Again and again one hears that wood ash can be used as fertilizer in the garden. We show you what you should consider when fertilizing with ash. Wood ash contains many nutrients but is unfortunately far too often not used for fertilization. This is a pity, after all the recycling of nutrients is an important part of sustainable agriculture and gardening culture. To enable you to use them, we have collected all the necessary information on the use of wood ash as fertilizer.
Many owners of stoves and fireplaces are uncertain whether ashes can be used as fertilizer at all. We answer this question and then explain the effect, the correct application, and explain which plants can benefit from fertilizing with ash.
Can Ashes Be used As Fertilizer?
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Not every type of ash can be used in the house or garden without hesitation. Plant ashes are basically suitable for use as fertilizer – meaning ashes from burnt wood, straw, or other plant material. Even normally printed, the non-glossy paper is suitable, so that ashes from your own fireplace can certainly be used – provided that only untreated wood has been burned.
Ashes from briquettes, coal, treated wood, and glossy printed paper must not be used. Because these materials contain substances that should not get into your garden soil.
How Do Ashes Work As Fertilizer?
Ash has a pH value of 10 to 13, it contains a lot of basic calcium and magnesium.
|Content in untreated wood ash
|Calcium Oxide (CaO)
|26 – 40 %
|Phosphorus oxide (P2O5)
|4 – 7 %
|Potassium oxide (K2O)
|7 – 12 %
|Magnesium Oxide (MgO)
|3 – 5 %
|Other trace elements
|In variable proportions
The nutrients contained in plant ash are readily available: If they were surrounded by an organic packaging of carbon before combustion, they oxidized during combustion and escaped as CO2. The calcium oxide contained is also known as quicklime.
Through combustion, this form of calcium is also easily soluble and acts quickly. It is therefore also suitable for liming heavy soils rich in clay. Burnt lime reacts with water in the soil, whereby the pH value of the soil solution increases and calcium is released. It is now ready for absorption by plant roots.
By the way, if the humus content is sufficiently high, calcium acts as a kind of electrical bridge between humus molecules and clay minerals, thus enabling the formation of stable soil crumbs. And these are a real blessing for many soil properties, one speaks of the so-called lime fermentation. It should be noted that ashes do not introduce organic material or nitrogen. It can be considered a nitrogen-free mineral fertilizer with a very high pH value.
How Does Ash Act As A Fertilizer?
- Ash has strong alkalinity and can increase the soil pH value very quickly.
- The plant nutrients contained are readily available – but no nitrogen is present.
- Light soils are slightly over-calcified with ashes, so the pH value is raised too quickly and too strongly. On heavy, clayey soils the application of ash is possible.
- The released calcium promotes the formation of stable soil crumbs (lime fermentation).
Which Plants Can Be Fertilize With Ash?
The alkaline effect of ashes should not be underestimated: Precisely because quicklime is so easily soluble, there is a not inconsiderable risk of overtime. A massive increase in the pH value due to an excessively generous distribution of ash can upset the soil chemistry to such an extent that the plants growing there can no longer absorb nutrients, are inhibited in their growth, or even die, at least in the short term.
This is especially true for plants that are actually used to acidic soils: Rhododendrons (Rhododendron), blueberries (Vaccinium myrtillus), false berries (Gaultheria), ferns, and many forest perennials, for example. Lime-loving plants and those with high calcium requirements tolerate ashes better and can even benefit from it. In the following table, we have compiled a selection of calciferous plants. By the way, you will find another selection of calciferous plants in our special article on fertilization with eggshells.
|Garden Silver Root (Dryas x suendermannii)
|Various maple species (Acer campestre, Acer negundo, Acer platanoides, Acer pseudoplatanus and others)
|Garden-ball thistle (Echinops ritro subsp. ritro)
|Hornbeam (Carpinus betulus)
|Rolling Wolf Milk and Colorful Wolf Milk (Euphorbia myrsinites / Euphorbia polychroma)
|Deutzia in its species and varieties
|Wild strawberry (Fragaria vesca var. Vesca)
|Ivy (Hedera in its species and varieties)
|Forest Master (Gallium odoratum)
|Mother-of-pearl shrub (Kolkwitzia amabylis)
|Blood-red stork-beak (Geranium sanguineum)
|Philadelphus / False Jasmine (Philadelphus in its species and varieties)
|Rispiges Schleierkraut (Gypsophila paniculata)
|Woolly, Ordinary, Japanese and Tongue Snowball (Viburnum lantana, Viburnum opulus, Viburnum plicatum, Viburnum rhytidophyllum)
Application Of Ashes As Fertilizer
Due to the strongly alkaline effect of ashes, a pH test of the soil should always precede any use. If your garden soil is only weakly acidic, neutral, or even slightly alkaline, ashes should be used with caution or not at all. If pure ash gets onto plant parts, it will burn the plant tissue. Because plant ashes do not contain any nitrogen or organic material, supplementary fertilization should be used to supply the plant with nutrients.
Basically The Following Applies:
- Spread ashes when there is no wind: This prevents lime-sensitive plants from being damaged in other parts of the garden
- Wear gloves when spreading ashes, as the high pH-value affects your skin
- Moistening the ash reduces dust formation
- Water the soil after application
- Never apply ash in combination with ammonium-containing fertilizers such as liquid manure, dung, or mineral nitrogen fertilizers – this would produce gaseous ammonia
- Never apply ashes in combination with mineral phosphates or phosphate fertilizers – this results in calcium phosphates that are difficult to dissolve and no longer available to plants
- Keep in mind that most garden soils have no need for lime or calcium because calcium-containing minerals are found in almost all locations. The use of ash or lime is usually only necessary for plantings with plants that explicitly love lime or for peat-based potting soils. After a few years, peat soils brought into tubs and beds are often severely depleted of calcium, since peat naturally contains almost no calcium.
How Much Ash Can Be Used As Fertilizer?
On heavy, acidic soils (pH value 4), 200 to 400 grams of plant ash per square meter can be spread every three to four years. Less acidic soils can be treated with 100 to 200 grams of plant ash every three to four years. On light soils, you should not spread ashes, as over-calcification can quickly occur. It is better to use ground eggshells as fertilizer, the effects of which are described in more detail in this special article.
Summary: Use of ash as fertilizer
- The use of ashes is only recommended for lime-loving plants, peat soils, or soils that are demonstrably too acidic or low in calcium. The latter occurs only very rarely
- Ashes should not be allowed to reach plant tissue in their pure state, as this can lead to burns
- As ashes do not contain nitrogen, additional fertilization with a slow-release organic fertilizer is necessary
- Ashcan also be spread on acidic rotting material on the compost to balance its pH value