What Kind of Potting Soil to Use for Seedlings
There are many options for potting mix, but only a few are suitable for growing vegetable seedlings.
An organic mix
A mix being organic or not mostly has to do with “wetting agents” that are used in non-organic mixes, which help retain moisture. In my experience, they’re not necessary.
Non-organic mixes may also have synthetic fertilizers added to the mix — read the labels, and choose what aligns with your food choices
Mixes for other purposes often have other base ingredients
Contains mycorrhizal fungi
This is not essential, but the good mixes will contain fungi. They help with nutrient availability for the plants, but the amounts in potting mixes are generally small… and we can add our own, if we want to
Labelled for seedlings or seed-starting
Avoid mixes that are “for indoor plants”, or for succulents, or for trees, for anything besides seed-starting
Mixes that are made for other uses often have coarser or finer fibers, varying levels of nutrients, varying densities (eg, the mix may be too heavy for germinating seeds), etc
Personally, I use organic Pro-Mix as the base for my seed-starting medium. I get it in large bales, but I’ve seen it available in bags at garden centres, which are suitable for home gardens.
Having said that, if you’ll be starting a lot of seeds, and would like a bale, get in touch, and I can see about picking one up for you.
Then, I add other sources of nutrition for the plants. This part can be as simple or complex as you want.
To keep it really simple (and effective), my top suggestion is to add worm castings.
If you don’t have your own bin of red wiggler worms to draw from, there are readily available bags of worm castings at various stores.
I’ve found that the Mark’s Choice brand of worm castings at Home Hardware is good. It feeds plants well, it’s moist (therefore, more active), a nice crumbly texture (as it should be), and it’s a reasonable price.
I’ve tried a couple other brands, and have seen other brands in use other places… and the range of differences in colour, odour, texture and effectiveness is very surprising.
Therefore, if you don’t want to experiment, start off with the Mark’s Choice brand, and get good results right away.
Worm castings can be mixed with the potting soil at a rate of about 2 - 10% by volume.
My rule of thumb is:
any plant that produces a “fruit” (such as tomatoes, peppers, cucumbers, zucchini, etc) get more worm castings.
any plant that is grown for its leafy greens receives the lower quantity of castings.
Other possible additions are: fine-ground lime (calcium), soft rock phosphate, kelp, or other trace nutrient additions.
For the sake of simplicity, and to still get good results, it’s probably best at the start to stick with the worm castings.
The seedlings can receive some natural fertilizers in their water as they grow, so the fertility in the soil mix doesn’t have to be their only source of nutrition. The soil fertility is to get them off to a good start, and then they can be topped up with other nutrients when they begin to develop true leaves.