Wednesday, 13 August 2008

Ashes to Ashes

In The Garden

Green with a gun has suggested using the ashes on the garden. I have until now avoided this, recalling warnings we were given when first starting out in gardening that it would alter the pH of the soil and damage it's structure. In the interests of finding the facts and deciding whether we really do have a winning solution I thought it worth investigating some more.

Research across a range of sources reveals the following elements may be present in wood ash, with quantities being dependent on the wood burned, and the ground that the tree grew in. In all cases hardwood ashes have more nutrients (and more ash) than softwood ashes. The values below are all from Northern hemisphere sources, and there's a chance our local Eucalyptus would come up with slightly different numbers.





15% - 30%

Generally in the form of calcium carbonate. This is the same stuff we get in lime, which is used to raise soil pH from acid towards alkaline.


3% - 9%

In the form of potash


1% - 3%

As phosphates.


1% - 3%


0% - 1%



A number of the trace elements are often classed as “heavy metals” and considered to be pollutants rather than valuable nutrients.

According to the PDF:


wood ashes are 40% - 50% as effective at altering soil pH when compared to lime. The other sources indicate that the constituents of ashes are water soluble therefore fast acting, in contrast to lime which takes longer to do it's job.

All sources indicated that wood ash should only be used on alkaline soils, and if using large amounts pH should be tested every year or two.

A number of other warnings were supplied, paraphrased, condensed and/or summarised below:

  • Don't use ash at the same time as applying nitrogenous fertilisers. The fertilisers will gas off the nitrogen as ammonia in the high pH, so you're wasting resources. Wait for a couple of weeks for the ash to do it's thing then apply the nitrogen.
  • Acid loving plants should not have wood ash applied to them e.g. Blueberries.
  • Alkaline should not have ash applied to them, and neutral soils should only have it applied very carefully in small quantities.
  • Ashes should not be applied where potatoes will be put as they may promote potato scab.
  • Don't use ash from stuff other than wood. You never know what you'll get.
  • Don't use ash on your seedbed at time of planting or on seedlings, ash salts are quick acting and relatively strong, so will burn the seedlings.

Another use for wood ash is as a pest control. It can be liberally dusted over infestations of pear and cherry slug to great effect. It was the solution of choice applied during my childhood, and I recall with some trepidation the act of spreading the stuff around.

Some representative sources on wood ash in the garden:


Totally Practical: Wood Ash

Wood ashes from your yule log can help your garden grow

Use caution with wood ash on your lawn and garden

Wood Ash in the Garden

WOOD ASHES - How To Use Them In The Garden

Is Wood Ash Good for Garden Soil?

In Soap and Fuels

Wood ashes (from hardwood trees) can also be used to make lye. Lye is generally made by soaking water through a barrel (with a perforated base) of ashes. The stuff that comes out the bottom is lye water, and needs to be concentrated for use by boiling off the excess water. This is pottassium hydroxide, rather than sodium hydroxide. This is good news if you're looking for a complete on-farm process for making biodiesel, as pottassium hydroxide is the stuff that is used to make ethanol-ester biodiesel, and ethanol can also be made on-farm, unlike methyl-ester biodiesel (which uses sodium hydroxide) See the following if you want to know more:

Making lye from wood ash

Ethanol biodiesel

A comprehensive guide with pictures covering soap making from lye is available at:

Traditional Soap Making

Another blog indicates that soap from potash lye is a jelly or soft soap, and requires hardening up with the addition of table salt if that is the aimed for product. According to the resource hard soaps are made with sodium lye.

Making Pioneer Soap


green with a gun said...

Yep, that's why I said just one heaped shovelful per square metre annually.

Geoff said...

Which isn't such a good idea if you've got alkaline soils ;-)

green with a gun said...

It is if you have the ash together with other stuff. Man does not live by bread alone, and plants do not live by ash alone.