How to Use Wood Ashes in Organic Gardens
Wood Ashes in History
Wood ashes have been used as a garden amendment since before the time of the Romans, for the wood when burned releases gases, but the powdery ashes that remain are still plant material. They are full of micronutrients plants need, including calcium, potassium, phosphorous, and trace elements. Before chemical fertilizers were developed, potash, or "pot ashes" swept from hearths used for cooking, were one of the main sources of fertilizers. The path from pot ashes to good garden soil gives new meaning to the idea of "ashes to ashes, dust to dust."
Before commercial toilet soaps were available, families made their own soap using lye, made from wood ashes soaked in rain water, boiled with rendered animal fat, either tallow, from cattle fat, or lard, from pig fat. Homesteaders and colonists who were among the first Europeans to settle in North America kept a lye barrel going, adding ashes periodically, and would boil soap out doors after rendering the fat, a foul-smelling process where the fat was cleaned and separated from the remains of meat and body tissue that clung to the fat after the animals were butchered for food.
Journals from the 19th century, such as Roughing It in the Bush by Susanna Moodie, an English gentlewoman married to an impecunious and landless younger son of English gentry, document the process. Moodie and her husband immigrated to Canada in the 1830's, drawn by the dream of owning their own land in the New World, quite unprepared for the physical labour and privation required to clear land, build a cabin from trees and sod, plant a garden, make or grow everything they used, and survive or die without medical care or even close neighbours. The book makes a fascinating read, and is full of odd recipes for household staples like bread from bran sourdough, soft soap, and smoked bacon. True to the pioneer spirit that still captures the imagination of many North Americans, back-to-the-landers, and survivalists, some of these recipes are circulating on websites like this one if you want to try making soap from ashes yourself.
pH of Common Products
commercial lye or caustic soda (sodium hydroxide NaOH)
wood ash lye (KOH)
hydrochloric acid (HCl)
Ashes for Gardens
When ashes mix with water they create potassium hydroxide, a form of lye. Lye is a strong base, and it raises the pH of soil chemistry to make soils alkali. Most plants thrive when soil pH ranges between 6 and 8. Some plants, such as blueberries, cranberries, rhododendrons and azaleas, prefer acid soil around 6.4 pH. Other plants prefer more alkali soil. Consequently, adding ashes to your garden may be useful depending on the chemistry of your soil. It is especially useful n rainy climates, where acid rain makes soil acidic, and on properties where there are many pine needles in the soil, or oak leaves, since these are acidic.
Since the ash particles are fine, and dust-like, they mix easily into the soil. They are light, so don't spread them on a windy day, when they easily blow around. Avoid breathing them in.
To deter snails and slugs from your plants, try circling plants with a ring of ashes. When they creep toward the new shoots and leaves, the ashes dry out their skin, and send them in another direction.
Soil pH Affects Plant Health
dianthus, boxwood, aucuba, daphne, lilac
6.5 and higher
pink flowers on hydrangeas
purple flowers on hydrangeas, or mixed pink and blue
5.5 and lower
blue flowers on hydrangeas
Ashes for Plants
Since wood ashes come from plants, they have many of the micro-nutrients plants need. They are a good source of calcium, potassium, phosphorous, magnesium, and trace elements. Their chemical make-up is equivalent to about 0-1-3 (N (nitrogen)-P (phosphorous)-K (potassium). Plants need calcium to develop strong roots, cell walls, and to complete protein formation. Potassium is necessary in photosynthesis, or the process during which plants manufacture food in green leaves in the presence of light. Potassium is also required for moving sugars, forming seeds, synthesizing proteins, and using nitrogen.
Fireplace Ashes for Compost
Adding small amounts of fireplace ashes to the compost helps maintain a neutral pH in the composter, to allow the active microorganisms to function most effectively. Use it sparingly, and sprinkle it on layers: a layer of green (grass clippings), a layer of brown (sawdust, dry leaves, coffee grounds from your local coffee house, or vegetable scraps), and then a light layer of ashes. For best compost, aerate the pile every few days by turning it with a compost turning tool. Keep the pile moist, but not wet. If necessary, add water. Compost that is working effectively gets very hot, has steam rising off it, and feels uncomfortably hot if you put your hand in.
Be sure to keep within the recommended quantities, for too much ash raises the alkalinity of the soil and accumulates salts that will harm your plants.
Ashes as Fertilizer
Mix wood ashes into your organic fertilizer instead of ground limestone. Recommended quantities are about 20 pounds (5 gallons) of ash/1000 square feet of garden/per year, which is equivalent to about 6 pounds of ground limestone in the same area.
Try this organic fertilizer recipe:
4 parts seed meal (alfalfa, soy, cotton, or canola--whatever is available in your area)
3 parts wood ashes or 1 part Dolomite lime
1/2 part kelp meal
1/2 part bone meal