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Pine Needles Cause Acid Soil – Fact or Fiction

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Believe it or not, this is fiction, a myth.  Pine needles do not make the soil more acidic.  This bit of garden lore is so common that almost everyone believes it, including many professionals.  It is true that the soil most pine trees grow in is acidic, sometimes only slightly, but acidic nonetheless.  Was this acidity caused by the pines?  Most evergreen species prefer soils that are on the acid side, as do oak, maple, and many other plants.  But they did not begin growing in an area and then change the soil to suit their requirements.  It would be great if plant could actually do that!

Now, I would like to do a little bit of clarification of this statement because I may have ruffled a few feathers.  Many materials are acidic before they are broken down.  Pine needles and oak leaves both have pH’s ranging between 3.2 and 3.8 when they fall from the tree.  If those materials are incorporated into the soil before decomposing, they may have a small effect on the soil pH.  If those raw materials are used on the soil surface as mulch, they will have very little effect on plant growth because the roots are not growing in this material.  As the leaves and needles break down, they are neutralized by the microbes that are doing the decomposing work.  Most compost has a pH of 6.8-7.0, which is very neutral.  Dr. Maynard from UCONN Ag Research Station has done several studies showing the effects of mulch on soil pH.

The general recommendation is to add no more than 10% of pine needles into a compost pile.  This has nothing to do with the pH.  Pine needles have a slightly waxy coating and are very dry, and they take a long time to break down.  For faster compost, limit the number of pine needles and sticks, shred leaves, and balance the “browns” with “greens” like grass clippings, food scraps, and coffee grounds.  The benefit of adding pine needles to a compost pile is they maintain good aeration.  They do not compact readily and keep air flowing through the pile, which is also important in fast compost creation.

Pine needles (and shredded leaves) are an excellent option to use as mulch materials around all plants.  They look good, will help keep down weeds, conserve soil moisture, and eventually break down and add nutrients back into the soil.  Pine needles are actually sold for this purpose in bales in many parts of the country.

Part of the reason this myth continues is because it is hard to grow anything under a pine tree.  Everyone assumes it is because of the accumulated pine needles making the soil so acidic that nothing will grow.  They reason nothing will grow is actually because the roots of evergreens are so numerous and shallow that they out compete other plants for water and nutrients.  They also tend to create quite dense shade which is a difficult growing situation for many plants.  I can address this issue in a future article.

Written by:  Sherry Combs University of Wisconsin Cooperative Extension

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