Seed Starting Mix – What Is In It and a Working Recipe
A seed starting mix has one job: hold moisture and air around a germinating seed, and let a root push through. Everything about it follows from that.
TL;DR
- Seed mix ≠ potting soil: finer, lighter, little or no feed.
- Home recipe by volume — 1 coir or peat : 1 perlite : 1 vermiculite.
- Never garden soil in a cell; pre-moisten before filling.
The three ingredients, and what each does
Peat or coir is the body: it holds water and gives the mix something for roots to grip. Coir rehydrates from a block, wets more easily once dry and is the more sustainable of the two.
Perlite is the drainage: white expanded rock that creates air pockets so a cell cannot stay saturated. In a small volume of mix this is what stands between you and damping off.
Vermiculite is the sponge: it holds water and is the traditional light covering over sown seed, because it does not crust the way soil does.
Commercial seed mixes are essentially these three in varying ratios, plus a wetting agent. That is why the home version works: there is no secret ingredient to miss.
Mixing and using it
By volume, one part coir or peat, one part perlite, one part vermiculite. Add a small handful of screened compost per gallon if you want the seedlings to have something to eat once they are up — but keep it modest, because a rich mix in a small cell pushes soft growth. Moisten the whole batch until it holds together when squeezed and no water runs out, then fill the cells and firm lightly.
Two habits make the difference between a mix that works and one that frustrates: pre-moisten (dry mix repels water and sends it down the gap between mix and cell wall), and fill to just below the rim so there is room to water without it running over the edge.
Why garden soil is the wrong answer
Garden soil in a two-inch cell compacts into something closer to brick, drains badly, and brings weed seeds and soil-borne pathogens with it. It also crusts when it dries, which is exactly when a germinating seed needs a soft surface to push through.
If you are filling a raised bed, garden soil and compost are right and this article does not apply. The distinction is container volume: the smaller the cell, the more the mix has to be engineered rather than dug.
Once seedlings have true leaves and are potted on, they move to a coarser, fed mix — the fertilizer guide covers when feeding starts, and the seed starting basics covers the rest of the setup.
Tools for this job
We may earn a commission from qualifying purchases made through the links below, at no extra cost to you. Marked as sponsored.
Coco coir block
Rehydrates into the base of a home mix, and cheaper per liter than bagged mix.
Related guides
Frequently asked questions
Can I use potting soil for seeds?
You can, but it is heavier, holds more water and usually contains fertilizer a seedling does not need yet. If it is all you have, mix it half and half with perlite and water more carefully — the risk is a mix that stays wet.
What is the difference between seed starting mix and potting mix?
Seed starting mix is finer, lighter and has little or no nutrition, so a small cell drains and roots can push through it. Potting mix is coarser, holds more water and is usually fed, which suits a plant that is already growing.
What is a simple seed starting mix recipe?
By volume: one part peat or coir, one part perlite, one part vermiculite, with a small handful of screened compost per gallon if you want some nutrition. Moisten it before filling cells; dry mix repels water and channels it down the sides.
Why not use garden soil?
It compacts in a small cell, drains poorly, carries weed seeds and soil-borne pathogens, and often crusts over so seedlings cannot push through. Damping off and slow emergence are the usual results.
Do I need to sterilize my own mix?
If it contains compost or garden soil, yes, or accept the risk. Commercial seed starting mixes are already treated. A home mix built from coir, perlite and vermiculite has nothing living in it to sterilize.
Can I reuse mix from last year?
Not for seedlings. Old mix has been drained of structure, may carry pathogens and often hosts fungus gnats. Use it in a garden bed, not in a seed tray.