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Seed Starting – What Type of Potting Medium to Use

From our April 2021 Newsletter

The Potting Medium I Use

Never use a potting mix that contains garden soil. Why? Because it contains natural fungi and diseases such as “damping off disease” that can harm seedlings. Garden soil is heavy and tends to compact and is not porous enough to supply the correct amount of air to the roots of little seedlings. A better option is to buy a soil-less seed starting mix which typically contains fine-grade peat, vermiculite, dolomitic lime (for pH) and a wetting agent.

I use Pro-Mix HP MYCORRHIZAE to start all of my seedlings; I buy this from Early’s in Saskatoon as 3.8cu.ft bale, but they are available in 42.5L bags as well.


Peat Moss

Peat moss is dead fibrous material that forms when mosses and other living material decompose in peat bogs. Most of the peat used in North America comes from remote bogs in Canada.

Peat moss first became available to gardeners in the mid-1900s, and since then it
has become highly valued by horticulturists for its ability to retain water and oxygen without becoming waterlogged or heavy. It holds several times its weight in water, and releases the moisture to plant roots as needed.

It also holds onto nutrients so that they aren’t rinsed out of the soil when you water. It is generally sterile and naturally suppresses fungal diseases that can afflict seedlings, making it a natural choice for seed starting.

There is some concern about the harvest and use of peat moss because it classified as a non-renewable resource due to the fact peat bogs gain less than a millimeter in depth every year.

On the link below I’ve shared an article that explains the concerns in detail and some alternative solutions that are on the market.

Washington Post Article: The Sustainability of Peat Moss


As plants have evolved to survive in challenging conditions, one important set of survival mechanisms they have developed involves creating mutually beneficial (symbiotic) relationships between plant roots and soil-borne organisms such as bacteria and fungi.

These associations between plant roots and fungi are called mycorrhizae. It is a symbiotic arrangement where plant roots are hospitable sites for the fungi to anchor and produce their threads (hyphae). The roots provide essential nutrients for the growth of the fungi. In return, the large mass of fungal hyphae acts as a virtual root system for the plants, increasing the amount of water and nutrients that the plant may obtain from the surrounding soil. This relationship between fungi and plant results in an overall improved plant growth.

The Pro-Mix soil-less potting medium I use contains a beneficial mycorrhizal inoculum (Glomus intraradices).


What is Vermiculite?

Vermiculite is the name of a group of hydrated laminar minerals (aluminum-iron magnesium silicates) which look like mica. Horticultural vermiculite is processed with massive heat that expands it into accordion shaped pellets composed of multiple layers of thin plates.

It has the benefits of improving soil aeration while retaining moisture and nutrients to feed roots, cuttings and seeds for faster, maximum growth.

What is Perlite?

Perlite is a naturally occurring mineral, and it exists in nature as a type of volcanic glass. When water saturates volcanic obsidian glass over time, perlite starts to form. Perlite is mainly water, and when manufacturers heat the material at high temperatures, it “pops” just like popcorn in the microwave.

Perlite is stable and retains its shape in your soil mix. It has a neutral pH, which makes it ideal as a soil amendment, and it contains no chemicals or nutrients. Perlite is a highly porous material and assists gardeners with water retention in the soil while improving drainage

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