Leaves and Soil pH

Frosty oak leafQuestion: Is it true that certain types of tree leaves are highly acidic? Should I think before shredding them and using them as mulch?

Answer: You’re probably referencing the old myth that oak leaves shouldn’t be used as mulch. Experts with University of Missouri Extension note that oak leaves are acidic, but pH is not a big concern with mulches, because mulch is put on top of the soil, not worked into it. Oregon State Extension concurs and adds that many organic mulches (besides oak leaves) are acidic, yet only very sandy soils are susceptible to pH changes due to mulch. Other soil types “buffer” potential changes. If the mulch is applied repeatedly, the top two inches of soil may become slightly more acidic, but this will not affect most plants. The best way to know and monitor your soil’s pH is to test it; you can also gauge soil pH by growing certain pH-indicating annuals.

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11 thoughts on “Leaves and Soil pH

  1. Michigan State University also did a study on pine needle mulch and reported at a Master Gardener conference last spring that it did not significantly change the soil pH.

  2. I’ve heard about black walnut leaves (Juglans nigra) specifically can be problematic to some plants that grow beneath them. However, I am having a hard time finding out if English walnut (Juglans regia) create the same problems with allelopathic qualities. Has anybody out there had experience with the English walnut leaves as a mulch?

    • Hi, Carmen. The problems associated with black walnut stem from a chemical called juglone, which is present in all parts of the tree. Juglone is also produced by other members of the walnut family, to various degrees. Here’s a link to a Q&A we ran on juglone. http://www.hortmag.com/featured/poisonous-leaves
      It includes info and links about susceptible and tolerant plants.

      And here’s a link to the Juglans regia page at the Missouri Botanical Garden. They note that the English walnut can be as problematic to certain plants as the black walnut.

    • Sounds like you need to perform an experiment with some dried English Walnut leaves. Grind them up and mix them with potting soil, then plant tomatoes, beans, etc. Don’t forget the control group, which would consist of the same plants, potted up in plain potting soil.

  3. Here in the West I would argue that leaves don’t even need to be shredded. With our sparse rainfall, most trees adapt by having small leaves (apple and aspen account for 90% of our yard trees). I routinely put 2-6 inches of leaves on my beds without shredding. By spring they have matted a bit, but are promptly broken up by birds looking for worms. I don’t even buy the argument about leaves hurting grass. I haven’t raked under our well established trees for two decades. Not only is the grass thriving, but the earth below is as soft as butter.

  4. How many intuitives, Garden Whisperers, are out there?

    Knowing entire regions of earth evolved, well before humans, to thrive with the humus of oak leaves. The giver of life: plant, fungi, insect, mammal, reptile & etc.

    How many are brave enough to use Leaf Litter Mulch?

    How many HOA’s approve of Leaf Litter Mulch?

    What forces are complicit in NOT using Leaf Litter Mulch?

    FYI, sadly, the great oak trees of Atlanta, GA are dying due to prolonged drought stress.

    Garden & Be Well, XO Tara

    • I love using my shredded leaves-(some grass clippings) as a winter mulch. I also make a pile of it and throw my vegetable peels in it through out the winter and by Spring I have a lovely pile of Garden Gold – my Thanks to the Sun and the Worms.

    • Maybe if the oak leaves had been left to compost and protect the soil under the trees from drying out they wouldn’t be dying of drought stress….Tara is right, we should be listening to mother nature…

    • Here in Dahlonega, GA I run our leaves through a lawnmower and then put them all around the trees and hydrangeas every fall and in the wooded area they stay where they fall mixed in with the pine straw from the pine trees, my awful red clay is now getting richer from three years of doing this. I also have lots of earthworms helping to break up the clay.

      • I take care of my mother’s yard. We just run the lawnmower over the leaves, mulch them and leave in place. Her lawn looks so much better since we have been doing this and can handle the dry months better. No more raking leaves!

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