Using wood ash

If you tend to accumulate wood ash from log fires over the colder months, you might be tempted to use it instead of lime to raise soil pH, as it contains between 45–50% calcium carbonate. It will certainly do that – but it should only be used on soils with a pH lower than 5.5 because the calcium in wood ash is in a highly soluble form that can change pH very quickly, and it is very easy to over-do the application. Ash from hardwoods contain one third more nutrients than ash from softwoods (e.g. pine).
Apply wood ash only to beds that are not going to be used for a while, using one handful per square metre. Test soil pH after 2 weeks to see if soil is in a more suitable pH range for plant growth. Wood ash should never be used near plants that prefer acid soils.
A safer way to use wood ash is to keep it in a covered container near your compost heap and dust it, instead of garden lime, between layers of other materials that you add to the compost heap. This will help to keep the compost heap sweet-smelling and ensure good microorganism activity. Mature compost holds nutrients in balance and compost containing wood ash can safely be applied to garden beds.
Wood ash can also be dusted over lawns that have become ‘sour’, especially where moss is growing. While the calcium improves the soil pH, the 2–8% potassium (potash) in wood ash will improve the lawn’s tolerance to heat and cold as well as improving disease resistant. However, if applying wood ash to lawns, avoid using seaweed fertiliser for the current season as seaweed also contains a considerable amount of potassium. Too much potassium can cause a magnesium deficiency.