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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