The Seasons of the Garden
A word of caution regarding wood ashes in the garden, also includes research briefs and gleanings.
by Greg and Pat Williams
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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/CaC0 3 ) should approximate
100%.
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/CaC0 3 . 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
510% K, 1-2% P, 17% Ca, and 60-70% NV/CaC0 3 . These ashes
are best used as fertilizing materials only and present
little risk of raising soil pH excessively.
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