Ash from wood fires, such as bonfires or wood burning stoves, can be a useful additive to the compost heap or can be applied directly to fallow ground and dug in. It can be a natural source of potassium and trace elements. It also has a liming effect, so wood ash can remedy excessively acidic soils.
When mixed with other components in the compost heap, the resulting alkaline compost can be used as a mulch around most ornamental plants and vegetables unless, like raspberries, rhododendrons and roses, they require an acidic soil. Fruit too performs best in slightly acid soil so wood ashes are unsuited for use in the fruit garden.
Vegetables grow best in soil with a pH of 6.5, so testing the level before adding the compost is recommended so as not to raise the pH too much (greater than pH7.0). However, where club root is present, wood ash can be used to raise the pH to as much as 7.5 to inhibit this disease.
Wood ash may contain useful levels of potassium (about three percent), a major plant nutrient associated with flowering and fruiting. However, the levels will vary depending on the age of the wood that was burnt; young wood such as from pruning will have higher potassium content than older, thicker branches.
When to use wood ash
Only add wood ash to the compost heap occasionally (every 15cm/6in of material), as heavier use risks high levels of alkalinity and soluble salts which could damage plants and soil.
If applying wood ash directly to soils, do this in winter and rake or dig it in. This will allow the compounds in the ash which could scorch plants to react with the moist soil and be rendered harmless before spring sowing or planting.
How to use wood ash
Apply wood ash in small amounts to the compost heap where, once mixed in, it will blend readily with other materials. As a general guide, you should not be able to identify it after mixing it into the compost.
Wood ash can be spread directly on soil in the vegetable garden in late winter at a rate of 50-70g per sq m (1.7-2.4oz per sq yd);
- Fork in, rake or rotovate
- It may be useful to sieve the ash before use to remove debris. Avoid breathing in the dust by using a face protection and limit skin exposure by wearing gloves, boots and work clothes
- Where wood ash is applied frequently, it is worthwhile to use a pH test kit to monitor changes in pH and prevent levels rising over pH7.5
- Wood ash may be especially useful in vegetable gardens where club root of brassicas is a problem
- Never leave wood ash in the rain, as the potassium (a useful plant nutrient for flowers and fruit) is in a soluble form and is easily leached out
- Ash produced from young sappy prunings contains a useful proportion of potassium and traces of other nutrients, while older wood tends to contain lower concentrations of nutrients
Wood ash is a useful by-product of bonfires, but there are a few things to avoid;
- Avoid using too much wood ash because an excess in alkalinity
- Avoid using ash from treated timber as they may contain potentially harmful residues consign such ash to the council refuse collection
- Avoid using wood ash on areas where potatoes are to be grown the following spring, as the alkaline conditions can encourage potato scab
Solid fuel ash
- Ash from coal or anthracite is best disposed of through the council rubbish collection since it has little or no nutritional benefit and is potentially harmful to soil, plants and consumers of edible produce
Ash from lumpwood charcoal can be used as recommended for wood ahses. Ash where other fuels have been used including briquettes is best discarded.