Wood Ash and Soil Fertility

I have written a series of posts that revolve around soil fertility, and so regular readers of the blog might think that another blog post is overkill. I hope not.

Last nights’ post, Crop Rotations, Soil Fertility, and Digging (part 2), looked at the amount of compost needed to grow the food to feed a couple. Even at the lower level there is still quite a lot of compost to make. I was a bit restless last night, and so stayed up late, feeding our wood burning stove.  I was aware that wood ash contained minerals from the tree, but a comment by renowned Permaculture Author Patrick Whitefield, led me to check my facts. My discoveries are likely to lead to a considerable saving in work.

Is there a loss of minerals when burning wood?

My first step was to check my facts. A quick internet search, enabled by another wintery day outside showed that the only minerals lost by burning wood are Nitrogen, and Sulphur. I have added some links to the sources of information that I read.


The link above is a general article about wood ash, but two of the references at the bottom were interesting. The link below is one of them. It is particularly useful as it has a table with figures for the mineral composition of ash, albeit average figures.


As Nitrogen is fixed from the air, the loss of it in burning wood is not too much of a concern. The sulphur is important for plant growth but and is not normally deficient, but I need to look at it’s provision, especially if wood ash becomes a significant part of my fertility regime.

What does this mean to me?

With 95% or more of plant material provided by sunlight, air, and water, only 5% coming from the soil, only a small amount of minerals are taken from the soil by plants. Returning most of the organic matter to the soil will replace much of what has been used, but a small amount will be always be lost. Minerals are also lost through leaching. One of the key elements lost in this way is Calcium, which is only lightly held by the soil colloids (clay and humus). This leads to an acidification of soils in the humid temperate regions, which is normally corrected by the application of lime. With most of the minerals present in wood remaining in the ash, this is a good potential source of replacement minerals, and as the Calcium content of wood ash is around 50%, it is a good way of correcting acidity.

A reduction in bulk

In my last post I quoted a figure of 160 wheelbarrow loads of organic material needed to produce the food for two people. This is remarkably close to the amount of wood that we burn over the winter, a figure about to rise as we start to use wood to cook, and for water heating. Wood is organic matter, and we use about 2/3 of a wheelbarrow load each day, for six months, making about 120 loads, so the wood ash that we are producing contains all of the minerals from 120 wheelbarrow loads of organic matter, which is potentially 3/4 of the minerals that we need to maintain soil fertility, less the Nitrogen and Sulphur. Not only that, but as the ash is 5% or less in volume of the original wood, that is a massive saving in bulk, and materials shifted. Sadly that doesn’t mean that I can stop composting, but it does mean that I may be able to reduce the scale of my composting operation.

The need to continue composting

Compost is not only a depository of plant foods, but its structure, and chemistry are vital to healthy plant growth, Replacing it with ash would be no different from using an artificial fertiliser, but as a renewable addition, either applied directly to the soil, or incorporated within my composting, it is likely to give a significant boost to the health of my soil, and therefore plants.

Practical Applications

Knowing that wood ash is valuable isn’t enough, finding ways to use it in a food production system is just as important.

As wood ash is highly soluble, dry, and powdery, it is important to consider how to apply it. Some of the sources that I have read suggest applying during moist conditions in the autumn. I’m not sure how efficient that would be. I suspect that a significant proportion of the active ingredients might be leached away by rain during the winter. A better option is likely to be to incorporate it into my compost by sprinkling a thin layer of it regularly as the pile is built. As long as the pile is protected from the rain, it should give time for the microbes to incorporate the minerals into their bodies, holding it there until the compost is applied. The colloidal structure of the humus ‘attract’ and hold minerals, especially the calcium, as will any clay added to the compost heap.

The articles for which I have provided links state that the mineral content of ash is highest when obtained from leaves and bark, followed by branch wood, followed by trunk wood. My plan is already to coppice smaller branches, which will help. I intend to use the twigs that are not used for kindling as a soil amendment, either directly, or composted. Our use of trunk wood will decrease as the larger trees are managed.

My reading about traditional Asian agriculture suggests another potential use, preparing the ground prior to planting fruit trees. If a pile of twigs and branches is built on the planting site of individual fruit trees, and then set alight, the fire will not only kill off weeds, grass, and weed seeds, but will add a boost of minerals to the planting site. Thus building soil fertility.This might avoid the need to use pesticides, or mulch mats. Any death of soil microbes can be rectified by the use of leaf mould, which will give a better soil microbial environment for the trees than that which exists in grass. Strange how some people think that ‘slash and burn’ agriculture, or swidden, was primitive.

A word of warning

All of my reading material has stressed the importance of checking soil acidity/alkalinity before using wood ash, in order to avoid making the soil to alkaline.

Further Reading

If you want to read a lengthier paper on the use of wood ash, the link below contains far more detail.

Wood Ash Link 3

Two interesting facts stand out about the phosphorous content. The first is that there is a dramatically higher concentration in the foliage, which is yet another good reason for using cut leaves and stems as a soil amendment. The second is that of the trees reported, Red oak had the highest levels of phosphorous. This was interesting as I have used red oak in my Chicken Scavenging Area, and it was one of the trees that I have considered adding to that space. I’d like to check to see if there is as much in common oak, but as the red oak grows much faster, it is looking like a good option to plant.

11 thoughts on “Wood Ash and Soil Fertility

  1. Sue Laverack

    Thanks Deano. I have found this series of posts very interesting as improving soil fertility is a subject that has been exercising my brain too. I think you may have found some of the answers for me.

  2. Ian Pearson

    I agree that adding to compost is a good way to utilise ash, but I try to keep some bagged up ready for direct application to any plants showing deficiencies, disease, or pest stress. I also use it as a preventative measure to any fast growing crops in late spring and summer, sprinkled on the surface before a heavy watering. The potash should harden up soft growth.

    1. Deano Post author

      That’s interesting Ian. I remember reading about a guy who used wood ash diluted in water, as a tree tonic for any trees that were looking ill. I think that I’ll keep some to one side.

  3. max

    Wood ash is allegedly a slug deterrent too, at least when it is fresh and dry , and therefore very salty and alkali for a slug to slither slowly upon. This could influence how and when you apply wood ash as fertilizer.

    I would avoid applying wood ash from fence posts and other sawn timber which may have been treated with wood preservative with a toxic mineral component ie CCA or copper-chrome-arsenate preservatives. Utility poles treated with ‘cobra’ also very toxic, and avoid ash from bonfires containing tyres, pvc etc. I guess old-fashioned creosoted posts would be OK if burned very hot in a stove. Best to stick with wood that is clearly made from bits of tree for fertilizing food crops.

  4. Pingback: Wood Ash and Soil Fertility « The Sustainable Smallholding

  5. sten

    Why burn the wood before preparing a site for a tree? Check hugelkultur that would preserve all minerals and other nutrients by slow degrading with the minerals in organic form all the time? If one burn the wood, why not use the old South American methods to make black soil by covering the fire with earth before it is burnt out to create “terra petra” or biochar at the site instead? But I think the infiltration of nitrogen rich manure/compost is required also to “load” the biochar. On the other hand a bonfire will damage the natural fungi and mycelium on the site. I would instead dig a larger hole, plant the tree in the middle and place dead wood around and then cover back with the top soil. Then also plant plenty of nitrogen fixing companion trees around main tree for chop and drop. A usable way ?


Leave a Reply

Your email address will not be published. Required fields are marked *