Master Gardener: Fertilizing with wood ashes

Jerry Vitalis
Jerry Vitalis

One of the indicators that the economy is struggling is the renewed interest in indoor and outdoor furnaces.  Along with this interest come questions on using wood ashes as fertilizer.  I asked Sue Humble, our Master Gardener coordinator, to help me.  Sue sent me information from Mother Earth News, the University of Illinois and a master garden program from Maryland.  Much of the information from those resources is used in this article.

Since the time of the Roman Empire, wood ashes have been recognized as a useful addition to the soil.  In the 18th century, North America exported wood ash to Britain as a fertilizer and even today eighty percent of the ash produced commercially in the northeastern states is applied to the land.

Since wood ash is derived from plant material, it contains most of the thirteen essential nutrients the soil must have for good plant growth and health.  Charles Frase, of Tennessee Technological University, analyzed ashes from seven tree species for potassium, phosphorus, calcium, magnesium, and sodium.  He estimated the extent to which the ashes could raise soil pH and found that the ashes of various tree species can differ widely in their mineral content.  Frase found that slow-growing hardwood ashes contain high levels of calcium and fast-growing hardwood have balanced levels of calcium, potassium and phosphorus.  These ashes are good to sweeten the soil thus raising the pH.  Softwoods are likely to yield ashes with low levels of calcium and high levels of potassium and phosphorus.  These ashes present little risk of raising soil pH excessively.

When wood burns the nitrogen and sulfur are lost as gases and calcium, potassium, magnesium, and trace element compounds remain.  The remaining carbonates and oxides will raise the pH and benefit soils that are arid and low in potassium.  Calcium and potassium are both essential to plant growth.  Calcium is needed for root development, strong cell walls and protein formation in the plant.  Potassium is an important catalyst in photosynthesis and is essential for the movement of sugars, seed formation, protein syntheses and the use of nitrogen in plants.

Wood ashes should not be put on acid-loving plants such as blueberries, cranberries, rhododendrons and azaleas.  They should not be applied to areas where potatoes will be planted as ash can promote potato scab.  For most garden soil, twenty pounds or about a five gallon pail per one thousand square feet can be applied safely each year.  That equals about six pounds of ground limestone applied to the same area.

The best time to apply wood ashes is in the spring when the soil is dry and before tilling.  Wood ash can be used to repel insects, slugs and snails because it draws water out of these invertebrates.  Sprinkle ash around the base of your plants to discourage surface-feeding insects.  Once ash gets wet it loses its deterring properties.  Too much ash can increase pH or accumulate high levels of salts that can be harmful to some plants, so use ashes carefully.

— Jerry Vitalis is a Chisago County Master Gardener.

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