University of Vermont Extension
Department of Plant and Soil Science
Anytime News
Article
PEAT
MOSS OR
COMPOST?
Dr. Leonard Perry, Extension
Professor
University of Vermont
Both peat moss and compost are
common soil amendments. When added to soils, each will improve them in
different ways. Here are a dozen
differences to consider when choosing which to obtain and add to your
gardens.
Peat moss may be more expensive,
especially if you have a local source of compost, buy in bulk, or
better yet
make your own. Generally the peat moss
we buy is harvested in Canada, so must be shipped from there, adding to
the cost.
Peat moss has few if any nutrients,
while compost is much better. However,
compost is not fertilizer. Compared to
fertilizer it is low in nutrients. The
nutritional value of compost often comes from its effect on soils and
the soil
microorganisms. Peat moss helps the soil hold nutrients by increasing
what is
called the CEC or "cation exchange capacity."
Peat moss has a low pH, so if you
use much, lime should be added as well.
Plants that do well in acidic soils, termed "ericaceous" such
as blueberries and rhododendrons, benefit from peat moss. Compost
usually has a neutral (pH 7) or
slightly alkaline soil reaction.
Peat moss doesn't compact, so can
last for years in soils, providing good aeration and water
holding.
Composts often compact, so should be added
yearly. Since composts lose their
nutritional value over time as well, yearly replenishment helps this
factor
too.
Both peat moss and composts hold
water, although peat moss tends to be better. This trait is
important in sandy or rocky soils that tend to dry out quickly.
Peat moss is hard to wet initially,
and to re-wet once it dries out.
Composts vary, depending on source, on how easy they are to
re-wet.
This trait seems contradictory, since once
wet these materials hold water well, releasing it to roots over time.
If peat
moss gets too dry, moisten it in a bag overnight. Using a couple
drops
of detergent in the
water may help, acting as a "surfactant". Warm water helps, as it
is
absorbed quicker
than cold water.
Peat moss has a uniform
composition. Composts often have
variable composition, especially among sources.
This is an important consideration when buying composts, which you
often
either have to learn on your own or from others who have used a
product. Composts also may contain contaminants,
depending on what was added.
Peat moss contains few
microorganisms. Composts are rich in
microorganisms. Most of these are
beneficial, improving soils in many ways, from aeration to
nutrition.
Peat moss contains no weed
seeds. Good compost
"shouldn't" contain weed seeds if it has been produced properly-- at
high enough temperatures in the compost pile to kill weed seeds,
covered to
prevent seeds from blowing in, and not made from weedy plants. If
you
don't know a particular source of
compost, or have any recommendations on it, put some in a pot.
Water
and wait a couple weeks to see if any
weeds germinate. There is nothing worse
than spreading weedy compost over a clean lawn or garden, ending up
with a
lifetime of weed seeds and weeding.
Peat moss has no disease suppressing
qualities, while compost (microorganisms) may suppress some
disease-causing
pathogens.
Peat moss is a natural resources,
obtained by "mining". This is
usually surface harvesting. Unlike the
past, most of this is done now after environmental impact analysis, and
in
a renewal and
sustainable manner. Composts, of course,
use recycled organic matter for the most part.
Finally, peat moss isn't really used
as a mulch, while composts are often used as a mulch
side-dressed
around plants. Unless used thickly,
however, composts wont suppress many perennial weeds. If peat
moss is
used as a mulch, it actually
may dry out soils by absorbing water from them.
Or, when dry, it may blow off the surface.
A solution to these differences
between peat moss and compost is to use both, getting the benefits of
each. Some incorporate peat moss and
compost when planting, then topdress perennial plants with compost in
subsequent
years. Peat moss reduces the tendency of
some compost to compact, and may extend the life of compost several
fold.
There are several other organic
materials you may consider adding to gardens or landscapes for various
purposes. These include mulches such as
bark or straw, (weed free, not hay), green manures or cover crops,
manures, and
paper. More details on all these are
available in a Cornell University fact sheet
(www.gardening.cornell.edu/factsheets).
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