When it comes to seed-starting mixes, most folks naturally default to using common "potting soil" as a growing medium. There is nothing wrong with using a pre-mixed product, and, in fact, there are a few newer ones on the market that I use regularly, as they are super convenient. For most of my Urban Farm School students who may be just starting out their food-growing journey, I most often recommend they start with a bagged seed-starting mix. It's important to note, however, that not all bagged mixes are created equal.
Although commonly labelled "potting soil", these products all have one thing in common: they contain absolutely no soil whatsoever. When it comes to starting your seeds, though, a soil-less mix is exactly what you want. Mixes for seed-starting need to be light enough to support easy formation of new root systems and porous enough to provide good air spaces and adequate drainage.
Some bagged products contain only 2 ingredients: peat moss and perlite.
Peat moss is sourced from naturally-occurring peat bogs, and although Canada has multiple sources, they are considered a finite resource and non-sustainable. There are some mixes that substitute coconut fiber for peat moss in an effort to provide a more sustainable product, however this product has to travel a very long distance to give us that option, so the question remains as to which product is actually more sustainable in the long run, and whether utilizing a local resource makes more sense than an imported one. Peat or Coconut fiber is used to improve aeration and water-holding capacity, and also to hang on to nutrients that otherwise would leach out. It is a naturally sterile product that contains little to no nutrients or micro-organisms, and as such has been the preferred medium for seed-starting. A sterile mix means little chance of weed seeds, insects or plant pathogens.
Perlite is essentially a natural mineral of volcanic origin that has been super-heated. It's granular structure is useful for providing high levels of aeration, as it helps maintain air pockets and prevents compaction. It is completely sterile and chemically inert, and does hold some water.
These two products, peat moss and perlite, are in almost every bagged mix on the market. But we can certainly do better than that for starting our seeds, especially in a northern climate, and I'll explain why that's important.
First of all, because of our short growing season, we will be starting some of our seeds as early as January. This means that our seedlings will be in pots under lights for quite a long time before they are transplanted out into the garden in late spring. Although each seed has enough energy and nutrient supply to ensure germination, once it does germinate it starts building a root system to scavenge for nutrients. If all it has to grow in is a sterile medium completely devoid of minerals, it will not thrive. What about using fertilizer to fill that need, or buying potting soil with fertilizer already in it (such as Miracle Gro), you might ask? The problem with using water-soluble fertilizers is that they train your seedlings roots to behave in an undesirable way. Plants actually develop 2 kinds of roots: some to uptake water (tap roots) and others to uptake nutrients. If you use only water-soluble fertilizers, your plants will not be inspired at all to develop "nutrient-uptake" roots (hair roots), as they can easily receive what they need simply by absorbing water through their "water-uptake" roots. Once these plants are transplanted out into the garden, they will not be equipped to function in the actual soil, and can become highly stressed.
Secondly, these incredibly basic sterile mixes are just that: Dead. No life. No micro-organisms. No nutrition/minerals. While on the surface a sterile medium may seem like a good thing, we now understand that the soil biome is absolutely essential for healthy nutrient uptake in plants. It makes sense that mass-produced products that may be traveling across country or across borders need to be sterile in order to avoid introducing unwanted plant diseases, pathogens and pests. The fact is, however, that without microbial relationships, which allow the plants to uptake an incredible range of minerals, we cannot grow nutrient-dense vegetables.
So what's the solution?
First of all, if you want to use a bagged seed-starting mix, choose a product that has more than just peat moss and perlite in it. One of the better ones is Pro-Mix Ultimate Organic for Vegetables & Herbs. Like all mixes, it is largely comprised of peat moss, and also contains compost, perlite, a little organic fertilizer, some gypsum, limestone and mycorrhizae, which is a highly beneficial soil microorganism that forms associations with plant root systems. I do use this mix sometimes, but almost always add more components to it before I use it. Because I want my seedlings to have really good nutrition, and I know that the only way to make that happen without relying on water-soluble fertilizers is to ensure there are beneficial microorganisms in the mix, I always add products that have the ability to introduce that life back into the mix. Doing this also supports the formation of those "nutrient uptake" roots that are so essential to healthy vegetables.
One product that fits the bill perfectly is worm castings. Loaded with microbes as a result of passing through the gut of the worm, worm castings are a fantastic addition to any seed-starting mix.
If you are making your own thermophilic (high-heat) compost, as I do, gather and screen some in the fall and set it aside in a bin indoors to add to your seed-starting mix in the spring. This high-heat composting method ensures that weed seeds, pests and pathogens are destroyed, and provides highly biodiverse populations of beneficial microbes that can be safely introduced into your mix. If you are practicing passive composting methods, however, I would NOT add your compost to your mix, as you will risk introducing unwanted disease and pest organisms to your seed-starting set-up. Like worm castings, compost brings not only biological life into your mix but also high-quality organic matter, improving water retention and contributing loads of nutrients.