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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.
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.
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);
Wood ash is a useful by-product of bonfires, but there are a few things to avoid;
Ash from lumpwood charcoal can be used as recommended for wood ashes. Ash where other fuels have been used including briquettes is best discarded.
Brassica club rootCompostingFertilisersLime and limingMulches and mulchingNutrient deficienciesPoultry manureSeaweed productsSoil: understanding pH and testing soilWoody waste: shredding and compostingWoody waste: using as a mulch
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