Prepare this recipe now and your plants will thrive in spring.

It may seem too early in the year to be thinking of starting seeds for the spring vegetable garden, but preparing a soil mix now can help ensure successful seed sprouting and hardy young transplants in the early spring.

Creating a seasoned potting mix

Most commercial seed starting mixes are a sterile mix of peat and vermiculite, which contain no nutrients. This is because newly sprouted seeds are sensitive to nutrient and water imbalances. By adding nutrients to your potting mix now, and setting it aside until early spring, the components can ‘mature’, giving a real boost to newly sprouted seedlings.

How to create your own seed starter mix

To prepare a nutrient-rich soil mix for starting seeds, combine:

  • 10 parts sterile potting starter mix
  • 2 parts finished compost, screened through ¼” mesh
  • 1 part glacial rock dust

Mix the ingredients together in a plastic bucket or tote and leave the lid slightly ajar to allow some air movement. Allow your mix to sit for 6 – 8 weeks or longer before using.

About the ingredients

Each ingredient in your seed starting mix has a purpose. Some help retain moisture, while others help deliver nutrients directly to your plants.

Potting mix

Used for starting seeds, your potting mix should be a ‘sterilized’ mix of peat moss and vermiculite or perlite.

  • Vermiculite is a natural volcanic rock, finer than perlite, consisting of small porous crumbs which act like small sponges which absorb water and release it slowly into the soil. Fine-grade vermiculite is preferred for seed starting, and coarse vermiculite is used to improve water retention in light, sandy soils.
  • Perlite is a natural volcanic rock that’s heated until it expands into rigid granules filled with tiny holes. The granules absorb up to 4 times their weight in water and then release it slowly which is ideal for young plants. This slow release of water helps maximizes nutrient intake. Its rigid crush-resistant structure also helps improve heavy soils by reducing compaction while increasing aeration and drainage.
  • Peat Moss is an organic material consisting of shredded, partially decayed sphagnum moss. The peat moss that is commercially sold in the US, as a soil amendment, is typically the decomposed product of Canadian sphagnum moss which has been growing for thousands of years in wetland areas called peat bogs. Because these peat bogs are ecologically-important wetlands, peat should be used sparingly. Relatively small quantities are needed for seed starting mixes.
    For the garden transplant beds, look for peat substitutes such as coir, which is made of coconut fibre, a material in plentiful supply. Composted leaves and green manures also serve as peat substitutes.


Compost is the principal supplier of nutrients to your seed starter mix. Use the fine, crumbly ‘finished’ compost from your compost pile and sift it through a ¼” mesh screen before adding it to your mix. Even though the compost appears finished, it may still be too strong for new sprouts. By setting your seed starter mix aside for 6 – 8 weeks, the compost and the glacial rock dust combine to fully mature the compost for ready absorption by the young seedling roots.

Related: How to Use Finished Compost

Glacial rock dust

A natural quarrying by-product which contains a broad spectrum of minerals in a form readily available to soil microorganisms. Rock dust improves soil structure and moisture holding properties, and enhances plant root’s ability to absorb nutrients.

Rock dust also promotes bacterial action in a compost pile, speeding up compost breakdown. It raises pH in acidic soils, increases phosphorus availability and corrects mineral imbalances in the soil. For more information or to purchase, glacial rock dust is available online from Eartheasy.

This simple New Year practice of preparing and setting aside your own enriched starting soil is an easy way to jump-start your garden when planting time arrives. And for the morose winter-bound gardener, getting one’s hands in the dirt, even if it’s mostly peat, is a welcome harbinger of a long-awaited spring.

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Editor’s note: This article has been updated from its original text published January, 2009.

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