I think that there’s probably a lot of confusion between the difference between seed starting mix and potting soil, because one of the most read posts on my blog is Potting soil vs seed starting mix. Back when I wrote that post, I was trying to figure the difference out myself. I think I have a better idea about it now, so I want to share my thoughts with you, in case it will help you with making your choice.
Seed starting soil is often made from just finely sifted peat moss, mixed with some perlite (for aeration and moisture retention). It has a lighter, finer texture than potting soil. It is meant to start seeds, but it is not life sustaining- its just a bunch of fluff. Seeds started in a seed starting soil should be transplanted into a larger container filled with actual potting soil within about 3 weeks.
Seed starter soil is more ideal for plants with very fine seeds (like lettuce or basil) but probably unnecessary for starting larger seeds (like squashes). It is also supposed to help the seedlings from becoming too waterlogged, which can cause the dreaded “damping off”. (Damping off is when seeds either fail to emerge, or die soon after they have emerged, due to diseases or moisture issues.) I’ve never had a problem with damping off, but I know many people who have. Seed starting mix is also usually sterile, which helps keep disease to a minimum.
A seed starting mix can also be useful if you want to start many seeds in one cup, and then be able to gently break apart the clump of young, tender plants for replanting. I have used this technique successfully with my tomato seeds.
Potting soil is usually a blend of several ingredients, including, but not limited to: compost or vermicompost (worm castings), peat moss or coconut husks (coir), fertilizers or amendments (natural like greensand, limestone or chemicals like Miracle Gro), perlite, vermiculite, or sand.
Potting soil is coarser and than seed starter mix. It also provides more nutrients, so you don’t have to transplant seedlings right away. There might be slightly more risks with using potting soil to start seeds, because there are more ingredients going in, and everything may not be sterile. Potting soil may tend to be heavier, have less aeration, and might not be as evenly moist, depending on the ratio of ingredients.
I have gotten a “bad” bag of potting soil before, which almost killed a houseplant, and caused problems for my seedlings. It was a national brand from a big box store, so I turned to a local source for my potting mix from then on. I have known others that have gotten “bad” bags of potting soil as well. I’m not sure how to prevent it, except by mixing up your own potting soil or finding some other reliable local source. I’m not totally sure what makes a bag “bad”, but my two best guesses are that either the ingredients weren’t mixed evenly in the batch (so that you get too little or too much of one input), or there was something wrong with the compost (not cooked enough, not sterilized properly, or bad ingredients made their way into the compost bin).
Soil blocker mix is just a type of potting soil. When I use my soil blocker, I use a homemade potting soil mix. I got the recipe from Eliot Coleman’s book The New Organic Grower. It includes peat moss, compost, greensand, lime, vermiculite, rock phosphate, blood meal, and sand. The exact ratio of ingredients is important to get get a mix that will hold well together in blocks. Now that I use a soil blocker and make my own potting soil mix, I won’t be using commercial seed starter or potting soil much anymore.
If you are trying to decide if you need to buy seed starting mix, the answer is “No”. Its not necessary in order to start plants from seed. Potting soil can work just fine for most casual home gardeners. However, if you are having trouble with damping off, disease, etc., then you might want to try seed starting mix. Just remember there are many other possible reasons why seeds fail to germinate properly (seeds are too old, disease from re-used containers, airborne diseases, watering issues, lack of nutrients, temperature is too cold, and more).
Do you use potting soil or seeds starter mix to start your seeds at home? Which is your favorite?