Quick method of acidifying soil for blueberry

Blueberry needs a soil ph of 4.5-5.5, but mine is around a neutral 7. I’ve tried to acidify it: I added sulphur to soil a couple months ago, and have been mixing tap water with vinegar before watering. Any suggestions on how I can improve the ph level more quickly, as the plants appear to be suffering?

There’s so much conflicting information wherever I read. Some say coffee grounds, but some say they don’t work. Some say 1 tbsp of vinegar to 1 gallon of water, some say 1/2 a cup of vinegar to 1 gallon of water!

I also have gypsum (somewhere; I’d have to search for it), but I’ve read that it doesn’t actually lower soil ph.

Elemental sulfur has to be broken down by the soil microbes and can take a year or more to reduce the pH significantly. If you added it in later winter the microbes will not have had much time to break it down. Standard application is 2 lbs / 100 sqft. (1 kg / 10 m2) one year prior to needing the lower pH to reduce from 7 to 4.5.

Iron sulfate is much more soluble and therefore faster acting. Application of 10 lbs / 100 sqft will normally reduce the pH by 1 unit. Recommendation is to wait 2 months between 10 lbs / 100 sqft. applications. It will stain things.

Ammonium sulfate fertilizers will also reduce the pH. However, too much can really mess up the soil microbiology so light doses should be used.

Adding organic material is the long term solution since the fungi and bacteria break it down with acidic compounds as a byproduct. Peat moss is particularly effective and can be used as top dressing, but that is slower than working it into the bed before planting.

Thanks, that was highly informative! I ordered iron sulfate and composted bark (sustainable alternative to peat moss). I’m going to try adding more vinegar for now, add the first batch of iron sulfate when it arrives, and mix the bark into the soil if I can manage.

Question, does the iron sulfate need to be mixed into the soil to work or can it be applied on top?

I would not directly apply vinegar. It has a rather low pH of around 3 and can kill roots and such if it makes direct contact. In fact we use vinegar to kill moss on tree trunks. You might consider diluting it 10:1 or so with water when you water so that you are applying a somewhat acidic water. I acidify my water (pH 8) with phosphoric acid to bring it down to about 6.5. Acetic acid (vinegar) has been recommended since the phosphorous can tie up the iron in the soil so I may make a change.

The iron sulfate can be applied to surface and watered in, but I would probably scratch it in or cover with your composed bark since it will dissolve faster if kept moist. I could also be dissolved in water and applied that way, but that will stain anything you apply it to.

I diluted the vinegar, perhaps about 5:1. I read vinegar can be as low as a pH of 2. Assuming a water pH of 8, this ratio gives a net pH of about 4.7. I’ve heard the water where I live can legally be as high as 9.5, and I haven’t tested it.

I would go to a pet store, look in the aquarium section to locate a pH test kit. Test the pH of the water as it comes out the tap. A good starting point for adjusting the pH is about 15 ml or 1Tbps per gallon of water. This will usually lower the pH to around 6.0-6.2 when the starting pH is 7.8-8.0. You may need to add more as you would want a lower overall acidity level. I would not, however, adjust the level below 5.5 I think 6.0 would be safer.


I have a pH test kit. The tap water is a ridiculous pH of 9; filtering lowers it to around 8. If I go purely by ratios, not understanding any complex chemistry that may be involved, I would require a dilution of merely 2 parts water to 1 part vinegar (which, afaik has a pH of 2) to obtain a pH of 6. I’m too scared to use such a concentrated level of vinegar.

I also tested the soil pH: it’s about 7.0. So I need to move pretty desperately to reduce it, or I’m concerned the plants will die. Given that, I’d be open to trying more extreme suggestions even if it means I might learn something from this while killing the plants.

The subject is definitely debated but I’ve seen convincing evidence (namely a detailed study done back in the 60’s or 70’s) that pine needle duff does indeed work towards acidifying soil over time, although the effect is slow. I would link the paper if I could but I can’t seem to find it. From what i recall burying the needles under a layer of soil seems accelerate the effect.

You would likely benefit from an RO system. The water is extremely alkaline, probably due to a high TDS content which is buffering the pH. Alternatively, rainwater collecting might be the way to go. Cheaper, no waste water, or expensive membranes to buy and replace.

Another thought, use a combination of acids. I’ve used lemon juice with vinegar to adjust the pH of municipal water.

Short of a complete soil change with a different water source, I don’t know of any other expedient ways to the job done.

Do you recall how much is needed?

I already have a filter. I’m not sure if it’s backed by a RO system, but it lowered the pH from 9 to 8. I’ve looked into rain collection, but since I don’t own the place I live in it’s tricky. Also, even though I live in the UK, we get surprisingly little rain outside of winter, at least in my experience.

Is there a benefit to mixing acids? Lemon juice is significantly more expensive than vinegar, I think?

I’ve also looked into the pH of different soils. Lowest I was able to find was 6.5.

I don’t think differing quantities of needles was something the study concerned itself with. That said I’d say just your typical 3 to 4 inches is a good bet. Plus you get the added benefit of weed prevention. But again the effect is slow, like a decade of consistent needle duff before significant change is seen.

Depends on what the base molecules are for the acids used. As examples:
Sulfuric acid= sulfur and hydrogen ions
Phosphoric acid = phosphorus and hydrogen ions
All acids have varying potency and some might work better in reducing the pH of the water you plan to use. Also some acids have a very finite lifespan when in solution. Nitric acid and carbonic acid will dissipate relatively quickly. Commonly used in hydroponic gardening is sulfuric acid, a little goes a long way with this one. Likely just a few milliliters per gallon would be used.
Use of gloves and eye protection is a must. I’ve seen a few hydroponic ph down solutions that use a combination of sulfuric, phosphoric, citric acid. All these are more potent acids than vinegar which is relatively safe and commonly used in food preparation and seasoning…basically safe for human consumption in small quantities.

Basically you might want to try a commercial ph down solution.

OK I ordered some pH down that contains Orthophosphoric acid. Do I need to use gloves for this stuff? I have never used it before.

Yes and safety glasses as well.

Several points about pH. The pH scale is logarithmic so a change of 1 pH (from 7 to 6 for example) is a factor of 10 change in the ions that are controlling the pH. The pH is given by log10(1/{activity of H+}). The activity of the H+ ions is approximately equal to the concentration for our purposes. [H+] is the concentration of disassociated hydrogen ions (mostly present as H3O+ hydronium ions or some other bound form). Therefore a pH of 5 indicates that the hydronium concentration is 10^-5.

The other key species for pH of pure water is the hydroxyl ion OH- ion. A first approximation is that [H+] times [OH-] is 10^-14. As a result at a pH of 7 in pure water the concentration of both ions is 10^-7. In soils other negatively charged ions like carbonate are important in determining the pH and will often neutralize hydronium ions.

I found a nice set of examples for calculating the pH of a mixture at ChemTeam: Two solutions of differing pH are mixed. What is the new pH? and the second example is most closely related to our situation of adding an acid to a basic water supply to reduce the pH, generally to a value below 7 (slightly acidic). The method assumes that the acid and base are both fully dissociated and the only ions are H+ and OH- which is a simplification, but it gets us into the right ball park. Here is a summary of the method using the values I had for my water and vinegar.

Acid Mol H+ = volume (liters) * 10^-pH = 1 x 10^-2.72 = 0.00191 mol
Base Mol OH- = volume (liters) * 10^-(14-pH) = 10 x 10^-(14-8.46) = 2.88x10^-5 mol
Excess H+ = mol H+ - mol OH- = 0.00188 mol H+
Concentration of H+ = mol H+ / total volume = 0.00188 / 11 liters = 0.00017
pH mixture = -log10(H+ concentration) = -log10(0.00017) = 3.76

I of course ran the experiment using 10 ml vinegar and 100 ml tap water and got a pH of 3.26 which is a fair bit lower than predicted. However, I could use the calculations to estimate the final pH and then check a mixture to refine the ratio of acid to water in order to obtain a desired pH. That is always a good idea since water chemistry is going to be more complicated because most of the acidic and basic species will only partially disassociate at the pH levels of interest to growing plants. Here are three quick examples at other ratios using my pH 2.72 vinegar and pH 8.46 water.

1 liter vinegar to 1 liter water – predicted pH = 3.02 (very acidic).
1 liter vinegar to 600 liters of water – predicted pH = 6.54 (close to my target of 6.5)
1 liter vinegar to 200 liters of water - predicted pH = 5.18 (probably fairly good for the blueberry patch)



Ooh, I completely misunderstood this. Thanks for clearing that up for me. It means I’ve almost certainly been over-acidifying my blueberries! Well, it’s been a learning experience if anything. I’ve got some new ones on order; hopefully they go better.

Remember that the soil will also react with the acid in the water and neutralize some of the H+ ions. That is why I suggested a pH of 5 for the water assuming that the soil is not that acidic.

Have you taught of using kanuma as your substrate? Ideal substrate for acid loving plants. Blueberry are in the same group as azalea and satsuki. I planted my big blueberry (40-50 years old) in 100% kanuma last spring and it did fine. I was worried how the kanuma would react to freezing since I live in Quebec and the tree spent the whole winter outside in a big (8 x 8) 2 inches thick styrofoam box where I keep all my native trees (Larches, spruces, rocky mountain Junipers, scott pines, jack pines…) The blueberry is now coming out of hibernation and preparing to flower. The soil looks fine and doesn’t seem to have lost it’s integrity.

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Had never heard of it before. Did you use it alone or mixed with something else? The one I found claims a pH of 5.5, which is in the (top end of the) range blueberries need.