what it contains, so that you get good value for money and know exactly what it is you are applying to solution.
Listed as Percentage Phosphoric Acid
If a pH down product is listed as % w/w H3PO4 use this equation to establish elemental P. 100% phosphoric acid contains 31.6074% elemental P. Therefore, if you were using a 75% phosphoric acid pH down product you can equate the percentage of P using this equation: 31.6074 (elemental P in 100% pure phosphoric acid) x 0.75 (% purity of your phosphoric acid) = 23.7% elemental P.
Listed as P2O5
If your product is listed as P2O5 % w/w; e.g. 60% P2O5, it is important to note that P2O5 is only 43.641% elemental P. Therefore, if you had a pH down product which was listed as 60% P2O5 you would use this equation to establish elemental P: 60 (% P2O5) x 0.43641 (P in P2O5) = 26.18 (% elemental P).
Step 2. Specific Gravity
Where working with % weight by weight (% w/w) listings, to find out how much elemental P a pH down product supplies to solution, at a given usage rate, we now need to look at the specific gravity of the phosphoric acid (pH down) product. See table following.
|H3PO4 % (Purity)||35%||45%||50%||75%||80%||85%|
|Specific Gravity *SG||
If you are uncertain of the specific gravity (SG) of your pH down product it is easy to measure the SG yourself, using scales and an accurate measuring flask.
How to Measure SG
The density of a substance is equal to its mass divided by its volume. You can measure the mass (weight) on a scale and record the volume of the liquid used. Use the equation “m / v = D” where m is mass in grams or kilograms, v is volume in milliliters or liters, and D is density.
- For example, if you had a sample that was 13.5 grams and 10 milliliters, your equation would be: “13.5 g / 10.00 mL = 1.35 g/mL which is 1.35 SG.
Now knowing the elemental P percentage and the specific gravity of your pH down product, to establish how much elemental P you are adding to the feed solution, via the addition of phosphoric acid/pH down, follow these steps:
- Using distilled water; make a 1% stock solution of your phosphoric acid (pH down) concentrate. To do this, half-fill a 1-litre measuring vessel with distilled water. Then carefully measure and add 10 ml of the concentrated pH down. Stir the mixture well, top up the container to the 1-litre mark and give it a good final mixing. You now have a 1% stock solution of your pH down. This diluted stock solution makes it easier to perform the test.
- Measure 10 litres of dilute (feed strength) nutrient solution into a container. This is your test solution. You are going to measure how much of your 1% pH down solution it takes to correct the test solution to your target pH.
- Using an accurate, calibrated, portable pH meter, measure the pH of the 10 litre test solution and write it down.
- Using a 10 ml pipette or other accurate measuring device, carefully add 1.0 ml of your 1% pH down stock solution to the nutrient test solution and stir well.
- Write down the amount you just added and stir the contents. Measure the pH and write the value down as well. Depending on the alkalinity of your test solution, the measured pH may have dropped drastically, just a little, or perhaps not at all. Use this as your guide for further acid additions. Each time you add another volume of acid, record the amount you add and record the resulting pH reading. You want to add just the right amount to achieve your desired pH. After you reach this point, make a note of it and continue adding acid volumes until you reach a pH level of about 6.0. If even this small first amount sent the pH below your target, then it’s back to the drawing board.
- You should perform this test 3 times for accuracy. If your results agree fairly well from one test to another then you are likely doing a consistently good job.
- With your test results in hand, you can now make a table of your results and calculate the acid concentration ratio that is needed. Let’s assume you had the following results:
Initial pH: 8.9 Target pH: 6.0
Acid stock solution strength: 1% solution of 75% H3PO4
Test feed solution volume = 10 litres
Amount of acid stock solution added to achieve pH 6.0 = 35 ml
Since the test solution was diluted by 100x from its original strength, the first thing we must do is divide our 35 ml by 100 to find out how much of the actual concentrated phosphoric acid (75%) we used: 35/100 = 0.35 ml acid concentrate per 10 litres or 0.035ml/L.
Based on this, we are adding 0.035ml/l of 75% phosphoric acid. The specific gravity of 75% phosphoric acid is 1.6, so 0.035ml weighs 0.056 grams (0.035 x 1.6 = 0.056). 0.056 grams per litre is 56 parts per million (1 gram per litre = 1000 ppm or 1mg per litre = 1 ppm). 75% phosphoric acid contains 23.7% P. Therefore, the amount of P delivered is 56 x 0.237 = 13.27 ppm.
Saving Money on pH Down. Purchase Phosphoric Acid from a Chemical Supplier
This one pretty much speaks for itself. Rather than purchasing phosphoric acid from a grow store, instead buy it direct from chemical suppliers to save on input costs.
Even for smaller growers, concentrated phosphoric acid (e.g. 75% food grade) can be purchased quite cheaply online, in smaller volumes (e.g. 1 gallon) through sources such as Ebay or Custom Hydro Nutrients @ https://customhydronutrients.com/phosphoric-acid-c-8_168_177_429.html
Author’s note #1: Where talking about phosphoric acid (H3PO4) and the P requirement of the plant it’s actually a bit more complex than talking about elemental P or H3PO4. That is, before H3PO4 can become available to plants it must first disassociate in the nutrient solution or growing substrate to become hydrogen phosphate (HPO42) and/or dihydrogen phosphate (H2PO4–). The ratio of these two forms is dictated by the pH, with H2PO4– dominant in acidic solutions (pH < 7), and HPO42- dominant in alkaline solutions. At pH 7.2, both H2PO4– and HPO42- are approximately equal in amount. At pH levels below 2.1, H3PO4 can become the dominant form, but at pH levels from 5.5 to 6.5 which are more favorable for plant growth, the amount of H3PO4 is negligible. Compared with either H2PO4– or HPO42-, it is only one out of 100,000 because it always dissociates to H2PO4– and further to HPO42-. Both H2PO4– and HPO42- ions are basic forms taken up by the plant, but H2PO4– is taken up more readily because in most growth conditions, soil solution pH is below 7. Once inside the plant, both ions are mobile.
Author’s note #2: some organic hydroponic growers use vinegar as pH down. This is not an advisable practice. A 2019 paper by scientists at the Oklahoma State University trialed pH down (phosphoric acid), vinegar and lime juice to pH correct the nutrient solution in a hydroponic growing situation. The nutrient solution’s pH was maintained between 5.5 and 6.5. pH Down (phosphoric acid) resulted in the most stable solution pH and required the least amount of product used when compared to lime juice and vinegar. The cost of using phosphoric acid or lime juice was greater than that of using vinegar. However, vinegar reduced the yield of all crops in comparison to pH Down. When compared to pH Down, lime juice reduced the yield of basil and Swiss chard but not that of lettuce. 
 Singh, Hardeep, Bruce Dunn, and Mark Payton. “Hydroponic Ph Modifiers Affect Plant Growth and Nutrient Content In Leafy Greens.” Journal of Horticultural Research, v. 27,.1 pp. 31-36. doi: 10.2478/johr-2019-0004