Newcomers reach for the richest, blackest bag on the shelf, reasoning that good soil grows good plants. For seed starting, that instinct is exactly backwards. A seed brought its own food, remember - it does not need a rich medium, and a rich one actively works against it. What a germinating seed and a young seedling want is the opposite of garden soil: something light, fine, clean and nearly empty of nutrients. Understanding why turns “buy the right bag” from a guess into a decision you can make on your own.
What a seed mix is really for
Cast your mind back to the three switches: water, oxygen, warmth. A seed-starting mix exists to deliver the first two perfectly. It must hold moisture evenly so the seed never dries mid-germination, and at the same time stay open and airy so oxygen reaches the seed and the emerging root does not suffocate. Holding water and holding air are usually in tension - the more of one, the less of the other - and a good seed mix is engineered to do both at once. That is its whole job. Think of it as a sponge that is also full of air pockets, not as food.
Two more requirements follow from the seedling being fragile. The mix must be fine and free of lumps, because a delicate root and shoot cannot push past a stone or a wad of bark - fine texture lets tiny roots run. And it must be clean, meaning free of the fungal spores and weed seeds that live in real soil, because the number one killer of healthy-looking seedlings is a fungal disease called damping-off, and starting in a sterile medium is your best defence against it.
Notice what is not on the list: nutrients. A seed-starting mix is deliberately low in fertility. The seed’s packed lunch powers germination and the first tiny leaves; by the time the seedling has used that up and needs feeding, you will either pot it into a richer mix or start a dilute liquid feed. Rich mix at the germination stage does nothing useful and can burn the emerging root with excess salts. “Nearly empty” is a feature, not a shortcoming.
Why garden soil and potting compost fail
This is worth being blunt about, because it is a classic and heartbreaking beginner mistake. Do not start seeds in soil dug from the garden, and be cautious with heavy general-purpose potting compost. Garden soil fails indoors for three compounding reasons. It is too dense - in a small container, without the structure and organisms of the open ground, it packs down into an airless brick that smothers roots and drives out oxygen. It is full of life you do not want in a warm, moist tray - fungal spores that cause damping-off, plus weed seeds that sprout alongside your crop and confuse everything. And it drains unpredictably, swinging between waterlogged and bone dry, exactly the instability germination cannot tolerate.
Standard bagged “potting soil” or “container compost” is better than garden dirt but often still too coarse, too rich and too water-retentive for seeds - it is formulated to grow established plants in pots, a different job. It can work in a pinch, especially sieved to remove the big lumps, but you are fighting it. The purpose-made stuff exists for a reason.
Reading the bag: what “seed-starting mix” contains
A product labelled seed-starting mix (sometimes “seed & cutting compost”) is blended for exactly the requirements above, and for a beginner it is the easy, reliable choice - buy it and move on. But knowing what is in it lets you judge quality and, later, make your own. The classic components are three:
- A moisture-holding base. Traditionally this is peat, prized because it is fine, sterile and holds water like a sponge while staying acidic and low-fertility. Increasingly it is coir (coconut fibre), a renewable by-product with similar water-holding and better wettability. We will come back to the choice between them.
- An aerator. Perlite (the white volcanic popcorn) or vermiculite is mixed in to keep the blend open and airy so oxygen reaches the seed and drainage stays free. Perlite mostly adds air and drainage; vermiculite adds air but also holds some water and is often sprinkled on top of seeds as a fine, moisture-retentive cover.
- Sometimes a starter charge of nutrients or wetting agent. Some blends add a very light feed to carry seedlings a little further, or a wetting agent because dry peat is famously hard to moisten the first time.
If you look at a bag and see essentially “fine peat or coir plus perlite/vermiculite, low nutrients,” you are holding the right thing. If the bag is dark, rich, chunky and smells strongly of fertiliser, it is potting compost wearing the wrong hat for this job.
A DIY mix that works
Once you start a lot of seeds, buying bags gets expensive, and mixing your own is simple and cheaper. A reliable, beginner-proof recipe by volume:
Two parts fine coir (or peat), one part perlite, one part vermiculite. Moisten it thoroughly before filling trays - dry coir and peat both resist water at first, so add warm water gradually and squeeze until the whole batch is evenly damp like a wrung-out sponge, not dripping. That is genuinely it. For crops you will grow on in the same cell for a while, some gardeners add a small handful of sieved worm castings or a pinch of balanced fertiliser to the batch, but keep it light - remember the seed does not want richness, only the older seedling does, and only a little.
The two rules that make any homemade mix succeed: fine and evenly moist. Sieve out lumps if your components are coarse, and never fill trays with dry mix and then try to water it - it channels and stays dry in pockets. Wet the whole batch first, then fill.
The peat versus coir question
You will meet a values choice here, and it is worth a clear-eyed paragraph. Peat is a superb seed medium and has been the industry standard for a century. Its drawback is environmental: peat is harvested from bogs that are ancient, slow-forming carbon stores and important wild habitats, and mining them releases stored carbon and destroys the bog. For this reason peat use is being phased out or restricted in some places, and many gardeners choose to avoid it.
Coir, the coconut-husk fibre, is the leading peat-free alternative: renewable, a by-product of an existing industry, and it wets far more easily than dry peat. It behaves a little differently - it holds nutrients slightly less well and can be saltier if poorly rinsed (buy a reputable brand) - but for raising seedlings it performs excellently and is what we would steer a new gardener toward. There are also peat-free blends based on composted bark, wood fibre and green compost; quality varies more between these, so if one disappoints, try another brand rather than concluding peat-free “does not work.”
None of this changes the biology. Peat or coir, the seed wants the same thing: fine, moist, airy, clean and nearly empty. Choose the base you are comfortable with, get it evenly damp, keep it clean, and your seeds will not know the difference. With a medium sorted, you are finally ready for the act itself - putting seed to mix at the right depth, on the right date - which is the next guide.