Seed starting instructions invariably instruct you to use a good soilless potting mix. What kind of potting mix doesn’t have soil, and what’s wrong with soil anyway?
Starting Seeds in Garden Soil
You certainly could use soil directly from your garden to start seedlings indoors, but garden soil comes with two major disadvantages:
- It contains undesirable elements: Disease and fungal spores, bacteria, nematodes, insects, weed seeds, and other unwanted components can easily hitch a ride with your garden soil. Natural predators and weather phenomenon help keep these things in check in the garden. To use garden soil indoors, you have to sterilize it with some sort of heat treatment (not recommended and difficult to achieve).
- It lacks drainage: Garden soil tends to be somewhat heavy and without tilling, either by you, earthworms, or other insects, it begins to compact after several waterings. Soil compaction is especially hard on the tender roots of young seedlings just getting established.
A soilless mix provides a cleaner medium and gives you more control. Besides being free of pests and diseases and other contaminants, you can blend in additional ingredients for preferred drainage, water retention, nutrition, and airspace. A soilless mix is also lighter in weight than garden soil, which you’ll appreciate when you have to lift and move containers.
Most soilless mixes are predominantly comprised of sphagnum peat moss. Sphagnum peat moss is lightweight and inexpensive. Just as importantly, it’s well-draining yet water retentive. Granted, until the peat is thoroughly moistened with water, its particles can be very dry and dusty. Peat moss is slightly acidic, and most seed starting mixes have a soil pH of about 5.8, which is fine for starting most seeds.
In nature, peat takes hundreds if not thousands of years to form. Alternatives to peat, such as coir, are being investigated. In the future, expect to see potting mixes come onto the market that omit peat altogether.
Amendments to Soilless Mixes
- Bark is added to improve drainage and airspace within the mix. However, bark will also decrease water retention. Bark mixes are better suited for mature plants that need to dry out between waterings and are often used to grow orchids. Bark mixes are not suitable for starting seeds.
- Coir is a coconut fiber by-product and works similarly to peat by providing good drainage while also retaining water. Coir is becoming a substitute for peat moss.
- Perlite is that white stuff that looks like pebbly StyrofoamTM. It’s a volcanic mineral, although it does not affect nutrient quality or soil pH. Perlite enhances drainage and air and water retention. Perlite is sometimes used in outdoor gardens to prevent sandy soil from leaching nutrients.
- Vermiculite is the golden-brownish flecks you see in potting soil. It’s a silicate material that has been heated and expanded to increase its water holding capacity. The particles soak up water and nutrients and hold them in the mix until the plants are ready to access them. You can also use vermiculite to cover recently-sown seeds to keep the seeds moist enough to germinate. When seeds are covered with soil, there is the potential for a hard crust to form if the soil dries out. You might see vermiculite for sale at home improvement stores to use in insulation or plaster. This grade vermiculite is not suitable for potting mixes because it does not absorb water easily. Purchase vermiculite in a local garden center or in the garden section of home improvement stores.
Fertilizer and Trace Elements in Soilless Mixes
- Seeds don’t require fertilizer to germinate, so the fertilizer is being wasted if you are using it for seed starting. By the time the seedlings have grown true leaves and require supplemental food, the fertilizer in the mix has begun to dissipate.
- Wetting agents are becoming increasingly popular. That’s understandable if you’ve ever worked with straight peat moss. Wetting agents are polymers added to the soil to greatly improve their water-absorbing ability. Certified organic wetting agents might not be possible, perhaps because by nature, a soil wetting agent can’t be quickly bio-degradable or they’d be useless. You can be successful without a wetting agent but make sure the mix is well moistened before placing it into pots or cell packs and don't let the soil dry out. A best practice is to water seed starting containers from the bottom by adding water to the tray in which the containers are sitting. Doing so enables water to be absorbed from the bottom of the containers and reduces the chance of seeds rotting from too much moisture.
- You might also see amendments in the mix, such as limestone or gypsum, to adjust the pH . Mixes will vary by manufacturer and region. Occasionally a particular plant will favor certain amendments over others, but for seed starting, a basic mix is generally sufficient. These will be labeled for seed starting or as a starter or germination mix.
The best way to evaluate a potting mix is to observe how well your seedlings grow. If your seeds have a high rate of germination, and the seedlings grow strong and green, all is well. Otherwise adjust your soilless mix. The first place to start is to make sure the soil pH is at the right level.
A soilless potting mix is preferable to using outdoor garden soil for several reasons, but if you need a large amount of mix or need a special blend, it is often easier to create your own potting mix.
Soilless Potting Mix Recipe
Potting Mix with Compost Recipe
2 parts compost
2 to 4 parts sphagnum peat moss or coir
1 part perlite
1 part vermiculite
Adding Nutrients to a Potting Mix
Add ½ cup each per every 8 gallons of the potting mix:
½ cup bone meal (for added phosphorous)
½ cup dolomitic limestone (raises soil pH and provides calcium and magnesium)
½ cup blood meal, soybean meal, or dried kelp powder (for added nitrogen)