University of Vermont Extension
Department of Plant and Soil Science
WOOD ASHES IN
Dr. Leonard Perry, Extension
University of Vermont
Winter in the north means wood ashes, even
more in recent years with the higher cost of heating oil and many
to wood stoves or wood pellets. Rather
than sending the wood ashes that are left to the landfill, they can
“judiciously” in landscapes and gardens.
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.
this means for wood ashes is that if your soil is 6.5 to 7 or above,
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
quickly than most liming products.
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
wood burns, it gives off nitrogen and sulfur as gases, but leaves
nutrients in small amounts. Hardwoods such as maple and oak provide
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
analyses), perhaps up to 10 percent “potash.”
wood ashes to plants is not a new practice, being first documented
ago by the Romans. Its benefits became
widespread in this country in the 18th century when the method was
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)
square feet of garden. This is the
amount you may get from one cord of firewood. You also may see
ashes “topdressed” or spread evenly up to one half inch thick.
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.
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
other fertilizer. This gives the ash
time to fully work, and not interact adversely with some nitrogen
wood ash is about a 0-1-3 analysis (nitrogen-phosphorus-potassium),
soils contain sufficient phosphorus already, perhaps the only
nutrient you may need to add is a source of nitrogen. But a
professional soil test will recommend
any additional fertilizer.
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
can so any hot coals in the fresh ash wont cause a fire.
option is to store wood ashes dry, then make a “tea” with them
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
at least 4 or 5 days.
dry, wood ashes can be spread around perennials such as hostas to
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
high levels of salts in the soil.
probably seems obvious, but bears mention, that ashes from burning
treated wood shouldn’t be used on gardens. They may contain
elements in high amounts, toxic to plants. Since wood pellets are
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
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
hide stains on paving, melt ice on walks, make soap, shine silver,
neutralize skunk odor on pets.