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pH of compost too alkaline

Posted by GWTaz Northern Virginia (My Page) on
Fri, Jan 24, 14 at 22:07

Hi...just found this website and am loving it. I have a farm with horses and am a florist so grow predominantly perineiis (sp). I have spent the past 5+ years adding truckloads of leaf mulch and composted horse manure to my flower beds. I have done a series of pH tests and realized that the compost and leaf mulch are making my beds too alkaline. I would like them to be around 6-6.5 but they are now more like 7-8. This is way too high for growing peonies and dahlias (not to mention my blueberry bushes); so guess I need to amend soil with sulfer or aluminum sulfate? There is snow on the ground now so wondered if I could put some sulfer around the drip line of my plants. since my beds are already planted (not dahlias...they are out for the winter) Suggestions on the best way to amend the soil? Can't wait to hear back. So excited!


Follow-Up Postings:

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RE: pH of compost too alkaline

Many people find that when a soil has adequate levels of organic matter it does have a pH in the 7.0 range. Mine went from 5.7 pH to 7.2 pH in about 4 years without any lime added. I have not seen any real problem due to that soil pH and all of the plants, annuals, biennials, and perennials grow and blossom quite well with very few insect pest or plant disease problems.
Where Blueberries, and other plants such as Rhododendrons and Azaleas, are to grow adjustments may well need to be made, although the Rhodies and Azaleas seem to do quite well in a soil well amended with organic matter with a higher pH.
If you do opt to add sulfur to lower the soil pH be sure to add enough to do what needs to be done and not just a bit sprinkled here and there. A good, reliable soil test is in order here.


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RE: pH of compost too alkaline

Thank you for sharing. My dahlias do wonderfully, peonies are smaller than I think they should be. Blueberries seem to be fine so was surprised with the results of the pH test. A neighbor has pH around 7 and grows great stuff. Will continue to add compost and will keep monitoring..should I check to see if lacking certain nutrients?


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RE: pH of compost too alkaline

  • Posted by ericwi Dane County WI (My Page) on
    Sat, Jan 25, 14 at 22:56

Agricultural sulfur will lower soil pH reliably, but it requires 2 or 3 months to take effect, and it can take 24 months for an application of sulfur to be completely broken down and metabolized by the bacteria that convert this material to acid. Sulfur is not toxic when used correctly, and the gradual lowering of soil pH may be advantageous, since it likely stresses the plants less than an immediate drop in pH. I use indicator dye, bromocresol green, to check soil pH around our blueberry shrubs. In the past I have used a glass bulb pH meter, successfully. Both methods work, but the indicator dye is less expensive.


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RE: pH of compost too alkaline

would I be better off using aluminum sulfate in stead of agricultural sulfer? I understand it is faster but can burn plants if not used properly.

Meanwhile, would adding pine bedding to manure help or does that go to neutral once composed.

So much to learn....sigh


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RE: pH of compost too alkaline

Back up.

Why do you want to lower the pH? What you have told us is that the plant, peonies, that prefers a higher pH, isn't doing as well as you would like.


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RE: pH of compost too alkaline

  • Posted by ericwi Dane County WI (My Page) on
    Sun, Jan 26, 14 at 16:40

Metal probe "pH testers" are not generally considered to be accurate enough to be used in any lab setting, that I am aware of. They might work OK in certain soils with certain moisture levels. Aluminum sulfate has been used with success to lower soil pH, however, aluminum is known to be toxic to fish and also to some bacteria found in the soil. Agricultural sulfur will also lower soil pH, and since it requires several months to take effect, it might be less of a shock to the plants. If you are growing blueberries with success, your soil must have pH values down around 4.5 to 6. I would not expect either compost or leaf mulch to raise soil pH dramatically, unless there was lime added in. If you have hard water, and you are watering the flower beds during the summer months, the limestone dissolved in the water will raise soil pH over time. We have hard water here in Madison, Wisconsin, supplied by the city. It comes from wells that are drilled into limestone aquifers below ground, and it has pH around 7.6. Since you have access to compost, manure, leaf mulch, and pine bedding, you might not have to get a soil test done. All of these amendments will improve the soil by adding nutrients that plants can use. If you can find out the pH of your irrigation water, that would be helpful.


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RE: pH of compost too alkaline

Don't use the probe since had no luck with that. Treated myself to the LaMotte soil kit for Christmas..readings are in the 7-8 levels for my compost...amended beds are around 7ph.

My understanding is that peonies and dahlias are happy around 6-6.5pH.. If I can get away without adding anything to my soil I would be thrilled. Concerned about the peonies tho. They are not happy. maybe its something else that is bothering them..will have to pop over to the Peony section...

Have well water which I do use for irrigation. Good idea to have tested. Never thought of that one.

Don't throw lime into manure piles. Keep that separate...

Thanks for your help!


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RE: pH of compost too alkaline

Just because they prefer 6.5 does not mean they will be sick at 7 or even 7.5. Plants have a range they will grow in.

I've had peonies that did great in pH 7+ soil. Eventually they got shaded out by trees. They do need sun. And we get some kind of blight around here in the hot humid summer weather. Leaves get brown spots and eventually die off.


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RE: pH of compost too alkaline

While most all plants will do most bestest growing in a soil with a pH in the 6.2 to 6.8 range and this range is where most all soil nutrients are most readily available, plants can grow quite well in soils with a higher or lower soil pH, especially if the amount of organic matter in that soil is adequate.
I would not be very concerned about a soil pH of 7.4 and not at all concerned if the pH of my compost was 7.4, as tested by a very accurate pH meter and not a cheap model.


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RE: pH of compost too alkaline

Okay then...will stay put and keep adding compost and hold off on additives...works for me!!
Off I go to the peony section..
Put Christmas tree carcasses on top of blueberries. Makes me feel like I am doing something.

Thanks!


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RE: pH of compost too alkaline

  • Posted by ericwi Dane County WI (My Page) on
    Wed, Jan 29, 14 at 11:07

I have re-read this string of comments, and I also checked out the LaMotte soil test kit, by visiting their website. If you used tap water to prepare your soil sample, it is quite possible that your tap water has pH between 7 and 8. In other words, the pH of the water used changed the pH of the soil, so the resulting reading was close to that of the water used. In the past I have used commercial steam distilled water for soil pH tests, but I found that this source actually had pH = 7.1, so I began using reverse-osmosis water from a vending machine in our local food co-op, here in Madison. If your well water has some dissolved limestone, then it is providing you with both calcium and magnesium, and therefore it is likely good for your health. So I am not saying that you have bad water, however, it may be throwing off your soil pH test results.


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RE: pH of compost too alkaline

Good call... I will check the pH of my water...Thank you.

Read another poston pH...is it possible that you can add too much compost to soil? I add some every year so my gardens are fairly dark, rich and loamy...Before was classic hard Virginia clay.


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RE: pH of compost too alkaline

  • Posted by ericwi Dane County WI (My Page) on
    Wed, Jan 29, 14 at 19:44

We have high clay soil here in Madison, Wisconsin. It might be possible to add too much compost, and get the level of organic material too high. However, I have never been able to accomplish this, in our garden plot. After 20 years of adding compost every spring, and tilling it in, the soil is much easier to work than in used to be. At least, the top 10 inches. But the compost does decompose and disappear over time, so the actual percentage of organic material will rise a bit, and then level off. I suspect that our garden soil is a bit low in nitrogen, because we have no source of manure, but that is a separate problem.


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RE: pH of compost too alkaline

It is hard to believe to have a compost (that is made of garden and kitchen waste) to be alkaline. Most of garden waste/scraps , grass, fall leaves tend to be on the acid side. UNLESS you added lots of things like wood ash, gypsum, ..


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RE: pH of compost too alkaline

Everything I have seen over the years tells me that the pH of finished compost is around 7.0, near neutral. The pH will also vary depending on the stage of digestion and the amount of water in the mix. Unless on adds a caustic (wood ash or lime) or an acid.
The pH of the compost I make is near neutral even though the mix is mostly Oak leaves and has never had wood ash, gypsum, lime, etc. added.


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RE: pH of compost too alkaline

This whole exercise has been most useful. Thank you all for your input..


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RE: pH of compost too alkaline

Deffly use distilled or RO (deionized) water when making a slurry for pH testing...glad someone brought that up, if it was not discussed earlier.


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