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Anyone who heats with wood and/or coal in the winter months can not only look forward to cozy warmth, but also has to dispose of a lot of combustion residues. Even our grandparents used ash directly as fertilizer and put the fine powder on the compost. But is this still relevant today and does ash actually have a fertilizing benefit?

A little ash on the compost has a positive effect on it

Wood ash in natural fertilizer is not uncritical

According to expert opinion, the disposal of large amounts of wood ash in the compost is not entirely unproblematic. This is due to the composition of the gray powder.

Wood ash consists of:

  • 25 to 45 percent burnt lime (calcium),
  • 3 to 6 percent magnesium and potassium oxide,
  • 2 to 3 percent phosphorus pentoxide,
  • as well as traces of iron, manganese, sodium and boron.
  • Depending on the origin of the fuel, there may also be heavy metals such as cadmium, lead and chromium. Sometimes even in critical quantities.

Ash should therefore only be used very sparingly as fertilizer in the garden. If you compost all the ashes in your household, the valuable fertilizer will inevitably be enriched with the substances mentioned above.

What are the implications?

Due to the high pH value, this fertilizer would no longer be optimal for the garden. Spreading the compost is then like liming the soil. In agriculture, such fertilizers are only used to improve unvegetated, very loamy soil.

In addition, without analysis, you know neither the exact proportions of trace elements nor how high the heavy metal content is. This can lead to you unintentionally enriching the garden soil with toxic substances.

Less is more

As so often, the same applies in this case: A little ash in the compost can improve the quality. Too much, however, must be avoided at all costs. The following applies:

  • Only add ashes from untreated wood to the compost. Varnish, glue or the plastic coatings of glossy magazines can contain dangerous, toxic substances.
  • Only use firewood whose origin you know. If the tree was on a busy road or in an industrial area, toxic heavy metals may have accumulated in the bark and wood.
  • Natural fertilizer enriched with wood ash is ideal for loamy or clay soils. Here it can be possible to get the high pH values under control with this fertilizer.
  • Sprinkle only a fine layer of the gray powder in the compost and cover it with a thick layer of green material.


In any case, you should dispose of ashes from the charcoal grill with the household waste. This ash contains fatty residues, such as the discredited acrylamide. These substances have no place in the garden soil.

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