As the temperatures drop, many Wisconsinites turn to wood burning furnaces to heat their homes. Is there a good use for the resulting wood ash? Can it be safely used in a garden application? In addition, many people have a “burn pile” used to burn branches, garden and yard waste, storm debris, and perhaps a few other odd items. What about that ash? Can it be used for anything?
Adding wood ash to gardens is not a new practice; it has been documented since Roman times. It is recommended as a soil amendment in cases where the soil has an imbalance in soil pH. Soil pH is measured on a scale of 1 to 14 with 7 being neutral, below 7 being acidic, and above 7 being alkaline. Because wood ash has fine particle sizes, it reacts almost immediately with the soil. The largest component is calcium carbonate in amounts up to 25%. This is a common liming agent and can significantly increase the pH of the soil.
Depending on the type of wood used, ash can contain up to 10% potassium and 1% phosphorous. These are two of the required macro-nutrients for plant growth. Nitrogen is the other macro-nutrient; however, there is no nitrogen present in ash. There will also be trace amounts of some micro-nutrients such as iron, manganese, copper, boron, and zinc.
In southeast Wisconsin, most of the soils tend toward neutral to alkaline, ranging from 6.8-7.6, but, for optimum growth, most plants prefer a pH of 6.0 to 6.8. Adding wood ash will quickly and significantly increase the pH to a level that prevents the plants from taking up necessary nutrients. There are some plants such as asparagus and juniper that can tolerate a more alkaline soil. However, wood ash should never be used on acid loving plants like blueberries and potatoes.
In addition, southeastern Wisconsin soils seldom need an application of potassium. The best idea is to take a soil test before adding any wood ash to a garden or lawn. Instructions and forms can be found at: https://uwlab.soils.wisc.edu/soil-samples/ The results will give specific details about amendments needed for the soil to achieve the best growing conditions. If the soil pH is below 6.5 adding wood ash would be highly unadvisable. If it is above 6.5 a small amount could be mixed into the surface of the soil.
The University of Alberta researched adding wood ash to a compost pile. They concluded that adding small amounts (less than 16% by volume) of wood ash to a hot compost pile and allowing it to run through the complete composting process will result in compost at or near neutral pH. The initial pH will be high, so do not use the material until it is completely composted.
As for that burn pile where miscellaneous items were consumed. Do not use the ash from it for any garden application. It could potentially contain hazardous material that would be detrimental to the landscape. Use only pure wood ash for any garden application.
If the garden and lawn is already fertile and is the correct pH, try instead using the ash for polishing silver, making soap, melting ice on sidewalks, or neutralizing skunk odors on pets.
Certified Master Gardener