Wood ash: what use is it in the garden? Is it a good fertilizer?

Wood ash, it is well known, has been used by gardeners for ages. It is reputed to render great service to the garden and we even read here and there that it can also be used to fertilize the soil. We are therefore going to discover together the main uses of wood ash in the garden and check whether it is really a fertilizer, but first let us look at the ashes that can be used in the garden without any risk and those that must absolutely be avoided.

Good and bad wood ashes for the garden

Not all ashes are suitable for use in the garden, and some even have a high degree of toxicity. This is among others the case:

  • Ashes from:

    • glued wood,
    • painted or stained wood (furniture, paneling, etc.),
    • wood treated against insects, UV rays or fungi (mold), etc.,
    • fireproof wood, i.e. having undergone a treatment capable of modifying its reaction to fire,
    • the burning of garden stakes, crates,
    • of a pallet fire, some of which may even have been treated with an extremely dangerous gas due to its toxicity and which is none other than methyl bromide. For information, these palettes stamped with the symbol MB are to flee.

  • Barbecue ashes if you lit the fire with the help of small ignition cubesbecause they are loaded with hydrocarbons.
  • Ultra-concentrated ashes that contain heavy metalssuch as those obtained following the combustion of pellets although they are initially natural wood,
  • Ashes from briquettes or ashes from mine coal recovered from the stove, because they are rich in arsenicheavy metals and sulphur.

It is therefore important to learn about the wood you are using in the wood stove, fireplace or barbecue because even if it looks completely natural, it may just be an appearance. If it has been treated, the products cannot always be seen with the naked eye. However, the ash of treated wood is contaminated by all these products. This is of course without counting the air pollution when these woods burn as soon as dioxin gets out of it.

In short, if we want use wood ash in the garden, you have to be sure of its origin. That of the barbecue, the fireplace or even the garden fire is quite suitable provided that care has been taken of only burn natural wood.

Uses of wood ash in the garden

If we can get a good natural wood ash, it would be a shame to throw it away because it is very useful in the garden. It is used for:

  • Reduce the acidity of the compost,
  • Block slugs,
  • Replace the healing mastic when a tree is found to be injured,
  • Correct too acidic soil,
  • Brush the trunk of fruit trees after preparing a ash milk to fight many diseases.

Preparation of a milk of ashes, a substitute for lime

Simply pass the ashes through a sieve first and then dilute it in water in order to obtain a fairly liquid paste which can then be coated on the branches and trunks of fruit trees or even sprinkled at the foot of the hedge and many vegetable garden products such as tomatoes, radishes, squash, eggplant, carrots…

The proper use of untreated natural wood ash in the garden

Using ash requires taking certain precautions.

  • Use it after it has completely cooled.
  • Use ashes preferably at the end of winter, after a period of heavy rain.
  • Scratch the soil on the surface beforehand.
  • Keep it sheltered so that it does not catch the rain.
  • Respect the recommended dosages by gardening specialists. For instance :

    • No more than 500 g for 50 L of compost in the decomposition phase,
    • 1.5 to 2 kg/m² for the garden,
    • 1 kg/m² maximum at the foot of each fruit tree,
    • 800 g to 1 kg for a 5 m² vegetable patch to scatter between the vegetable beds.

It is recommended not to spread it at the foot of very young plants such as seedlings which have just been transplanted, for example.

It can be noted that calcium oxides and others are very present in the ashes. Consequently, these are very alkaline. They therefore raise the Hydrogen potential (pH) of the soil. Ash should not be used in soil that you want to keep sufficiently acidic, as is the case, for example, with heather soil in which, for example, raspberries, blueberries, hydrangeas, magnolias, Japanese andromeda, azaleas are planted. and other rhododendrons.

Fertilizing the soil with wood ash: info or intox?

It cannot be said that it is strictly speaking a fertilizer. However, it can be used as such and to understand why many gardeners use wood ash as fertilizer, let’s take a look at its composition. There are for example:

  • Silica,
  • You sodium,
  • sulfur,
  • Phosphorus,
  • two potassium
  • Magnesium,
  • Calcium (in the form of lime or calcium oxide),
  • Iron and other trace elements in less quantity.

The calcium is by far the most important component of wood ash since it can represent up to 50%. But beware, we can never repeat it enough: for it to be beneficial to plants, it is essential to use exclusively ash from natural woods that have not been treated.

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