Perhaps one of the most widely used soil amendments in the continental U.S. This being true, I find it odd that it is also one of the least understood. What is it? Where does it come from? More importantly, what does it really do for the soil? With a little research, you will immediately find the term peat moss is often used to describe three different materials. That in itself is confusing. Let’s have a closer look.
Moss- Moss is usually a tiny, very ancient plant form. Moss grows on stone, bark, decaying wood, and on bare soils. Mosses are a very large family of plants. Sphagnum moss is one of the larger sized species of moss that covers huge areas of Earth in cold latitudes. Sphagnum moss grows in peat bogs. As it dies and slowly decays, sphagnum moss adds to the volume of the peat bog. Moss, in and of itself, is not used as a soil amendment.
Peat- True peat is described as being the beginning stage of coal. Peat bogs that become buried through siltation, seismic action, and other catastrophic events or any combination of such events will eventually form coal if enough pressure (overburden) is applied.
Peat can be formed by any organic matter (not just moss) including animal remains. Plant and animal remains that fall into bogs decompose very slowly. This is called anaerobic (lacking or without oxygen) decomposition. Combined with cold temperature which decreases microbial activity, it can take thousands of years for organic matter to fully decompose. This is made evident by the discovery of many prehistoric animal and human remains in peat bogs. Some still retain flesh and even hair after thousands of years. Some peat materials have been carbon dated to be as old as 360 million years.
True peat can be dried and burned as fuel. This is the most common use of peat. Being high in carbon, concentrated minerals, and humic acids, true peat would make an excellent soil amendment, but that is not what is being sold at the local garden center.
Sphagnum peat moss- This is the material commonly sold to gardeners. Peat bogs composed largely of decaying sphagnum moss are easily accessible in the Canadian provinces. These bogs take thousands of years to form. It is said that these sphagnum bogs grow only one millimeter per year. Thus a two foot layer of peat moss has taken a thousand years of anaerobic decomposition to form.
Sphagnum moss that is harvested as a live plant has been used as a growing medium for orchids and other epiphytes (plants that grow on other plants), for hanging baskets, and as a decoration. Sphagnum moss is not considered a soil amendment.
Sphagnum peat moss is considered a good soil amendment mainly for its capacity to absorb and hold water. However, other qualities have been highly overrated. The environmental impact alone does not justify mass harvesting of sphagnum peat moss. We are harvesting millions of tons of a substance that took thousands of years to form; this is definitely not a sustainable practice.
In my opinion, sphagnum peat moss rates on the lower end of materials available as soil amendments. In other words, if fresh compost rates a ten then sphagnum peat moss would rate about a two.
In the first place consider volume. What you see is what you get. Although a compressed block of peat moss will fluff up to make a good wheelbarrow full of loose material, what is compressed out of that bale of peat moss is merely air. Peat moss will become compressed again once it is added to the soil. Let’s say you paid $15 for a bale of peat moss that weighed fifty pounds. What you got is only fifty pounds or 2 cu. ft. of material. That 2 cu. ft. will amend about 18 sq. ft. (2” deep) of average soil. If you spent that same $15 on bulk compost you would be getting about one thousand pounds of material that would amend about 150 sq. ft. of the same soil. Do the math using whatever scales you might have and you will find sphagnum peat moss is just not cost effective for large areas.
As stated earlier, the best quality of peat moss is its water holding capacity. Recent studies done on clay soils showed that peat moss would absorb moisture away from clay particles. We all know that once clay is wet it tends to stay wet and once completely dry it will actually repel water. Dry peat moss will also repel water. What most plants want is evenly moist soil and good percolation. Adding peat moss to improve clay soils may be counter productive altogether. It does seem to work better in sandy soils.
I live in an area where soil ph ranges from alkaline to very alkaline. I was led to believe that peat moss was really good at lowering ph levels. True, peat moss is acid by its very nature and when tested in its pure form. However, due to the absorbent nature of peat moss, it will absorb chemical compounds from the surrounding soil. This means that over time sphagnum peat moss will become as alkaline as the surrounding soil. Any gain on soil acidification will be short term at best.
Next consider that sphagnum peat moss is really just very, very, old compost that is anaerobic. I hope all of you understand by now that anaerobic decomposition does not make good compost. It smells bad, breeds disease organisms, and takes a long, long time to break down. We turn our compost piles to introduce oxygen. Oxygen stimulates fast reproduction of decomposing microbes causing the release of energy (heat) which kills off disease organisms, and thwarts insect reproduction. Well made compost is full of beneficial microbes and available nitrogen as well as all minerals and elements needed for plant growth. Anaerobic compost has few beneficial organisms, little if any useable nitrogen, and may harbor insects and disease. Sphagnum peat moss is thousand year old anaerobic compost which contains very few active components at the point of sale.
Still, sphagnum peat moss is useable in sandy soils and as a component in various potting mixes. However, the cost involved in harvesting, shipping, and loss of habitat is causing this to change. When you consider the fact that anything (including soil mixes) you can do with peat moss is really done better with compost, then any company producing soil mixtures will sooner or later have to look at composting local materials rather than shipping peat moss out of Canada. Perhaps there was a time when cheap oil and affordable equipment made harvesting of peat moss lucrative. Those times are all but gone.
Twenty years ago, if you walked into any store that sold gardening products you would expect to see great stacks of peat moss bales. During that same time, oddly enough, there was little commercially available compost and absolutely no such thing as locally produced compost in bulk form. Like everyone else, I used plenty of peat moss and believed what I was told about the quality of it. Today we see the exact opposite. Most garden centers offer several choices of bagged or bulk compost plus we don’t see so much baled peat moss. I sold our last bale somewhere around 1993 as more commercial compost hit the market although I had learned some about the true pros and cons of peat moss before then. I suppose there are still quite a few gardeners out there who believe it is worth buying. I hope this article will help change that. Give me compost; you get more for the same money plus greater benefits. I’m done selling peat moss.