Increasing Soil Acidity

Robert Pavlis

There is a lot of advice on how to make make acidic soil both in print and on the net. You can use coffee grounds, pine needles, and sulfur to name a few. This advice has two problems. Firstly, the recommended product may not actually acidify soil. For example in Do Pine Needles Acidify Soil I show that pine needles do not make acidic soil. Coffee grounds don’t acidify soil either. The second problem is that before such advice is given it is important to know the soil types (ie soil texture) being treated. Let’s take a closer look at this.

acidic soil texture
Soil texture is important when trying to acidify soil

Soil Texture

Your soil has been made over millions of years using the rocks that were present at your location. It might have a lot of sand, or a lot of clay. It will also contain minerals based on the type of rock that was degraded to make your soil. The ability for any soil amendment to change soil acidity depends very much on the soil type you have. Let’s look at a couple of examples.

Soil that is very sandy usually does not contain much in the way of minerals. If you add a small amount of acidic material to the soil it will become acidic, at least for a short period of time. The problem with sand is that minerals and added acid leach away quickly; so the acidification of sand is a short term event – your soil will not stay acidic for long.

If your soil contains significant amounts of loam or clay, the soil could be naturally acidic or alkaline. It will contain minerals that will react with the added acid. Any acidification of the soil depends very much on the composition of these minerals. The minerals may be able to neutralize, or buffer the added acid. The importance of this buffering ability is discussed in Liming Acidic Soil. Soil testing is the only way to determine the pH buffer value.

Plant Science for Gardeners by Robert Pavlis

Soil Acidity

Most of Southern Ontario is a clay loam. The base rock here is limestone and there is a lot of limestone, both as rocks and as minerals in the soil. These minerals are able to neutralize any acid that is added.

Consider this fact. Rain dissolves CO2 from the air as it falls to earth producing carbonic acid (this is not due to pollution). This rain, even without the added pollution has a pH of 5.5. This acidic rain has been falling in Ontario for millions of years and even after such a long time of ‘acidifying the soil’ our soil pH is still 7.4. How can this be? Our soil contains a lot of neutralizing minerals due to the limestone. As soon as an acidic material is added to the soil it is quickly neutralized so that it has no net effect on the soil pH.

In Northern Ontario and Quebec, the base rock is granite, not limestone. Granite is very stable and hardly reacts with acid. The soils in these areas are generally acidic and the addition of more acidic material will make the soil pH more acidic. In fact the pollution over the last 50 years has made the rain more acidic (ie pH lower than 5.5), and this has resulted in the soil in some areas becoming more acidic.

I have split Ontario into two parts, northern and southern, but in each area there are exceptions to the above statements. You can find acidic soil in the south and alkaline soil in the north.

 Conclusion:

The acidification effect of any material on soil depends very much on the soil types you have. Simply saying that “material ABC” acidifies soil is not correct. It may acidify some soils and not others.

References:

1) Photo Source: Mikenorton

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Robert Pavlis

I have been gardening my whole life and have a science background. Besides writing and speaking about gardening, I own and operate a 6 acre private garden called Aspen Grove Gardens which now has over 3,000 perennials, grasses, shrubs and trees. Yes--I am a plantaholic!

20 thoughts on “Increasing Soil Acidity”

  1. The most thorough information I found on acidifing soil. I’m going to plant rhododendron for the first time. I live on live water. It boils out of the ground. Have no idea what the soil ph is…

    Reply
  2. Would one of the soil test kits that use an inert white powder to draw the pH reagent to soil surface offer higher accuracy? The white powder covers soil sample to eliminate soil color from interfering with pH reagent color. Something like the Lovibond soil pH test kit
    that measures in 0.5 pH increments? https://www.forestry-suppliers.com/product_pages/products.php?mi=30681&itemnum=77181
    You are still only testing a very small amount of soil. I create my own mixes for starting seedlings and the ph of peat moss can be as low as 4.5 and as high as 6. I typically add lime to balance the acidity of the peat however if I do this blindly I can push the ph very high.

    Reply
    • Probably better than cheap garden kits, but “White powder covers soil sample to eliminate soil color from interfering with pH reagent color. “, makes no sense to me.

      Reply
  3. Wow , great site and really informative. I to have the blueberry bush bug here in England and used ericaceous soil to plant in pots but the PH was still 7.I added a white vinegar and still mineral water to it and it has brought it down to 4. I guess this will have to be done regularly?

    Reply
  4. I’m loving your posts!
    I used to have goats and added a lot of barn lime to their paths and in their barns. Not thinking about pH at the time, I also composted their droppings and the barn sweepings. Here in Georgia, the soil is acidic clay. I haven’t had ours tested but I assume the places where I added lime are going to be too alkaline for growing the tomatoes and peppers I’ve started. Do you think using the compost now is a bad idea? I would have it tested but it is such a big pile and isn’t thoroughly mixed. I don’t want it to go to waste though. Would adding sulfur, horse manure, or bark mulch help? Thanks!

    Reply
  5. Hi Robert,

    Thank you for the evidence based articles. I too landed here because of blueberries. What are your thoughts on raised beds with peat moss if your soil is alkaline? I thought I made a good move planting blueberries in Georgia with our acidic red clay but I tested pH and it runs 6-7 on a hill below pine trees…

    Reply
    • You can grow blueberries in a container or raised bed – which is just a larger container, provided you keep the soil acidic.

      Reply
  6. Hello Mr. Pavis! I am a solid, new fan!! Excellent excellent answers especially for those that are complete newbies. I’ve got some questions for you…I am on a site ‘Stack Exchange, Gardening and Landscaping’…and I am sort of the black sheep/rebel. This is just one. I am deficient in chemistry and I have to look stuff up to remember the details down at the cellular level. Oh well…

    What I was researching this morning was from a picture of this ‘kiwi tree’ that had blackened margins with white crusty, salty looking stuff…mixed with the black. One of my big things is salt buildup from too much fertilizer and tap waters. I know that the first thing that happens is the margins and tips of leaves yellow, brown, blacken. Lots of ‘crust’ on clay pots as well as the top of the soil.

    What would cause this bright white stuff mixed with black that pretty much as decimated the leaf margins? The leaf itself looks very green, shiny and healthy. Am unable to see closely enough for insect habitation…would that be possible? Could I send you a picture from this site?

    Reply
  7. Hello. Very interesting reading. Always good to see another perspective. Can I ask a question? Just wondering if you have any recommendations for acidifying an alkaline heavy clay soil? I have been looking at flowers of sulphur, but wondering if there are other options. Clearly as you say, pine needles and coffee grounds will have limited impact. Thanks

    Reply
  8. Hi. I also live in Southern Ontario. My garden soil contains a lot of clay. I have just planted blueberries. Without testing the soil, can I assume I need to add acid or should I just wait and see how the fair?

    Reply
    • If you can take them back – do so. It is real hard to grow blueberries here in S. Ontario. Our soil is alkaline and nurseries should know that and not sell these plants without a warning notice.

      If you decide to keep them – start adding sulfur to the soil to acidify it.

      Reply
  9. My soil is very alkaline and is chalky. Will adding peat moss help to fix this problem? Will concentrated vinegar help?

    Reply
    • Chalky soil usually has a lot of lime in it, and this is alkaline.

      Lots of people claim peat moss will make soil more acidic. I have not researched this, but I doubt it will have much of an effect. The best way to lower the pH is to add sulfur.

      Reply
  10. here’s another question you may know the answer to.i have my blueberry’s in concrete pots.will the concrete (made from lime of course) neutralize any acid added?

    Reply
    • Depends on how old the concrete is. New concrete does leach alkaline chemicals and will have some effect on pH. Older concrete will not do this. How old does it need to be? I don’t know. Based on most of my reading little leaching happens after a year.

      Reply

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