How to Make Seed Starting Mix with Your Own Compost: Three Recipes That Actually Work
By Ku · Updated March 2026 · 8 min read
The first time I started seeds indoors, I used regular potting soil. Half of them didn't germinate. The ones that did came up spindly and yellow. I blamed the seeds, then the window, then my watering schedule. It took me a full season to figure out what actually went wrong.
Potting soil is the wrong medium for germinating seeds. It's too dense, too rich in nutrients, and often harbors pathogens that devastate seedlings in the warm, humid conditions of a seed tray. Seed starting mix is different — it's designed specifically for the first two to three weeks of a seedling's life, when the requirements are almost the opposite of what established plants need.
The good news: if you're already composting at home, you have the most important ingredient. Here's how to use it.
What seed starting mix actually needs to do: According to Oregon State University Extension horticulturist Brooke Edmunds, a good germinating mix must be fine and uniform, well-aerated, loose and free of pests, diseases and weed seeds. It should also be low in fertility and able to hold and move moisture. That last point surprises most people — low fertility is intentional, not a flaw. Seeds carry their own nutrient reserves to power germination. They don't need fertilizer until their first true leaves appear.
Why Regular Soil — and Too Much Compost — Kills Seedlings
The two most common mistakes in DIY seed starting both involve using the wrong growing medium. Understanding why they fail makes it easy to avoid them.
Garden soil: wrong texture, wrong biology
Backyard soil is too compacted for seedling trays. It doesn't drain freely, forms a crust that prevents seedlings from pushing through, and almost certainly contains weed seeds and pathogens. Michigan State University Extension puts it plainly: avoid filling pots with native soil that becomes compacted and often contains pathogens. In a warm, moist seed tray, those pathogens thrive. The result is damping off — a fungal collapse of seedlings at soil level that spreads rapidly through a tray.
Pure compost: too rich, potentially too active
This one is counterintuitive. Compost is excellent for garden beds. In a seed tray, used at high concentration, it can be harmful. The Garden Professors, a peer-reviewed gardening science resource, explains why: unfinished compost still contains active decomposition microorganisms that require nitrogen. In the absence of other nitrogen sources, those microbes can pull nitrogen from the seedling itself. Even finished compost, used at too high a ratio, creates conditions that favor the pathogens that cause damping off.
The University of Illinois Extension explicitly recommends against using straight garden soil for starting seeds indoors, citing drainage, compaction, and disease risk. The same logic applies to unmixed compost: fine as an ingredient at the right ratio, problematic as the sole medium.
The safe ratio for finished compost in a seed starting mix is 20–25% by volume — enough to contribute nutrients and beneficial microbes without creating the rich, moist conditions that favor pathogens.
The Ingredients and What Each One Does
A good DIY seed starting mix requires three types of ingredients working together: a fine organic base for moisture and nutrients, an inert drainage material for aeration, and optionally a moisture-retaining fiber. Here's what's available and why each one matters.
Finished compost (screened)
Your homemade compost — sifted through a fine screen to remove chunks — contributes slow-release nutrients, beneficial microbial activity, and moisture retention. Screen it through hardware cloth or an old colander before using. Chunks and unfinished pieces belong back in the pile, not in a seed tray. Use at 20–25% of total mix volume.
Perlite
Expanded volcanic glass that looks like small white pellets. Perlite creates air pockets in the mix, prevents compaction, and ensures drainage even if a tray is slightly overwatered. It doesn't hold nutrients and doesn't decompose. MSU Extension includes perlite as a standard component of quality seed starting mixes. Use at 30–35% of total mix volume.
Vermiculite
A naturally occurring mineral that retains moisture and provides some aeration. Vermiculite holds more water than perlite, making it useful in mixes for seeds that need consistently moist conditions during germination. Some recipes use it in combination with perlite; some use one or the other. UNH Extension's standard seed starting recipe uses vermiculite as the primary drainage amendment.
Coconut coir (or peat moss)
A fiber that holds moisture and provides structure without compacting. Coconut coir is the sustainable option — it's a byproduct of coconut processing, renewable, and performs comparably to peat moss. Peat moss is still widely used and available but comes with environmental concerns related to harvesting from non-renewable bogs. OSU Extension notes that both have environmental consequences; coir is the preferred choice for sustainability-conscious gardeners.
Worm castings (optional upgrade)
If you're running a worm bin, adding a small amount of worm castings (no more than 10–15% of total volume) improves both nutrition and microbial diversity in the mix. Worm castings are gentler than standard compost — they're less likely to burn seedlings — and their microbial population can actually suppress damping off. This is one of the best direct uses for vermicompost in a home garden.
Three Recipes: Simple to Complete
Recipe 1: The Basic Two-Ingredient Mix
Adapted from Prairie Road Organic Seed's approach — as minimal as it gets while still working reliably.
- 1 part screened, finished compost
- 1 part coarse vermiculite
Mix thoroughly. This works well for experienced gardeners who know their compost is fully finished and properly screened. Fast to make, low cost. Less forgiving of overwatering than mixes with perlite.
Recipe 2: The Standard Three-Ingredient Mix
Based on the OSU Extension formula recommended by horticulturist Brooke Edmunds and aligned with the MSU Extension recommendation.
- 1 part screened, finished compost
- 1 part perlite or coarse sand
- 1 part coconut coir (rehydrated) or peat moss
Mix compost and coir first, then add perlite and combine thoroughly. This is the most versatile recipe — good drainage, good moisture retention, low enough nutrient level to be safe for even small or sensitive seeds. This is my default for most vegetable and herb seeds.
Recipe 3: The Worm Casting Upgrade
For gardeners running both a compost pile and a worm bin, this mix combines both outputs.
- 2 parts coconut coir
- 1 part perlite
- 1 part worm castings
No screened compost required — the worm castings provide the organic fraction. This mix has more consistent microbial activity than standard compost-based mixes and is gentler on sensitive seedlings. Particularly good for tomatoes, peppers, and herbs that will spend several weeks in trays before transplanting.
How to Use Your Seed Starting Mix
Moisten before filling trays
Mix should be damp — like a wrung-out sponge — before you fill your trays. Dry mix added to trays and then watered from above compacts unevenly and can displace seeds. Add water gradually while mixing until the mix holds together when squeezed but releases no dripping water.
Fill trays, press lightly, sow seeds
Fill cells or trays to within ¾ inch of the top. Press gently to eliminate air pockets without compacting. Sow seeds at the depth specified on the packet — generally 2–3 times the seed's diameter. Very small seeds (lettuce, basil, thyme) can be surface-sown and pressed gently into contact with the mix.
Cover to retain moisture during germination
Cover trays with a clear plastic dome or slip them into a clear plastic bag until germination. This maintains the consistent moisture seeds need without requiring frequent watering. Remove the cover as soon as the first seedlings emerge — continued high humidity after germination is the primary trigger for damping off.
When to start fertilizing
Seeds don't need fertilizer. Seedlings don't need fertilizer until their first true leaves are well-developed — typically two to three weeks after germination. At that point, a half-strength soluble organic fertilizer applied weekly is appropriate. UNH Extension notes that young seedlings are easily damaged by too much fertilizer, especially under any moisture stress.
Seed Starting Mix vs. Potting Soil: When to Switch
Seed starting mix isn't designed for the long term. Once seedlings develop their first true leaves and you're moving them to larger containers before outdoor transplanting, the mix changes.
| Stage | Right Medium | Why |
|---|---|---|
| Germination → first true leaves | Seed starting mix | Fine texture, low nutrients, good drainage |
| Potting up to larger containers | Potting mix with compost | More nutrients needed for active growth |
| Transplanting to raised beds or garden | Garden soil amended with compost | Full nutrient profile for mature plant production |
For the "potting up" stage, the Almanac recommends a mix of 2 parts compost to 1 part coir with added perlite — essentially a richer version of the seed starting mix. At this stage, worm castings or a slow-release organic fertilizer can be added without risk of burning seedlings that now have an established root system. If you're transplanting directly into a raised bed filled with quality soil, the seedlings will find everything they need once their roots reach the bed's growing medium.
Frequently Asked Questions
Can I reuse seed starting mix from last year?
Not for germination. Used mix may harbor pathogens from last season, especially if you had any damping off. It also compacts over time, reducing the aeration that seedlings need. Used seed starting mix works well added to garden beds or mixed into larger container plantings — just don't put it back in seed trays. Start each season with fresh mix.
Do I need to pasteurize my homemade compost before using it?
OSU Extension recommends pasteurizing compost before using it in seed starting mix — this involves heating moist compost in an oven at 180–200°F for 30 minutes to kill pathogens and weed seeds. It's the conservative approach. In practice, many gardeners skip this step with fully finished hot compost and have good results. If your compost came from a well-managed hot pile that reached 130°F+, pathogen and weed seed kill is likely already complete. If it was a passively managed cold pile, pasteurizing is worth the extra step.
My seedlings are falling over at soil level. What went wrong?
This is damping off — a fungal disease that girdles seedlings at the soil surface. It spreads rapidly in warm, humid, poorly ventilated conditions. Causes: overwatering, using non-sterile or compost-heavy mix, keeping dome covers on too long after germination, or poor air circulation. Once damping off appears, affected seedlings cannot be saved. Remove them immediately to prevent spread. For remaining seedlings, improve air circulation, reduce watering, and dust the soil surface lightly with cinnamon (a natural antifungal) or fine sand.
How far in advance can I mix and store seed starting mix?
Dry ingredients can be mixed and stored indefinitely in a sealed container in a cool, dry place. Don't pre-moisten and store — moisture activates microbial activity and can lead to mold growth or pathogen buildup over weeks of storage. Mix dry, store dry, moisten just before use.
The Bottom Line
Seed starting mix is one of the most direct ways to put your homemade compost to use. Screened, finished compost at 20–25% of total volume gives seedlings a biological head start without the risks that come from using it at higher concentrations or in unfinished form.
The setup is inexpensive: coconut coir, perlite, and vermiculite are all available at any garden center and a small quantity goes a long way. Once you make your first batch, you'll have the ingredients to make more for years. And unlike bags of commercial seed starting mix, you know exactly what went into it.
What seeds are you starting this spring? Drop a comment — I'm especially curious whether anyone is using worm castings in their mix and whether they've noticed a difference in germination rates or seedling vigor compared to compost-based recipes.
— Ku
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