Peat Moss or Mulch

by Leonard Perry

email: lpperry@uvm.edu

In extension I serve as an advisor and consultant to the greenhouse and nursery industry, primarily in Vermont but throughout
the region and beyond as well.

I give presentations on my research to the industry, and to home groups.

In Research, my focus is "herbaceous perennial production systems".

His website is at http://www.uvm.edu/~pass/perry/index.html

Leonards zone of gardening: home with my trials, generally USDA 4a. Campus in Burlington is 5.

March 14, 2010

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.
Here is some more information for you,
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 ).