Throughout the course of our testing garden assumptions series I have been asked a number of times about the use of wood ash as a soil amendment in vegetable gardens. This interested me as often I will add the ash from my small fire pit into my compost as it is something that I have seen family do in the past.
Today I thought I would look into the practice to see if using wood ash has benefit in the garden while diverting it from landfill.
Today’s general hypothesis is that Wood ash has benefit when used in a vegetable garden.
In order to understand this issue a little more we will need to ask some much more specific questions
Is wood ash a fertilizer?
Will wood ash change the pH of my soil
Is wood ash a fertilizer?
The University of Wisconsin Extensions produced a information bulletin on the use of wood ash as an agricultural amendment and substitute for lime. In their research the sampled 16 ash samples from local sources and found 12 of the 15 commonly tested for elements that are essential or beneficial for plant growth.
Notably there were significant concentrations of Calcium, Magnesium, Phosphorus and Potassium all nutrients required by plants in higher concentrations.
They also noted some other elements that could become toxic however their concentrations are low enough to represent what you would expect to find in most soils naturally.
It would seem that wood ash does have value as a fertilizer however most notably it is missing nitrogen which is lost during the burning process.
Regardless there are many studies evaluating the use of wood ash and correlating over many years to increased production of commercial crops. This proves to be most effective in acidic soils when the researchers were growing forage crops for livestock that grow best in neutral or slightly alkaline soils.
Will Wood ash change the pH of garden soil?
This leads us to the next question will wood ash change the pH of garden soils?
Wood ash is very effective at increasing the pH of soils.
Many sources site the use of wood ash as a replacement for lime in agricultural practices. The purpose of which is to raise the pH often from acidic conditions to a near neutral pH of 7.
This is done to bring the soil pH into the optimal range for the crop. Having the pH of the soil close to optimal allows the crop to access the available nutrients resulting in higher yields.
If the pH of the soil is outside of the optimal range the nutrients in the soil may not be available to the plant.
With this in mind most of the common crops grown in back yard gardens prefer pH on the acidic side of the scale. Crops such as tomatoes, spinach, peppers and peas grow optimally in pH ranges between 5.5 and 7.0.
The addition of wood ash to garden soil that is already in the optimal range may have a detrimental effect on nutrient availability. Wood ash generally has a very small particle size and even if applied in the mulch layer can quickly move into the soil having an immediate effect on the soils pH. This is in contrast to things like coffee grounds and pine needles that although are acidic they have to break down before they can influence the soils pH and have often neutralized their acidity by the time they can do that.
The soil will likely buffer out the wood ash returning the soil to a more neutral pH over time however the immediate change to the pH may cause damage. Quick changes in pH often disrupt or kill the beneficial organisms, block nutrient absorption and burn plant roots.
So application in the mulch layer unless you know the pH of your soil is very acidic may not be the best course of action in a home vegetable garden.
As I anticipate questions about the use of Lime in vegetable gardening I will likely address the topic in another installment of the Testing Garden Assumptions series.
That said wood ash from your fireplace is a waste product and does have a good spread of nutrients with in it. Is there a way we can use it in the garden without it harming our plants or soil?
There have been a number of studies that looked at the effect of adding wood ash to compost and its effects on the pH of the finished compost.
The results vary likely due the pH of the initial compost material. Most studies found that at a maximum wood ash application rate of 16% by volume the finished compost ended up with a pH near neutral. At the beginning of the composting process the pH would start high due to the ash however over time the pH drops and often settles in the optimal range for vegetable gardens before the composting process is completed.
This is consistent with the neutralization of more acidic materials in hot compost such as spent coffee grounds. They tend to start much more acidic however through the composting process neutralize.
There are some claims that the use of wood ash both in combination with other materials and by itself help to manage pests. The information is less than consistent and generally with the passive pest management methods I practice I have not needed to actively manage them in my garden.
In summary wood ash can be used in vegetable gardens as a fertilizer however it is recommended to first add it to a hot compost and allow time for it to neutralize before application.
If you have would like to catch up on the testing garden assumptions series where we put products, process and methods to the test make sure to check out the playlist on screen now. If I have not addressed a topic please make sure to suggest it in the comment section below. Government of Alberta Fact Sheet on the use of wood ash for agriculture
http://www1.agric.gov.ab.ca/$Department/deptdocs.nsf/all/agdex3435  Wood ash effects on plant and soil in a willow bioenergy plantation
http://ac.els-cdn.com/S0961953404001783/1-s2.0-S0961953404001783-main.pdf?_tid=d4c73406-4b5b-11e5-9372-00000aacb35d&acdnat=1440529528_26f194ae9c8e4a27c5d6713a7991acfb Using Industrial Wood ash as a soil amendment
http://learningstore.uwex.edu/assets/pdfs/A3635.pdf  Wood ash as a liming amendment
http://www.tandfonline.com/doi/abs/10.1080/00103629209368661#.Vdy-IPlVhBc  Perdue University Info Sheet
https://www.hort.purdue.edu/ext/woodash.html  Wood ash in yard waste compost
http://www.tandfonline.com/doi/pdf/10.1080/1065657X.1997.10701864#.VeCOAflVhBc  Wood ash admixture to organic wastes improves compost and its performance
http://ac.els-cdn.com/S0167880908000650/1-s2.0-S0167880908000650-main.pdf?_tid=de5b3f36-4da3-11e5-a294-00000aab0f27&acdnat=1440780371_c837bb8b8861885db1857790982e1b4b  Evaluation of three composting systems for the management of spent coffee grounds