University of Vermont Extension
Department of Plant and Soil Science
Dr. Leonard Perry, Extension
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
Peat moss may be more expensive,
especially if you have a local source of compost, buy in bulk, or
make your own. Generally the peat moss
we buy is harvested in Canada, so must be shipped from there, adding to
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
microorganisms. Peat moss helps the soil hold nutrients by increasing
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
Composts often compact, so should be added
yearly. Since composts lose their
nutritional value over time as well, yearly replenishment helps this
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
This trait seems contradictory, since once
wet these materials hold water well, releasing it to roots over time.
moss gets too dry, moisten it in a bag overnight. Using a couple
of detergent in the
water may help, acting as a "surfactant". Warm water helps, as it
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
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
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,
prevent seeds from blowing in, and not made from weedy plants. If
don't know a particular source of
compost, or have any recommendations on it, put some in a pot.
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
lifetime of weed seeds and weeding.
Peat moss has no disease suppressing
qualities, while compost (microorganisms) may suppress some
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
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
around plants. Unless used thickly,
however, composts wont suppress many perennial weeds. If peat
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
years. Peat moss reduces the tendency of
some compost to compact, and may extend the life of compost several
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,
paper. More details on all these are
available in a Cornell University fact sheet