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.
Two other products that will inoculate your mix with beneficial microorganisms are Earth Alive's Soil Activator and Mykes for Vegetables. Both products are in a powdered/granular form that makes them easy to use. Since I have been adding inoculants directly into my seed-starting mixes, I have observed a noticeable difference in the vigor of my seedlings as well as overall productivity of my plants once they are out in the garden. What I love about this is that I know that my seedling roots are developing healthy microbial associations right from the beginning, which means they are also forming complex and highly efficient root systems that will enable them to hit the ground running once they are transplanted out into the garden. It ALSO means that my garden soil is getting a regular infusion of beneficial microbes wherever my plants are being placed.
For my heavy-feeding crops like tomatoes and peppers, I will also add in a little glacial rock dust or kelp meal, to ensure my microbes have some minerals to work with to give my plants what they need. For tomatoes I will also add some pulverized eggshells to provide much-needed calcium. This will help balance the plants water management functions, which helps avoid a common tomato malady known as blossom-end rot.
When it comes to transplanting my seedlings into larger pots, I make up an identical batch of custom mix for my vegetables, to ensure they have all they need for their next stage of growth.
Do avoid the addition of concentrated composted manures to your seed-starting mix, as all that nitrogen-rich substance will do is promote a ton of foliar growth at a time when your plants need to be developing strong root systems. It's ok if your compost contains some manure, or if you use a balanced all-purpose organic fertilizer in your mix that contains some nitrogen. Try to use products that are in their most basic form, so that the microbes have something to work with. One product I can recommend is Gaia's Green All Purpose Fertilizer. It will break down slowly in the potting mix, and won't bloat your plants with a flood of water-soluble fertilizers.
Making your own Seed-Starting Mix
If you are planning on making your own mix, start with the basics - Peat moss, perlite and compost, all of which are available in bags. If you're not making your own high-heat compost, as I mentioned above, it's fine to use a bagged compost. Be sure you're not just buying a bunch more peat moss, as some companies add a ton of that into their "compost" mix. It likely will be sterilized, but at least you're getting some organic matter and hopefully some nutrients. All you have to do next is add in some of the other ingredients to bring your mix to life.
It's important to wear a mask when working with and adding products to a peat moss base. In order to incorporate all these ingredients together thoroughly, you need to mix everything in a dry state. This produces a lot of fine particulate dust, which can seriously irritate your lungs, so it's a good idea to protect your mouth and nose while mixing.
I typically use the following proportions for my mixes, but you can tweak this to suit your particular needs and preferences.
7 parts peat moss or coco coir
2 parts perlite
1-2 parts compost or worm castings
+ additional amendments according to package instructions
Once everything is mixed, thoroughly wet down your mix before using, so that when squeezed, you can barely produce 1 drop of water. Fill your containers and get started planting your seeds!
A well-balanced seed-starting mix will give your plants the fantastic foundation they need to support their growth, from seedling stage to maturity, providing you with vigorous, healthy and nutrient-dense food for you and your family.