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Increase soil pH

Posted by johnnc7b z8aNC (My Page) on
Mon, May 26, 08 at 11:53

How can I raise the pH of my acid soil? I want to plant red hydrangeas and assure that they will stay red. I have considered digging a large planting hole and lining it with oyster shells then fill it with a lime adjusted potting mix. Any other suggestions? Also, what would be the best high pH mulch to use to insure that the pH remains high?

I posted this message on the Hydrangea forum also.

Thanks for any help,


Follow-Up Postings:

RE: Increase soil pH

First, I would have a soil test done to find out just how acidic your soil is (so you know how much to increase it). You could call your local Cooperative Extension Service (see site below), and they will know where to send it. A basic soil test only costs about $8, and it will tell you the pH of your soil, and what kind of product to use to raise it. If your soil is also low in magnesium, they will suggest one thing, but if it isn't low in magnesium, they will tell you what else is best for your area.

Even though NC soils tend to be acidic, knowing what kind of lime to use, and how much, is very important. Without a test, you won't know. Overliming, in many cases, is just as bad as not liming.

Finely-ground lime will still take three years or so to be broken down into the soil, approximately 1/3 of it per year.

Most mulches tend to be slightly alkaline, so they would be suitable for your purposes. Materials to avoid are oak leaves, pine needles, pine bark and sphagnum peat moss, all of which will increase acidity.

Also keep in mind, even after your soil is limed, that if your water is acidic, that can affect your plantings. If you collect rainwater and have acid rain, try hanging a bag of limestone chips in the water barrel to neutralize the acidity.


RE: Increase soil pH

Thanks for the response. I have checked the pH with one of those pricey stick in the ground meters and found my pH to be 6.2 to 6.5. Since I'm trying to get red hydrangeas I really need aluminum rather than magnesium in the soil. As I understand it, if the pH is high enough then the hydrangeas can uptake aluminum giving the red color, at least for those plants whose color responds to pH. I guess the soil test can give me the aluminum content of my soil and what I need to use to amend it if necessary. Better call my Extension agent tomorrow, huh!

As to mulch, how about ground wood chips? I have a wood chipper that I can feed wood to and make a mulch, of course, then the question becomes "which wood?".


RE: Increase soil pH

  • Posted by jean001 z8aPortland, OR (My Page) on
    Mon, May 26, 08 at 15:38

You wrote: "Since I'm trying to get red hydrangeas I really need aluminum rather than magnesium in the soil."

That's backwards.

As soil pH decreases, more aluminum is available to plants. With hydrangesas, they become more blue.

With increase in pH, you get red hyrangea blooms. As was said, adding lime helps.

As to the pH of various mulch materials, for the most part they're very near neutral.

RE: Increase soil pH

While aluminum is an essential plant nutrient it can be toxic to plants if just a little too much is put in the soil. Before you believe that soil testing probe check it against known values, such as vinegar and baking soda to see what it reads. I have more often than not found these will read the same thing no matter which it is testing.
If you need to raise the soils pH, a soil pH of 6.2 to 6.5 is very good and does not really need any adjustment, then you need lime, but whether you need calcitic or dolomitic lime can only be determined by a good, reliable soil test, one you meter will not do.

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