When wood ash is mixed with other components in a compost heap, the resulting alkaline compost can be used as a mulch around most ornamental plants and vegetables unless, like raspberries and rhododendrons, they require an acidic soil. Fruit too performs best in slightly acidic soil, so wood ash is unsuited for use in the fruit garden.

Vegetables grow best in soil with a

This describes how acid or alkaline soil is, and is expressed as a number between 0 and 14. Soil with a pH of 7 is neutral, an acid soil has a lower value, and an alkaline soil has a higher one. Most plants grow best in soil that’s within a specific pH range, so it’s useful to know your soil’s pH. Testing kits are easy to use and widely available in garden centres and online.

Can refer to either home-made garden compost or seed/potting compost: • Garden compost is a soil improver made from decomposed plant waste, usually in a compost bin or heap. It is added to soil to improve its fertility, structure and water-holding capacity. Seed or potting composts are used for growing seedlings or plants in containers - a wide range of commercially produced peat-free composts are available, made from a mix of various ingredients, such as loam, composted bark, coir and sand, although you can mix your own.

compost

Wood ash may contain useful levels of potassium (about three per cent) – 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 a 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 that could damage plants and soil.

If applying wood ash directly to bare soil, do this in winter and rake or dig it in. This will allow the compounds in the ash that could scorch plants to react with the moist soil and be rendered harmless before spring sowing or planting.

It may be useful to sieve the ash before use, to remove debris. Avoid breathing in the dust by using a face mask and limit skin exposure by wearing gloves, boots, trousers and long sleeves.

Apply wood ash in small amounts to the compost heap – once mixed in, it will blend readily with other materials. As a general guide, you shouldn’t be able to identify it after mixing it into the compost.

Wood ash can be spread directly on bare soil in vegetable plots in late winter, at a rate of 50-70g per sq m (1.7-2.4oz per sq yd). Fork, rake or rotovate it into the soil. It can be especially useful where brassica club root is a problem.

Composting

Composting

How to test soil for texture and pH

How to test soil for texture and pH

Wood ash is a useful by-product of bonfires, but there are a few things to beware of:

• Avoid using too much wood ash, as it could make the soil too alkaline

• Avoid using ash from treated timber as it may contain potentially harmful residues – instead, dispose of it in the council refuse collection

• Avoid applying to soil where potatoes will be grown the following spring, as alkaline conditions can encourage potato scab

Solid fuel ash

Ash from coal or anthracite is best disposed of through the council rubbish collection, as it has little or no nutritional benefits and is potentially harmful to soil and plants, and might contaminate edible crops.

Ash from barbecues

Ash from lumpwood charcoal can be used as recommended for wood ash. But if other fuels have been used, including briquettes, the ash shouldn’t be used.