1

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?

7
  • 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 9 '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 Greens included boxes of outdated produce from a local high-end grocer, coffee grinds, alpaca manure, a small amount of grass clippings and some shrub trimmings.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 3:08
  • 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 3:09
  • Do you have a pH meter? Nov 10 '20 at 3:32
  • 1
    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. Nov 10 '20 at 16:12
4

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.

5
  • 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 3:15
  • 1
    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. 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 0:37
2

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.

0
0

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.

5
  • 1
    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 3:51
  • 1
    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
  • 1
    "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. 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
-1

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.

1
  • 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 at 19:48

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.