Since the 1940s or 1950s peat moss has been a widely-used component of potting soils and related commercial soil mixes. However, as noted in the Garden Shed article Should We Stop Using Peat?, peat bogs, which account for 3% of the earth’s surface, provide 15-30% of land-based carbon storage. Unfortunately, digging up peat moss releases much of that stored carbon into the atmosphere, contributing to climate change.
Many climate-sensitive gardeners would like to convert to peat-free potting soils. However, exactly what to change to is confusing, and the many different mixes on the market make it unclear what the best alternatives are. Let’s take a look at market choices and Cooperative Extension recommendations to try to determine the best options.
Key Functions of Growing Media
There are several potting mix characteristics that are important to its success as a growing medium:
- Supplying plant roots with nutrients, air and water. Most require some fertilizer additions.
- Allow root growth
- Provide physical support to growing plants
- Be free of pests, pathogens and weed seeds
- Have a pH appropriate for the plants to be grown.
Peat moss supports several of these elements, including lightening soils, allowing air and water entry, and holding moisture without over-saturating the plants. Its key weakness is that it lacks necessary nutrients, typically requiring the addition of other organic materials and/or fertilizers. Also, it tends to have a low pH and may require the addition of lime to reduce acidity. Certainly, any replacement material or mix of materials should perform as well as peat-based products in these areas.
What is available commercially?
An examination of local dealer inventories shows a head-spinning variety of options. Many still offer products that include some amount of peat. Products include varying combinations of multiple materials including peat moss, reed-sedge peat (a darker, more decomposed material from reeds, sedges and grasses), recycled forest products, rice hulls, perlite, vermiculite, coconut coir (pronounced coy-er), worm castings, fertilizers, and limestone (to adjust pH).
Examples, without including brand names, include:
- 50-70% coconut coir plus 30% perlite and fertilizer
- 50+% forest product (tree bark and ground wood) plus peat or compost, perlite and fertilizer
- Various mixes that may include forest products, reed sedge peat, rice hulls, perlite, limestone.
- Compost-based materials including worm castings, bat guano, ground oyster shells, feather meal, soybean meal, bonemeal, lobster meal, kelp meal and other additives that are local to the manufacturer location.
Obviously, the best substitute for the long-established peat-based potting mixes isn’t clear, and in fact, may vary by intended use and location, but there are a number of non-peat products available for trial.
What do the Cooperative Extension Services recommend?
There are specific recommendations available from multiple Extension Services, but before getting specific, it probably makes sense to note the benefits and drawbacks of the various components already mentioned:
- Based on published articles, the most favored non-peat primary components are coconut coir and ground tree bark/wood.
- Coconut coir is the most-recommended non-peat component.
- Almost all of the new potting soils on the market recommend adding compost to the mix. Compost adds missing nutrients while supporting the other necessary functions of the potting soils. In fact, using straight local or home-made compost as a potting soil for vegetables, herbs, and flowers is one recommended solution.
- Items like builders’ sand, perlite, and vermiculite help boost air and water accessibility, but add little or no nutrients and may drain moisture faster than desirable.
- While fertilizer needs vary according to other ingredients, some amount is generally needed, and organic products like bone meal, blood meal, and fish meal are among the preferred additions.
- pH can vary significantly, and pH preferences will depend upon the plants to be grown. A lime-based fertilizer is often recommended to raise pH and reduce acidity. Sulfur-based products can increase acidity as needed.
In its publication No Peat Potting Soil Options, the University of Minnesota Extension offers several mix recipes using coir. Key coir characteristics include:
- Holds moisture well
- Wets more easily than peat
- Drains well
- Decomposes more slowly than peat
- Resists compaction
- Less acidic than peat
- Low nutrient content and high cation exchange rate, that tends to hold on to stored nutrients, creating a need for nutrient additions from other ingredients and/or fertilizers.
- Also, when purchasing coir, check the label to be sure that it has been washed to remove salts. It appears that suppliers have added salt flushing washes to their processes, but it is worth confirming since salts can harm plant growth. If the salts haven’t been removed, the coir should be washed prior to use.
The Extension-recommended mixes, which are based on a blend of coconut coir and compost, include:
- For Seedlings
- 1 part compost
- 2 parts coconut coir
- 1 part builders’ sand
- For Ornamentals
- 1 part coconut coir
- 1 part compost
- 1 part top soil
- 1 part builders’ sand
- For Edibles
- 2 parts compost
- 2 parts coconut coir
- 1 part builders’ sand.
- Similar formulas using ground bark and wood materials are mentioned by other sources, although less commonly than coir.
In each case, additional fertilization is likely needed, and pH should be checked to match plant needs.
Are the mixes re-usable?
If last year’s potting soil had issues with disease, it should not be re-used in the garden. If it was basically weed- and disease-free, it is suggested that last year’s soil be removed from pots and mixed 50/50 with a fresh batch of potting mix prior to re-use.
Take-Aways
This brief run-through of possible substitutes for peat-based potting mixes indicates that there are lots of possibilities available, both commercially and home-made. Compost is a highly-valued component and is included in most options. Coconut coir seems to be the most favored peat substitute, although ground up wood products have some backers as a longer term solution that is less expensive and more sustainable than coir, which is imported from Asia and Central America.
Because of the variety of possible products and the importance of finding an effective solution, some professionals recommend taking a multi-year approach, experimenting with different mixes and a slow phase-out of peat, to offer the best odds of successful change with minimal disappointment.
In any case, I hope this discussion helps clarify possible solutions and helps environmentally-responsible gardeners make a successful change that supports our growing efforts while positively impacting the climate crisis.
SOURCES:
Featured Photo: Ralph Morini
“Coconut Coir vs Peat Moss,” Coconut-coir.pdf, Washington State Extension
Coir is sustainable alternative to peat moss in the garden | OSU Extension, Oregon State University Extension
Soilless Growing Mediums | okstate.edu, Oklahoma State University Extension
Growing Media (Potting Soil) for Containers | umd.edu, University of Maryland Extension
Peat-free potting mixes – Maryland Grows Blog / umd.edu, University of Maryland Extension
“No-Peat Potting Soil Options,” University of Minnesota Extension, Hennepin Master Gardeners
“COCONUT COIR AS AN ALTERNATIVE TO PEAT MEDIA FOR VEGETABLE TRANSPLANT PRODUCTION,” swfrec.ifas.ufl.edu, Southwest Florida Research and Education Center, University of Florida
Why Gardeners Should Stop Using Peat, and What to Use Instead – The New York Times (nytimes.com) (Margaret Roach, Feb. 6, 2022)
