Wood ash can pile up during a cold winter, and it would be nice to have a practical use for it. It has often been used as a soil amendment in gardens. In recent years, gardeners have had mixed feelings about the safety and value of using wood ash on their garden soil.
Is it safe to use wood ash in the garden? As with most gardening questions, the answer is "it depends." You have to know a little about both your soil and the ashes themselves. Read on to find out if wood ash is good for your needs.
Benefits for Soil
Ashes from fireplaces and wood-burning stoves can be a good source of calcium and potassium. To a lesser degree, they also provide some phosphorus, a bit of aluminum, magnesium, and sodium, and a few micro-nutrients, such as boron, copper, molybdenum, sulfur, and zinc. The nutrient level in wood ash is not particularly high, and it depends on the type of wood burned. In general, if your soil has a potassium deficiency, wood ash might be a good amendment.
Benefits for Soil pH
Calcium makes up about 20 percent or more of wood ash. That's a fairly large amount, and it is the primary ingredient in garden lime. If your soil is very acidic (pH 5.5 or lower), amending with wood ash can raise your soil pH level. Approximately four cups of wood ash can be substituted for one pound of aglime or agricultural lime.
However, if your soil is neutral or alkaline, adding wood ash could raise the pH too high, interfering with the plant's ability to take in nutrients. Wood ash should also be avoided around acid-loving plants like Rhododendron, azaleas, blueberries, red maples, and birch trees.
How to Use Wood Ash
You never want to overdo it with wood ash. Before you apply it, test your soil's pH. You can get an inexpensive pH meter online or at your local gardening or home improvement center.
If you are looking to make the soil more alkaline, at a maximum, use no more than 15 to 20 pounds (approximately a five-gallon pail) per 1,000 square feet per year.
Sift wood ash to remove large charcoal pieces and any active embers. Spread wood ash evenly over the vegetable garden bed, established perennial flowerbed, lawn, or landscape area during the winter. Wood ash particles are fine and can easily be blown by the wind; avoid applying when windy. It's best to apply wood ash to moist soil, and if possible, work the ash into the soil using a rototiller, spade, or rake in early spring.
Wood ash can potentially cause skin, eye, or respiratory irritation due to its high alkalinity. It is caustic. Wear protection when working with it, such as long pants, long shirt, gloves, eye goggles, and a fine particle mask.
The safest way to store wood ash is in a metal bin or container with a metal lid. Keep the container tightly shut; it will not be combustible if it does not have a source of air or oxygen. Store the metal bin on concrete, stone, or brick surface, and never on wood. And, keep it outside. Take these extra precautions, because tiny embers can sometimes hide in wood ash piles and reignite when exposed to air or a flammable substance.
If you are looking for what you can do with wood ash and do not want to apply it to your plants, it has other practical uses. It can be used for making soaps or as a household cleaning agent, and it's a good amendment for controlling a compost pile's acidity.
Negative Effects in the Garden
Unfortunately, wood ash can also be a source of heavy metals such as cadmium, chromium, or lead, which you don't necessarily want in your garden. However, most studies have not shown that if the soil pH is above 6.0, the plants do not take in the heavy metals in measurable amounts. And since wood ash raises the soil pH, heavy metals should not be a problem. If you have a regular source of wood ash and are worried about the extended use in your garden, you should consider having it tested in a lab.
However, adding too much wood ash or adding wood ash to a soil that is already very alkaline can adversely affect your plants. Too much alkalinity prevents plants from absorbing nutrients, and a sign of that is chlorosis, or yellowing plant leaves.
Plants That Like Wood Ash
Many plants thrive with a sprinkling of wood ash, primarily plants like some extra nutrients or plants that stay in the ground longer than most. Potassium in wood ash often boosts flowering and fruiting.
- Garlic, onions, scallions, chives, leeks: Plants in the allium family can benefit from a top dressing of wood ash by deterring soil-borne pests and onion worms.
- Stone fruit trees: Fruit trees like cherries and plums grow well in soil that is moderately alkaline. Avoid using wood ash around other fruit trees that like acidity, like apples, pears, and peaches.
- Most root vegetables: Carrots, parsnips, turnips, and radish appreciate the potash in wood ash, because it helps the plants use water and resist drought. Avoid using it on any potatoes because they like acidic soil, and wood ash can cause potato scab.
- Peas, beans, asparagus, lettuce, and leafy greens: These vegetables grow best with nutrient-rich soil and like extra potassium.
- Some flowers: Lavender and Hydrangea are affected by alkalinity in different ways. Lavender grows best in soil that is more alkaline than most and will benefit from a sprinkling of wood ash each season. Hydrangea grow in more acidic soil, however, gardeners use wood ash to raise the soil pH to change the coloration of the flowers for aesthetic reasons.
As we look toward sustainability and reusing what we can, It's always best to ask before throwing something out, "Can I do something with this?" In the case of wood ash, it has several alternate uses.
- Pest control: Slugs and snails are repelled by wood ash, and if they come in contact with wood ash, it is a salt that kills them by desiccation. Wood ash can also be used to smother aphids. Dust a fine layer onto infested plants, coating the aphids. You can hose the ash off the plants once it has done its job.
- Compost tea: Wood ash added to compost turns the compost into "compost tea," a liquid version of compost or plant food for plants that need enrichment.
- Ice melt and traction: Another use for your wood ash is traction on ice. It has potassium chloride, a salt that also melts ice. Although it is not as effective as sodium chloride, it can do the trick if you're in a bind. A drawback is that the ash is sooty, so it often leaves a mess behind.
- Making soap: Wood ash from hardwoods like ash, hickory, or beech was once often used to make soaps. When mixed with water, wood ash from those hardwoods becomes lye, a key ingredient in soap.
- Glass and metal cleaner: Mix wood ash with a little water to form a paste. It becomes a mild abrasive to buff tarnished metals, clean dirty glass, and remove adhesives and sticky residue. Use a cotton cloth to apply the paste, and wear gloves to protect your skin. Test a small spot before you start.
Using Wood Ash in the Home Garden. University of Wisconsin-Madison Extension.