By Marilyn Homewood, Master Gardener

At a recent horticulture meeting, the subject of using wood ash in the garden came up.  As the days grow shorter and the temperatures drop, I was reminded of this.  As someone who used to heat the house with wood, I am no stranger to finding uses for my ashes.  Although most of the time, the ash went onto icy paths and the lane, there was always some left in the spring.  Those were spread in a thin layer over some of the soil in the garden.  As with most things, ashes in moderation can be a good thing, sometimes!

Why use wood ash in the garden? From my perspective there are two reasons for wanting to use ash in the garden.

1.  Recycling.  Better to see the ash returned to the earth than ending up in the landfill. 

2. Benefits to the soil.  Constituents of ash can vary depending on what type of wood is burned but generally the largest component is calcium carbonate (20%).  Next is potassium (10%), phosphorous (1%) and possibly some trace amounts of micro-nutrients such as iron, manganese, boron, copper and zinc.  Used in moderation, ash does act as a fertilizer but note it is not a complete fertilizer as it doesn’t contain nitrogen.

Keep your eye on Soil pH

Ash is alkaline due to its calcium carbonate component and has the ability to affect soil pH.  For those growing acid loving plants such as azalea or blueberry, the addition of wood ash will not be appreciated.  Folks with neutral to slightly sweet soil probably will not notice any deleterious effects however if you are struggling with an elevated pH, ashes could end up tying up more micronutrients such as iron and result in even more chlorosis in plants.

How do I use wood ash and how much?

As a general rule of thumb, an application rate of 20 pounds (approximately a 5 gallon pail) per 1000 square feet per year (it takes about one cord of wood to produce this much ash).  I have seen recommendations on applications for both spring and fall.  Just recall that wood ash is very fine and blows away quite easily.  It seems best to apply a thin layer to moist soil or to dig it in.  Wood ash can also be added to compost piles at a low rate (sprinkle one or two cups on top of pile when adding green or brown material to the pile but do not exceed more than 5% of the total volume of the compost).

Cautions when using wood ash

Ash is caustic.  Wear a mask to avoid breathing it in and wear appropriate PPE.  Never mix ash with nitrogen fertilizers such as urea or ammonium sulphate as such a mixture will produce ammonia gas.  There are certain things you should not burn if you plan on using your ashes in the garden as they could contain heavy metals such cadmium, chromium or lead.  These things include:  Treated lumber, painted or stained wood, cardboard, fake fireplace logs, coal, BBQ briquets and any wood that was ignited with a fuel such as gasoline.  Some references caution against using the ash from wood that was grown near industrial locations for fear of heavy metal contamination.

Since wood ash does have some benefit as a soil amendment, it seems like this is one more way to reduce the amount of waste going to our landfill. Why not give it a try?

Resources

https://gardening.usask.ca/articles-and-lists/articles-healthysoils/wood-ash-in-garden.php

hort.extension.wisc.edu/articles/using-wood-ash-in-the-home-garden

https://www.canr.msu.edu/news/wood_ash_in_the_garden

Using Wood Ash in the Home Garden

Using Wood Ash in the Home Garden