Dr. Leonard Perry, Extension Professor
University of Vermont
Winter in the north means wood ashes, even more in recent years with the higher cost of heating oil and many homes converting to wood stoves or wood pellets. Rather than sending the wood ashes that are left to the landfill, they can be used “judiciously” in landscapes and gardens.
The main benefit of wood ash in the soil is to raise the soil pH, or make it less acid. Soil pH is a measure of acidity on a 14 point scale, with 7 being neutral. Below 7 is acid, and above is alkaline. Most our northeast soils tend to be acidic, often 5.5 to 7. Slightly acidic is ideal for many plants, as this is the range in which most nutrients are available to them. Generally, wood ash is from 25 to 45 percent calcium carbonate, a common liming agent, so you can use twice as many ashes as you would this lime.
So what this means for wood ashes is that if your soil is 6.5 to 7 or above, don’t add them. If you’re not sure of your soil pH, do a soil test in spring when the ground thaws (kits are available from your local Extension office and many garden stores), but in the meantime go lightly on the wood ashes. Adding too much may do more harm than good, particularly since wood ashes change the soil pH much more quickly than most liming products.
Keep in mind a plant or crop’s preference for soil acidity, and so wood ashes. Asparagus, conifers, and juniper tolerate more alkalinity, and so wood ashes. Potatoes, blueberries, and rhododendrons prefer a fairly acidic soil (less than 5.5), so don’t add any wood ashes for these or where they’ll be grown.
When wood burns, it gives off nitrogen and sulfur as gases, but leaves behind other plant nutrients in small amounts. Hardwoods such as maple and oak provide up to 3 times as much ash, up to a third more calcium, and slightly more nutrients, than softwoods such as fir and pine. The main nutrient added is potassium (the third number on fertilizer bag analyses), perhaps up to 10 percent “potash.”
Adding wood ashes to plants is not a new practice, being first documented centuries ago by the Romans. Its benefits became widespread in this country in the 18th century when the method was discovered for making potash– potassium bicarbonate– from wood ashes. In fact, this method of basically soaking ashes in a pot received the very first U.S. patent in 1790.
Although the amount to add will vary with soil and crop, a good rule is 20 pounds (roughly a 5 gallon pail) per 1000 square feet of garden. This is the amount you may get from one cord of firewood. You also may see recommended ashes “topdressed” or spread evenly up to one half inch thick. Tomatoes in particular like potassium, and so wood ashes. For lawns, go a bit lighter—10 to 15 pounds per 1000 square feet. For individual plants, spread one-half to one pound of ashes evenly around a mature shrub or rose bush. For trees such as apples, you can spread around them up to an inch thick.
Clay soils usually tolerate more wood ashes than sandy soils. Don’t leave the ashes in piles or clumps, as concentrated nutrient salts can leach from these and damage roots. It’s best to spread them in winter or early spring, a month or so before planting or adding other fertilizer. This gives the ash time to fully work, and not interact adversely with some nitrogen fertilizers.
Since wood ash is about a 0-1-3 analysis (nitrogen-phosphorus-potassium), and many soils contain sufficient phosphorus already, perhaps the only additional nutrient you may need to add is a source of nitrogen. But a professional soil test will recommend any additional fertilizer.
Wood ashes also can be added to compost piles to keep the acidity more neutral. Sprinkle some on each layer of compost as you build the pile. If stockpiling wood ashes to use later, keep them dry. Rain will leach the nutrients, and so benefit, from them. If storing, use a metal can so any hot coals in the fresh ash wont cause a fire.
Another option is to store wood ashes dry, then make a “tea” with them during the growing season for watering plants and so providing some nutrition. Put about 3 pounds in a permeable bag or wrapped in burlap into a 30-gallon garbage can of water, leaving it there for at least 4 or 5 days.
When dry, wood ashes can be spread around perennials such as hostas to physically deter slugs and snails (they don’t like to cross dry ashes). But when they get wet they lose this effectiveness, so you would need to reapply. If done too often, this could end up adding too many ashes and so too high levels of salts in the soil.
It probably seems obvious, but bears mention, that ashes from burning trash or treated wood shouldn’t be used on gardens. They may contain chemicals, or elements in high amounts, toxic to plants. Since wood pellets are made from sawdust or wood particles held together with natural lignins from the wood when heated in the molding process, they’re fine to use.
So this winter as you snuggle by a wood stove or fireplace, or see a neighbor burning wood but who doesn’t garden, consider recycling the ashes from this natural product back to the soil. Just make sure to not use too much, and avoid spreading around acid-loving plants. If your soil is already fertile or of the correct pH, wood ashes can be used to hide stains on paving, melt ice on walks, make soap, shine silver, or even neutralize skunk odor on pets.
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