Frase's data indicate that the manner in which wood ashes are used in gardens should be based on the kinds of ashes available and on the pH of the soil.
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The Seasons of the Garden column shares seasonal gardening information and tips with MOTHER EARTH NEWS readers, including wood ashes for plant nutrition, deterring Christmas tree thieves from the garden and an organic deer repellent.
Wood Ashes in the Garden
It has long been thought that wood ashes are a good source
of potassium for plant nutrition, but recent research has
shown that you should use a great deal of caution
in adding them to your garden.
Charles Frase, of Tennessee Technological University,
analyzed ashes from seven tree species for potassium,
phosphorus, calcium, magnesium, and sodium; he also
estimated the extent to which the ashes could raise soil
pH. His results show that ashes of various tree species can
differ widely in their mineral content.
For example, shagbark hickory (Carya ovata) and
white oak (Quercus alba) ashes have much less
potassium (K) and phosphorus (P) than does hemlock (Tsuga canadensis ), while ashes of white
ash (Fraxinus americana) and silver maple
(Acer saccharinum) have intermediate levels of K
and P. On the other hand, the shagbark hickory and white
oak ashes have the same liming potential—pound for
pound—as high-quality ground limestone!
Frase's data indicate that the manner in which wood ashes
are used in gardens should be based on the kinds of
ashes available and on the pH of the soil. Although
detailed information about the mineral composition of most
tree species is not available, some tentative
generalizations can be made.
It appears that slow-growing, dense hardwoods yield ashes
that are best used as liming materials. Based on Frase's
research, such ashes can be expected to contain about 2% K,
0.3% P, and 30% calcium (Ca). Their soil-neutralizing value
relative to pure calcite (NV/CaC03 ) should approximate
Fast-growing, low-density hardwoods yield ashes that are
typically balanced in their levels of calcium relative to
their levels of potassium and phosphorus: They contain
about 4% K, 1% P, and 20-25% Ca, and have a liming
equivalent of 85% NV/CaC03. These ashes can thus be used
as liming or fertilizing materials for soils with
low or near-neutral pH.
Softwoods are likely to yield ashes with low levels of
calcium and high levels of potassium and phosphorus: about
5-10% K, 1-2% P, 17% Ca, and 60-70% NV/CaC03. These ashes
are best used as fertilizing materials only and present
little risk of raising soil pH excessively.
Two other considerations: Gardeners in arid regions should
watch out for the sodium level of wood ashes, to avoid
contributing to soil salinity. Sodium content ranges from
about 0.3% in slow-growing hardwood ashes to about 1.5% in
softwood ashes. In addition, research at the University of
Rhode Island has shown that large applications of wood
ashes can result in high concentrations of the toxic heavy
metal cadmium in certain garden crops. Wood ashes contain
lead, copper, and zinc as well as cadmium, but
none of these heavy metals appear to be taken up
by plants if the soil pH is between 6.0 and 7.0.
Thus, the most important consideration in applying wood
ashes to garden soil is to avoid putting on too
much. (The Rhode Island researchers recommend no more
than 20 pounds of wood ashes per 1,000 square feet of
garden area annually.) Large applications can lead to soil
alkalinity and plant accumulation of heavy metals. The key
to safe use is annual monitoring of soil pH-keep it between
6.0 and 7.0!
Seasonal Gardening Research Briefs
Thwart Christmas tree thieves with "ugly mix." Sad but
true, each holiday season brings out the Christmas tree
"rustlers"—and any landscaped area that includes
evergreens is at risk. In fact, Cornell Plantations in
Ithaca, New York, had trees worth up to $3,000 each cut
down and carted away by midnight entrepreneurs . . . until
an employee invented a way to make evergreens so ugly
(temporarily, of course) that nobody would want to
steal them! To make your own batch of "ugly mix," stir
together 20 ounces of hydrated lime (from a building supply
store) and 4 ounces of Wilt-Pruf anti-transpirant
(available at garden stores) in a nonmetallic bowl. Slowly
fold and stir that into two gallons of room-temperature
water in an aluminum pan. Mix in two bottles of
pink food coloring . . . and spray the dye onto
your trees with a garden sprayer. Do this during dry
weather; rain gradually washes the coating away.
Fumes from fresh-cut wood can harm houseplants! Growers of
houseplants belonging to the family Gesneriaceae, including
the "true" gloxinia (Sinningia species),
Gloxinia perennis, and Drymonia species,
report yellowed leaves and defoliation within a week of
stacking freshly cut oak firewood in the same room with the
plants. To be safe, watch for symptoms on any
plants close to your firewood.
Deer problems? Use soap! If you live in an area with
abundant deer (as we do!), your orchard is probably a
favorite winter browsing spot. A good fence is a sure,
expensive cure for the problem, but we've found a
cheaper organic deer repellent remedy. Hang pieces of deodorant soap (plain soap
doesn't work—the deer actually eat some kinds!) in
the trees. We use one piece for small trees, and four or
more pieces for large ones. The result? Over the past two
years, we've had no deer browsing on
Green up stored cabbages. A Canadian agricultural
researcher exposed stored cabbage to light for a week at
50 degrees Fahrenheit. Afterward, the heads looked greener, and vitamin
C in the outer leaves was increased nearly tenfold!
EDITOR'S NOTE: Greg and Pat Williams raise most of their own food on a small farm and publish HortIdeas, a fine newsletter on gardening research and products (available for $10 a year from G. & P. Williams, Gravel Switch, KY).