Ash as a fertilizer - possibilities and dangers

Ash as a fertilizer - possibilities and dangers

Is ash suitable as a fertilizer?

Ash is a natural fertilizer that is easy to make. Any hobby gardener can make it with a wood stove or a fireplace. Ash is rich in lime and potassium. It also contains iron and phosphate, which are essential for healthy and vigorous plant growth. Ash fertilizer is mainly used for liming over acidic soils. As ash with a pH value between 11.0 and 13.0 has a very basic effect, it should not be used without restrictions for all plants.

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Wood ash contains:

  • 25 to 45 percent quicklime (calcium oxide)
  • three to six percent magnesium and potassium oxide
  • two to six percent phosphorus pentoxide
  • mineral trace elements (iron, manganese, boron and sodium)

What ashes can I use?

Not all ashes should be used to fertilize the plants. Fireplace ash made of wood or burned charcoal is suitable, but here too you should know exactly where the wood comes from. Both hardwood and softwood are processed into charcoal. This can be contaminated differently depending on the source.



The solid fuel is created when air-dried wood is strongly heated in the absence of oxygen. In various phases of heating, gases escape and ultimately charring. These processes reach temperatures between 150 and 500 degrees Celsius. Good charcoal has a black sheen, with the structure of the wood still clearly visible. In contrast, badly charred coals are red to brownish-red in color. In the production of these so-called red coals, temperatures of 300 degrees Celsius are not exceeded.

This is what you should pay attention to

When additives are burned in the wood, harmful substances can develop and remain in the ashes that poison the soil. It can contain dioxin or other toxic substances. If the wood comes from an industrial region or from near a motorway, it can be contaminated with heavy metals.

How to make perfect wood ash:

  • Burn untreated and natural wood
  • Use wood without lacquering and glazes (59.87 € at Amazon *)
  • use dry branches, leaves or nutshells for lighting

Do not use brightly printed newspaper for lighting, as these contain chemical substances and can contaminate the ashes. Even black and white newspaper, egg boxes or toilet paper rolls should not be used. The material is often made from recycled paper, which can be contaminated with mineral oil components from colorful brochures. Also, be careful not to mix wood ash with ashes from coal.

Unsuitable ashes


Depending on the origin of the wood, the ash can contain heavy metals which are harmful to health and which also poison the soil and plants. Lead, cadmium or chromium can often be detected in critical concentrations. Ash made from lignite, hard coal or coal briquettes is also unsuitable as a natural fertilizer because of its heavy metal load.

Heavy metals are naturally contained in wood, as the tree absorbs substances from the environment as it grows. The toxic metals also come from tool wear during production by logging machines.

To be on the safe side, you should dispose of the residues of burnt charcoal in the household waste, because the origin is often unknown here. In addition, ashes from the grill contain fat residues that have arisen from burning. Decomposition products such as acrylamide are harmful to health and have lost just as little in the soil as cigarette ash.

PelletsWood wastehigh heavy metal pollution
BriquettesBrown or hard coalcontains traces of radioactive elements
tobaccoFoliage leaves of the tobacco planttoxic soot and heavy metals, hardly any nutrients
coalfossil plant remainscontains heavy metals and radioactive substances

Where can I use ash fertilizer?

If you can make sure that your ashes are clean, there are many uses of the natural product in the garden. It improves the lawn and can be spread in beds and under bushes. You also benefit from positive side effects.

This causes an ash fertilization:

  • removes moss and algae growth
  • lets root weeds die off
  • disinfects open wounds

Only use wood ash that is sound. Otherwise, you run the risk of toxic substances being absorbed by crops.

Which plants tolerate ash?

Ash prevents potassium deficiency. In small quantities, you can fertilize various plants with pure ash. The particles are washed into the ground by rain and dissolve within a short time. This means that the substances are quickly available to plants. Bog bed plants and plants that prefer acidic soil should not be provided with ash.

Particularly suitable plants:

  • Vegetable garden : tomatoes, Brussels sprouts, leeks
  • Perennials and shrubs : roses, gladioli, phlox
  • Flower beds : geraniums, fuchsias
  • Fruit trees : grapevines, gooseberries, raspberries
  • Indoor plants : all lime-tolerant plants


Basically, you can also fertilize potatoes with ash. However, caution is advised because the ash fertilization promotes potato scab.

In the forest

If you have your own forest, you will appreciate the high lime content of wood ash. The majority of all German forest soils are too acidic. In a well-dosed concentration, wood ash can contribute to improving the soil in the long term. As a result, the trees can root deeper, which increases their stability.

Advantages of the ash fertilization:

  • Extension of the growing season for deciduous trees
  • reduced needling in conifers
  • increased fine root formation in the topsoil

Ash fertilization problems

If ash is applied in too high concentrations or in unfavorable weather, it can have negative consequences. These affect both soil life and plants. Incorrect fertilization can cause great damage, so ash should be used in low concentrations.

Plant damage

The high content of calcium, which is present as quick lime in its most aggressive form, ensures that the soil is limed. This highly alkaline quicklime can cause leaf burns if residues are left on the plants. In agriculture, calcium oxide is only distributed on uncultivated areas that have a loamy to clay subsoil.

Lucky bag

Another problem is the often unknown composition of the various substances in wood ash. The proportion of minerals, like the heavy metal content, can fluctuate greatly. Without a precise analysis of the ashes, fertilization tailored to the soil is not possible. You run the risk of the earth being enriched with more toxic substances than improved.

Soil damage

If quicklime is distributed on light sandy soils, it can cause massive damage to soil life due to its low buffer capacity. Unsolidified wood ash dissolves particularly quickly if precipitation occurs after it has been applied. This can change the soil chemistry, so that the plants growing there can no longer absorb nutrients for a short time. Their growth stagnates and sensitive plants can die. In order to reduce the solubility and to better control the changes in the pH value in the soil, the ash should be pelleted before application.


Let wood ash swell in water so that the fine-grained particles soak up. Then place the pot on the stove and wait for lumps of ash to form. These dissolve more slowly.

Have ashes tested

If you want to be on the safe side, you can have your ashes analyzed in a laboratory. There are quantitative tests that check the ash for ten to twelve common heavy metals. Ten grams of wood ash are sufficient for a precise analysis.

If you burn different tree species, you can send in the ashes as a mixed sample. However, it is then not possible to assign the ingredients to the respective ash. If you send in several samples, you must expect higher costs. A test costs between 100 and 150 euros, depending on the laboratory.

frequently asked Questions

Is ash suitable for plants that prefer acidic soils?

Ash increases the pH of the soil and should only be used for plants that prefer calcareous soils. Hydrangeas, ferns, rhododendrons or peonies like a more acidic humus soil, which is why ash is not suitable as a fertilizer. You can fertilize these plants with coffee grounds instead.

How do I fertilize with ashes?

Choose a windless day so that the dusty ash does not spread in the garden. To be on the safe side, you can moisten the ashes a little. The high pH can damage the skin so you should wear gloves. After the application, the soil is watered. Ash should never be mixed with ammonium-containing fertilizers such as manure or liquid manure, as gaseous ammonia can be formed. Also, do not mix ash with phosphate. This can lead to the development of calcium phosphates that are difficult to dissolve and are not available to plants.

How much ash should I use for fertilizing?

You can fertilize the soil with ash every three to four years for liming and soil improvement. For heavy soils with a pH value of 4.0, 200 to 400 grams per square meter is sufficient. In less acidic soils, reduce the amount to 100 to 200 grams. Light soils can quickly be limed over and should therefore not be fertilized with ash. Ground eggshells are sufficient here.

Can I put ashes on the compost?

Many microorganisms become more active at a high pH. Adding ash can accelerate the decomposition process on the compost, especially if it is acidic compost. However, you should only sprinkle the ashes on the compost in small quantities to avoid over-lime formation. If you are not sure which source the wood ash comes from, you should dispose of it in the household waste because of possible pollution.