Step 2a: Understand What Makes Great Seed-Starting Soil High-quality seed-starting soil is light, sterile, well-drained, and moisture-retentive—not rich or heavy. Seeds already contain the nutrients they need to sprout; the soil’s role is to manage air, water, and cleanliness. Heavy garden soil or compost can suffocate seeds and promote disease. Many gardeners rely on proven, garden-center-approved seed-starting mixes to remove guesswork. For DIY growers, a trusted basic recipe by volume is 50% peat moss or coco coir for moisture retention, 25% perlite for drainage, and 25% vermiculite for aeration—creating the perfect environment for germination.