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I have water that is at a pH of 8.0. I want to acidify the water to a pH of 4.0 (for the purpose of watering acid-loving seedlings).

I have two products that I can use to acidify the water:

  1. Lemon Juice:
    • $12.47 * 13% tax = $14.09 per container
    • The volume of the container is 3.8 L
    • It takes 15 ml to acidify 1 L of water to 4.0 pH
  2. Citric Acid:
    • $22.99 * 13% tax = $25.98 per bag
    • The weight of the bag is 2.27 kg. Note: according to my measurements, the dry granules equal a volume of 2.3 L (I did not add water to form a solution for this measurement).
    • It takes 1.25 ml of dry granules to acidify 1 L of water to 4.0 pH

Which product is more cost effective at acidifying water?

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    Is this a maths question? Or a riddle? I'll try: If you only have one litre (as you said) then the lemon juice is best, because it's more than enough to acidify one litre, and it's cheaper than the Citric acid. ---- If you have huge quantities of water (more than 253.3 litres) to acidify, the citric acid is cheaper by far. One bag lets you acidify 1277.7 liters of water, so more than a thousand liters more than the lemon juice. --- How to calculate: price-for-container/(volume-in-container/volume-for-one-liter) = price-for-acidifying-one-litre. But remember the minimum water-to-acidify!
    – bukwyrm
    May 10, 2018 at 20:11
  • Starts or baby plants do just fine with neutral. Make sure you test the pH after you've added vinegar or lemon juice. Changing the pH of the water is not enough to change the pH of the soil. Acidifying soil is temporary and must be maintained. There are complex reasons for the pH of the water, the soils in a locality...what plants are you taking about? And just to emphasize; if plants like a slightly acidic soil, more acidity will never be better. Same with alkaline. Easier to change acidic to alkaline however. What again is it you want to do. This is not normal for starts or watering
    – stormy
    May 11, 2018 at 5:46
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    And the only way to change the pH down or more acidic is the use of sulfur...and that is temporary at best. Acidifying your water for your plants is counterproductive. Honest.
    – stormy
    May 11, 2018 at 5:48
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    Both those acids will be eaten by fungus/bacteria. Get yourself a bottle of 85% phosphoric acid. It's not a horribly dangerous acid, like sulfuric, nitric, or hydrochloric. One drop per gallon is likely all you'll need. Used to be able to buy buffered phosphate salts, which would be perfect, but I haven't seen them in years, and making them at home is troublesome.
    – Wayfaring Stranger
    May 11, 2018 at 13:38

2 Answers 2

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I used to use white vinegar, but it made my cactus smell like pickles (for a little while).

I have been using granulated citric acid for a number of years now, which as your calculations show is pretty efficient in making alkaline things acid.

Storage is another consideration; dilute acids like lemon juice or white vinegar take up a lot more storage space and generates a LOT more trash/recycling.

Note: don't store acidulated water, it reverts back to near starting pH in a couple of hours. Rainwater does the same thing. Hits the ground acid and becomes more alkaline as the CO2 dissipates.

Commercial growers use concentrated sulphuric acid for this purpose. They use a system that injects into the stream of water before it goes into the distribution system; mix on the fly.

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  • water with lemon juice or citric acid won't become (much) less acidic over time. After all, lemon juice is water+citric acid(+sugar etc.) and that stay acid in the bottle.
    – Chris H
    May 10, 2018 at 21:17
  • Check this: ralph.cs.cf.ac.uk/Cacti/Cactus%20and%20Alkalinity.pdf
    – Tim Nevins
    May 11, 2018 at 15:12
  • OK, if you're specifically dealing with bicarbonate you have to get into the behaviour of CO<sub>2</sub> as equilibrium returns. Rainwater might become less acidic due to loss of CO<sub>2</sub> (e.g. if it warms) but won't actually become alkaline of its own right because it doesn't contain bicarbonate (it might dissolve stuff already present in the soil though).
    – Chris H
    May 11, 2018 at 15:52
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It is certainly more hazardous, but sulfuric acid also cheaply acidifies water. I found an interesting article about variations on the practice.

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