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I want to lower my soil pH from 7 to 4.5 with dilute sulphuric acid. It's time to buy potted blueberry plants here in El Dorado County, CA (Zone 9A) so I would like to adjust the pH quickly rather than waiting for elemental sulphur to convert to sulphuric acid in the soil over several months.

My soil is a 50-50 mix of clay soil and rich compost with perlite added, and no commercial potting soil. I will be growing my blueberries in 20 in dia containers shaped like half barrels.

I would appreciate a recommendation on the level of dilution for 93% sulphuric acid (I'm thinking 3 tablespoons of acid to a gallon of distilled water) and the application rate. Should the application rate be based on the surface area or the volume of soil in the container? After obtaining the desired pH, perhaps after several applications as determined by monitor pH, how long should I wait before adding 2 potted blueberry plants to the container?

Rohit Gupta
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Anthony
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  • When you say 'rich compost', is that compost you've made yourself? And is the clay soil just from the soil in your garden? – Bamboo Nov 09 '20 at 20:27
  • Yes, I made the compost myself. Greens included boxes of outdated produce from a local high-end grocer, coffee grinds, alpaca manure, a small amount of grass clippings and some shrub trimmings. Browns included sphagnum moss, cedar shavings, shredded cardboard (non-glossy) and shredded paper, and twigs. The 3 ft pile was kept moist, turned ever few days and reached temperatures between 130-140 deg F for 5 days. The temperatures cooled as the pile finished composting in about 6 weeks. I sifted the compost and removed any remnants of non-composted materials before mixing with the soil. – Anthony Nov 10 '20 at 03:08
  • The mostly clay soil is from topsoil I brought into my garden area 16 years ago. I've had no problems growing a lawn, shrubs and trees ever since, but I have not grown vegetables or blueberries in this soil before. When I added soil to water and tried to separate it into layers of sand, clay and loam, it was primarily a monolithic layer which I believe is clay...based on the way it feels and clumps when dry. – Anthony Nov 10 '20 at 03:09
  • Do you have a pH meter? – Polypipe Wrangler Nov 10 '20 at 03:32
  • Yes, but I also ordered strips that read pH between 4.5 and 9.0 which I think may be more accurate. – Anthony Nov 10 '20 at 03:54
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    You will burn holes in your gardening clothes as the water dries off the H2SO4, leaving concentrated acid behind. The stuff is *nasty* that way. I'd do a test run on 100 ml of soil before applying. That'll give you an application rate. You have to treat the volume. – Wayfaring Stranger Nov 10 '20 at 16:12
  • I have seen pH-adjusting soil amenders, labled "pH up" and "pH down". The latter is the one you may use. They are available at Hydroponic stores. They are said to lower the pH to that needed for acid-lovers like azaleas and heathers, but I don't know how low they can go. I have never used them as I don't grow acid-loving plants. – Christmas Snow Nov 10 '20 at 19:42

8 Answers8

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IMO the only place 93% sulphuric acid belongs is in a chemistry lab or an industrial plant. It is a very hazardous material.

Using sulphuric acid directly at any concentration is probably a bad idea, because it will react with organic materials (e.g. cellulose and carbohydrates) in the compost, and won't do anything good to the microorganisms in the soil either.

Two relatively safe ways to "instantly" decrease soil pH are aluminium sulphate (sold horticulturally for producing blue hydrangea flowers on alkaline soils) and ferrous sulphate. Both have the disadvantage of reducing the availability of phosphorous in the soil to plants, but that might not be an issue for blueberries which grow in impoverished soil in any case.

alephzero
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  • I know how to safely handle concentrated sulphuric acid from days I prepared solutions of different normalities as far back as high school 40 years ago. Yes, caution is required. As far as any concentration goes, adding sulphur to soil slowly becomes sulphuric acid with water and time. So in the proper concentration, I know it will work. – Anthony Nov 10 '20 at 03:15
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    If you know how to safely handle it, I hope you have a suitable fume cupboard and other safety equipment available. I know "health and safety" standards were generally lower 40 years ago (and I'm of a similar age group to you) but I doubt very much that you ever personally handled 93% sulphuric acid in high school. Asking for the dilution in "tablespoons per gallon" in the question didn't suggest that you were aware of what you were proposing to do. – alephzero Nov 10 '20 at 13:39
  • alephzero: You may doubt my experience all you want, but it doesn't change the facts. I find your demeanor offensive and certainly not helpful. You make unwarranted assumptions and come to inaccurate conclusions. You have done nothing to answer my question. – Anthony Nov 10 '20 at 15:42
  • Elemental sulphur can also be used, but it takes a winter for soil bacteria to convert it to acidic compounds. – Wayfaring Stranger Nov 10 '20 at 16:14
  • Well, if you don't like my answer or my comments, flag them and let the mods decide what to do with them. – alephzero Nov 11 '20 at 00:37
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Last winter I planted 40 bushes (2 each of 20 varieties) in my blueberry patch - 4 rows of 10. I had prepared the soil with elemental sulfur (about 5 lbs per row) the previous fall, but it wasn't enough time. I tested the soil in multiple places and it was in the 7.0 to 7.5 range at planting and throughout the summer - yikes!

The plants were okay during the spring, but didn't show much growth during the summer and some started to look very sick by the end of the summer. So I bought a Chemilizer (liquid fertilizer/chemical injector) to integrate with my drip irrigation.

I bought some 16 oz bottles of 10% sulfuric acid (H2SO4) from the Science Company (Lakewood, CO). I added a full bottle to about 2 gallons of water each time I irrigated. I have 2 gallon/hour emitters for each plant and the Chemilizer distributed the diluted H2SO4 in a little over an hour.

I noticed a big improvement in the health of the bushes within two weeks - darker, greener leaves and significant new growth. I plan to administer more elemental sulfur in early spring, but this time I will dissolve it in warm water instead of just working it straight into the soil. I also plan to maintain my protocol of injecting the 16 oz bottles of 10% H2SO4 this growing season, but am hopeful that the elemental sulfur will eventually be adequate.

I realize that I haven't given any specifics regarding flow rates and concentrations at the emitters, but I got very positive results with what I did, and that's good enough for me. I hope this info helps someone else.

You could use battery acid, but it may contain impurities such as heavy metals. Pure H2SO4 from a lab is the closest thing to microbes excreting H2SO4 after ingesting sulfur. ;-)

Ed in Maryland
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I asked about the compost and the clay soil for a reason, because you want to use it in containers. Your compost should be pathogen free because its been produced aerobically, but the soil from the garden is another matter. Garden soil may contain pathogens which, all the time they're in open ground are fine, but may not be fine if transferred into containers. That is why potting soils are sterilised, to prevent any such problems.

I wouldn't recommend using sulphuric acid for the various reasons mentioned by other people already - I would recommend buying ericaceous (acid) potting soil instead, which is formulated for acid loving plants. If you want to add some of your own compost to it, don't add too much because composted materials are usually more alkaline than acid.

Bamboo
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I am acidifying two circles in my yard to plant mango trees. My soil is pH 8.5. I buy the sulphuric acid from the pinch a penny swimming pool shop. It is called non fuming and is 38% vitriol into water. I'm using a blue rainwater barrel and my well water that has a pH of 7.5 unfortunately.

I put 500 ml of acid in the barrel and fill it. I irrigated the 20ft diameter with 6 barrels so far. I think my average pH drop is currently 1.3. I feel I'm on the right track with current pH at average 7.2. My target is 6.2 to 6.5. I want the depth affected by the acid to be as deep as possible and hence the choice of dilution and high volume applied. I think I could have acidified the surface much faster with a liter of acid in the barrel but that could be getting into dangerous concentrations regarding my submerged pump, my clothing and last but not least, my skin.

I plan to do the same again to get where I want to be. The alkalinity will slowly return so this will probably be a regular job to maintain a healthy pH.

Rohit Gupta
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Bill Holden
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Having done that , I would say you can't do it . That is , it is not reasonably possible to make that large a change in a natural soil pH. I had a clay , sand , gravel soil in IL and I wanted blue berries . I put in a lot of organic mulch, a lot of gypsum ( Ca SO4) , sulfur dust and I forget what else ( I worked in a lab so many things were available ) . After 2 years of marginal growth and marginal pH change , I decided to cure them or kill them. I got a bottle ( 3 L ? ) of concentrated sulfuric . I poured it on 6 in. snow depth in a circle around 6 blue berry plants. Immediate snow melt - I could see where I poured. Some brown foaming on the soil surface. Next spring the blueberries grew very well . But the following year it was back to poor growth . So I decided it was not worth the trouble. PS ; several miles away in a very sandy soil , commercial blueberry farm did fine , they had a pick-yourself deal so that is where I got blueberries. PS 2 : Pretty sure conc, sulfuric is 98% ; in a refinery alkylation unit they try to keep sulfuric at 95% + or it starts dissolving steel.

blacksmith37
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    Thanks for sharing your experience. Can you tell me what pH you achieved initially and how it changed over 2 years? I understand it requires regular monitoring and retreating to maintain a favorable level. I think I have a better chance by growing them in containers where I have the best control over soil chemistry, moisture and sunlight,so I'm willing to give it the old college try. I appreciate you warning and the time you spent responding. Yes, concentrated sulfuric acid is 98%...the product available to me listed its concentration as 93%. – Anthony Nov 10 '20 at 03:51
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    This experience isn't surprising. Concentrated acid will simply leach out of the soil in rain water unless it is incorporated into other organic compounds, and it will most likely kill the microorganisms which could have done that. Sulphur can be processed (slowly) by bacteria into a form which won't leach away. – alephzero Nov 10 '20 at 13:43
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    "Conc. sulphuric" usually means anything higher than 93%. As the answer says, the chemical industry prefers 98.3% for bulk transportation, because it actually more stable and less corrosive than lower concentrations containing more water. 10M sulphuric acid (commonly used for laboratory work) is only 62% concentration. – alephzero Nov 10 '20 at 13:51
  • I measured the pH many times and places , I don't recall all specifics only that it became acidic with acid addition ,then returned to alkaline. – blacksmith37 Nov 10 '20 at 16:10
  • Thanks, blacksmith37. I'll monitor the pH changes and adjust the treatment accordingly to maintain a favorable level. I expect diluted sulphuric acid will bring down the pH fairly quickly, and I believe that addition of periodic ferrous sulfate heptahydrate will be useful in holding the pH down for the long term. This may seem like a lot of effort but having only a few containers makes the job feasible.. – Anthony Nov 10 '20 at 22:06
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That makes any difference to y’all. I would measure out 2 gallons of water, put in 1 tablespoon of sulfuric acid at a time and stir. Test the water with my pH tester and readjust if necessary until I got 4.5 to 5.5. And I would do that about every three weeks for my blueberries and it worked perfect.

Tim
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Using diluted 93% sulphuric is the most cost and time effective way to lower soil pH but needs to be approached methodically and gradually so that you don't overshoot on lowering the pH and maintain consistancy while not destroying biota in the soil. One factor in areas with wet and dry rainy seasons with alkaline soil such as in Florida where I am, the acid rain precipitation will definately lower your soil pH during the wet season more than during the dry season. That is why vegetation becomes chlorotic during the winter dry season. Always add acid to water and not vice versa to avoid extreme ionic reaction.

Steve
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I was a molecular biologist for 20 years and I have had more years of botany labs than I care to remember.

dilute Sulfuric acid is the quickest and safest way to lower pH - elemental sulfur is great if you want to wait a year. Plants love sulfates. Vinegar used at around 1/4 dilution works but only for a couple weeks - due to acetate effect after soil transformations.

I am searching for the proper dilution as well...but I know 1/4 vinegar in water works great for about 1.5 pH decrease in my crappy calciferous clay soil full of ca carbonates.

So simply determine the Normality of 1/4 vinegar and replicate. Probably about 2 tablespoons per gallon of water.

Stephie
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Brett
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  • Do you have a reputable source to back up your assertion that, in a horticultural context, "dilute Sulfuric acid is the quickest and safest way to lower [soil] pH"? – Peter4075 Jul 27 '21 at 19:48