Give your seeds a good start with a homemade starter mix.

Formula for Mixing Starter Soil

by Janet Beal

Starting seeds indoors or in a cold frame can extend the gardening season, ensure healthy plants and expand your garden stock beyond nursery offerings. Growing your own seeds can save money as well, and you can increase the savings by making your own seed-starting mix from easy-to-obtain ingredients. Gardeners may differ on the best recipe, but a basic formula and some possible additions will help you create your own seed-starting mixture.

1. Seed-Starter Requirements

Usually soil-free, seed-starter mix must be both dense enough to support young plants and porous enough to maximize the growth of fragile roots. A light texture is essential to ensure drainage, but the most important quality of seed-starter mix is its moisture-retention capacity. This is one reason to keep starter mix soil-free. Even good garden soils can become crusty if allowed to dry, making it hard for seeds to sprout.

2. Traditional Basic Formula

For the most basic light-textured moisture-retaining seed-starter, combine 2 parts peat moss, 1 part vermiculite and 1 part perlite. Peat moss or sphagnum peat moss is lightweight, dried vegetable fiber with good moisture-holding properties. Vermiculite is a heat-expanded mica product, which also holds water. Perlite, a heat-expanded form of volcanic obsidian, maintains the air spaces critical to new root development as well as adequate water in the mixture. Although the mix is low in nutrients, it provides a weed-free, disease-free medium that sustains seeds for the usual four- to six-week period they need to become seedlings.

3. Replacing Perlite and Vermiculite

Coarse builder's sand can be used in place of perlite, part for part. Sand provides good drainage and also makes the mixture heavier. The extra weight may be desirable with long-stalked seedlings such as bush beans (Phaseolus vulgaris), which can get tippy as they shoot up. Environmentally concerned gardeners point out that perlite is a non-replaceable mined material, while sand is constantly renewing. The same concern applies to vermiculite, which is a by-product of mining mica. For lightweight vermiculite, some gardeners substitute rice or other grain hulls, or ground newspaper.

4. Replacing Peat Moss

Like perlite, bog-harvested peat moss is the product of centuries of change and is being used at non-renewable rates. While sphagnum peat moss is more renewable, coir fiber from coconut husks is becoming increasingly available as a sustainable substitute. Organic gardeners also substitute rotted sawdust as a homegrown replacement for non-renewable peat moss and non-native coir.

5. Adding Compost

While traditional seed starter contains no fertilizer or other nutrients, adding 1 part compost to your mixture may give your seeds a better start and can sustain them for a longer time if transplanting is delayed by the weather. Compost is avoided by some gardeners because its exact nutritional content cannot be determined and, unless properly aged, it may contain disease-producing soil organisms or still-viable weed seeds. Organic gardeners note that commercially sterilized soil-starter mixtures, even if they contain compost, are no more nutritious than traditional basic starter. Pasteurized compost, however, heated to 180 F rather than the 212 F required for sterilization, retains many nutrients. To pasteurize compost, bake approximately 1 quart at 180 F for 30 minutes. You will still not be completely certain about nutrients, but pasteurization will kill pathogens and weed seeds. With experience, you may add more or less compost, depending on what kind of seeds you are growing.

6. Seed-Starter vs. Potting Soil

Some gardeners add screened garden soil to their mixture, while others advocate adding slow-release fertilizers. Both additions can be useful if you plan to use the same mixture for starting seeds and growing container plants. Soil and fertilizer nutrients will be needed to sustain maturing container plants, which will also benefit from a heavier mixture. You don't need to add soil or fertilizer if your mixture is strictly for starting seeds.

7. Revised Basic Formula

Factoring in a wide variety of opinions, an enhanced basic seed-starter formula would contain 1 part peat moss, sphagnum peat moss, coir fiber or rotted sawdust, 1 part vermiculite, rice or other grain hulls, or ground newspaper, 1 part perlite or sand, and possibly 1 part pasteurized compost, garden soil or leaf mold.

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