Wednesday, 18 January 2017

Is mulch made from conifer wood chip or bark acidic?

I have seen it often suggested that organic mulches such as wood chip and bark from conifers is acidic.  The pH is a measure of the amount (the negative log of the concentration)of hydrogen ions and this measurement must be done in a liquid.  You cannot measure the pH of a solid unless you dissolve it in water.  As most of the solid from wood chip and bark  is insoluble in water, it is virtually impossible to measure the pH of these materials.

The woody chippings or bark could be giving off substances that acidify the soil solution as they decompose.  However, neither pine bark nor pine chippings have been found to have any effect on soil pH.  (Tahboub, Lendemann and Murray 2008) There is no significant change in soil pH for wood chip incorporated into soil measured over a three year period.   Bare soil is more likely to have a low pH (be acidic) than organic mulches.  Shredded bark and woodchip have been found to be the least acidifying of the organic mulches.

Regardless, wood chip could be shaken up in water and the resultant solution tested.  This should be done with distilled water or deionised water to make sure that you are testing the pH of the dissolved substance not the pH of the water.  Tap water contains a lot of substances although it usually has a neutral pH of around 7. 

Now, I haven't tested the pH of  woodchip solution, however I would conjecture that it would be fairly neutral or possibly slightly alkaline.  There are some that suggest that the phenolic substances secreted by the above ground structures of a plant may be acidic. I don't know. 

What I am going to do is test the soil beneath an estimated 2000 year old yew tree.  The tree is in my local nature reserve.  There is nothing growing under the tree within about a 50 food diameter. 

I would suggest that the pH of the soil is no different from the rest of the wood so I will take samples from outside the yew trees influence as well as under its canopy.  After 2000 years of falling litter surely it would have affected the soil underneath it. 

My suggestion is that there will be no difference between the soil pH from beneath the canopy of the yew tree and the soil from outside the canopy.   

The fact that there is no build up of litter under the tree would suggest to me that some invertebrates, such as worms, are feeding on the organic matter from the tree.  Worm's preferred habitat is one with a neutral or alkaline pH.

Usually scientific consensus is pretty solid, however sometimes explanations that purport to be scientific are merely based on hearsay and anecdote and need to be challenged by experiment. 


  1. What a fine load of chippings. Although I agree with you about the general benefits of these mulches. I do think that other than interest, doing a pH test for the chipping is a waste of time.
    Many of any effects of mulch tends to be longer term and consequential on the properties of the material rather than its actual measured immediate pH
    I noted your comment about pH of uncovered soil and yes it is sometimes more acid than when mulched. Some soils- perhaps very few - have been shown to be more acid if managed by minimum cultivation

    1. Hi Roger,
      The only reason I am bothering to try to do a pH test on chippings is to prove to myself that it is a total waste of time. I think that when the chippings decompose, it is a little like burning. Carbon dioxide is given off as microbes respire, leaving potassium, phosphorus and other salts which will make the soil solution alkaline. Uncovered soil will be exposed to rain which may be a weak solution of nitric, carbolic or sulphuric acid (acid rain) which will make the soil solution in the top soil more acidic. The mulches will tend to neutralise acid rain. Digging will tend to increase decomposition of organic matter in the top soil producing carbon dioxide and leaving salts in the soil. This is why annual plants seem to do so well in dug soil. Digging will tend to increase the pH. With "no dig" decomposition will be slower allowing time for acids to build up. In a vegetable garden we want the soil to be slightly acid - around pH 6.5. to make as many nutrients as possible available to the plants. The advantages of mulching far outweigh the slight disadvantage of it raising the pH. As you noted Roger, I use a lot of chippings to mulch the allotment. It was almost completely mulched last year.

  2. I've always found woodchip to have an effect on the soil so much so I tend to dug in a barrows worth into my blueberry bed ever since I've started to do this I find I get a larger and sweeter crop year on year.

  3. Although there is some evidence of nitrogen draw down if woody chippings are incorporated into the soil, it does seem to have lots of benefits. All the evidence that I have read says that wood chip from whatever source does not change the pH of the soil especially if used as a mulch. However, there is evidence that it reduces bulk density, increases water retention, introduces large quantities of organic matter, increases microorganism density and eventually adds nutrients to the soil. All these factors could be giving the results that you have seen Gerry. I would continue to add woodchip to your blueberry bed.