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Compost Creates Acidic Soil

Does compost make acidic soil? It is a common claim made for compost but does it really work? Is it a good option for making alkaline soil more acidic? Let’s have a closer look.

does compost acidify soil

Compost pH madness – does compost acidify soil?

Compost pH

What is the pH of compost? The answer is more interesting than you might think—it is not a simple answer.

To start with the pH of compost depends very much on the materials you put into the compost. If you use wood products like saw dust they will make the finished compost more acidic. If you use more manure or add in some ashes from the fireplace it will be more alkaline. So the pH of any particular compost depends on the material being composted.

As the compost is being made it goes through pH swings. In the initial stages, organic acids are formed and these make the compost pile more acidic—the pH drops. In these acidic conditions fungi grow better than bacteria and they take over the pile and start to decompose the lignin and cellulose in plants. As this process continues the pH rises and bacteria become more populous. What this means is that the pH of your finished compost also depends on when you consider it to be finished. If you rush things, it might still be more acidic. If you wait longer, it will be more alkaline.

Most sources report the pH of compost between 6 and 8, without specifying a source for this information. Reference 1  shows the pH of various composts and most are between 7 and 8. For example: yard debris 7.7, mixed manure 7.9 and leaf 7.2. The ones below 7 include horse manure at 6.4 and bark compost at 5.4. Reference 2 studied home composts and found a mean pH of 7.0 to 7.5.

Yard debris and leaves are the  most likely sources of organic material for the home owner, so you can expect your compost to be slightly alkaline.

So how will this alkaline compost acidify your soil???

Acidic Soil and Compost

If compost is neutral or slightly alkaline, how can it acidify soil? The simple answer is that it doesn’t, at least not to any great extent. Since compost can be either slightly alkaline or slightly acidic, it is just as likely to reduce acidity as increase it.

Adding a soil amendment that has a pH of around 7 will not affect the soil pH. Secondly, you are not adding compost in huge amounts. As you will learn in a future post too much compost is not good for your garden.

The third reason compost will not have a significant effect on soil pH is that much of the material in compost is still not decomposed. See my post Compost – What is Compost for more details.

The full story is a bit more complex than I have stated in above. You might remember that one of the benefits of compost is that it buffers pH (Benefits of Composting). It absorbs ions and tends to keep the pH of soil from changing too much. In some alkaline or acidic soil, compost can absorb enough ions to move the pH closer to neutral. These will be small changes in pH. Don’t expect huge drops in pH by adding some compost.

In conclusion, compost may move the pH of your soil closer to neutral, but for the most part it will not affect the soil pH very much. For most gardeners who are growing a variety of plants, the shift in pH is a non-issue.

If your soil is very alkaline and you need to lower the pH, compost is not your best choice—use sulfur.

References:

1) Compost Acidification for High Bush Blueberries: http://www.pnva.org/fview.src?ID=137&

2) Compost Analysis:  http://www.google.ca/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0CCMQFjAA&url=http%3A%2F%2Fwww3.imperial.ac.uk%2Fpls%2Fportallive%2Fdocs%2F1%2F33729701.PDF&ei=YQ76VI6TLIKNyATSs4G4CQ&usg=AFQjCNHGU2wrfDK8BWnJat66ImDpcxCgyw&sig2=-HZUdjFNQBvcAh0gxQOWRA

2) Photo Source: Anathea Utley

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Robert Pavlis
Editor of GardenMyths.com
I live in southern Ontario, Canada, zone 5 and have been gardening a long time. 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!

I hope you find Garden Myths an educational site that helps you understand your garden better.

18 Responses to 'Compost Creates Acidic Soil'

  1. Dr D. J. Udoh says:

    Good source of information on managing compost. Keep the Ste active pls.

  2. Barbara says:

    I have access to spent grains from beer to use in my compost along with used coffee grounds. I just don’t know how much of each to add to my compost. Seeing as both these products are acidic, I don’t want to over acidify my compost. I use garden products such as clippings, leaves, small cut branches, soil, water. Compost bin is made up of 3 sections and each section holds about 3/4 cu yd of material.

    • As things compost the pH will be adjusted by the microbes. I doubt spent grains of beer are very acidic, but even if they are, microbes will use the organic acids as a food source.

      • Grayley McWatters says:

        Hi Robert. My fern garden browns out too early each year and I would like to extend its bounty. Any ideas? Also what should I expect in terms of the length of my fern season each year. I live in Barrie Ontario. Cheers

        • Not sure what a fern season is, but if you mean the duration that they stay green, then the secret is to grow more evergreen ferns. Some ferns will stay green all winter in zone 5.

  3. Heather Booth says:

    I have a problem with very alkaline compost. Food scraps in an anaerobic bin with plenty of tiger worms give me virtually solid worm castings which are highly alkaline (>10). So are vine leaves left in a plastic bag for a year. I cannot use this compost on my garden. Is there anything I can do to turn it into usable compost? And why is it so alkaline in the first place?

    • If you have an anaerobic bin – you don’t have tiger worms – they would die in such conditions.

      Worm bins are usually under pH7, and some people have trouble keeping the pH above 6. At pH 10 the worms are probably dead. How are you measuring pH? I wonder if the problem is one of measuring pH.

  4. Neal says:

    What can I do if I can’t find any sulfur or iron sulphate to reduce the ph of my clay soil? We only get magnesium sulfate and copper sulfate, do you think they would work?

    • Your soil probably has lots of magnesium so that is not a good choice, and copper can become toxic – never add much of it.

      Sulfur is used in horticulture and it is used for treating horses. You might try farm stores that sell horse supplies.

  5. Mike says:

    I have very sandy soil I did a ph test it shows slightly acidic and neutral can I add like humus and manure and triple 12 fertilizer.

  6. Dusty miller says:

    I live on the edge of a desert with lousy soil. Have been adding very large amounts of lawn clippings and horse manure weekly and turning (aireating)on a daily basis. With ambient temps around 40 deg c it only takes about a week to break down but ph has increased from 5.5 to9 I do not have access to sulphur but do have sulphuric acid. Will it do the same job

    • I would suggest you stop turning the soil. The more you mix it, the faster the organics will decompose.

      Sulphuric acid will also work since the microbes convert the sulfur to sulphuric acid anyway. Be careful with the acid, and make sure you know how to use it properly. It is dangerous and for that reason I would not recommend its use for the general public.

  7. Great info! Thank you. I was thinking of getting a couple of blueberry plants and maybe some azalea plants, to put in a raised bed. For the soil, I was gonna use a mix of compost, clay and sand. And for the acid, I wasn’t sure…. Is adding sulfur the best way to do things? What would you recommend? Thanks!

    • A lot depends on the pH of the ingredients you will be using. Sand will probably not affect the pH very much, provided it is washed sand. The clay I am familiar with is alkaline, but there are many kinds of clay.

      The best way to acidify soil is with sulfur. Some sources recommend aluminum sulfate, but I would never use this. Aluminum can be toxic. Sulfur is slow acting. The only way to know how much sulfur to add is with a soil test. Or you can add some, and see how the plants do. I have no posts on using sulfur, but there is lots of info on the internet. Don’t believe sites that tell you that you can use peat moss and/or pine needles.

      My soil is 40% clay, and pH is 7.4. I can’t grow either blueberries or azaleas very well–they live, but don’t prosper.

      The following links might be of help to you:

      http://www.gardenmyths.com/soil-ph-testers/#more-1453

      http://www.gardenmyths.com/increasing-soil-acidity/#more-1256

  8. Roger Brook says:

    Gosh is that really you Robert. I had you down as a very sober gentleman!
    What a ridiculous idea that compost acidifies the soil. There is some slight acidification in the decay process but it is trivial with regard to the effect of your compost on the garden. I don’t do much composting myself as I prefer to recycle my organic matter more directly such as mulch mowing or leaving all weeds to die on the surface but when I do compost I chuck a few handfuls of dolomitic limestone on the heap more to speed up the process rather than bother about any slight acidity.
    I don’t know if you have issues with peat over there but this is an excellent way to acidify the soil if you wish it and of course sulphate and/or ammonium fertilisers acidify the soil.

    • The claims by others for compost is that it acidifies soil, and the title of the blog says so, but I think the contents of the blog show it does not acidify soil.

      I don’t think adding limestone to a compost pile will speed things up – just my opinon. It is important that the pile becomes acidic so that fungi start decomposing the lignin.

      Does peat acidify soil? I have to write about this one day. Peat moss, or what is called Canadian peat here, is acidic. But it is not a good additive to acidify soil around here, because our soil contains limestone. http://www.gardenmyths.com/increasing-soil-acidity/#more-1256

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