Seed starting instructions invariably say 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 Soil from the Garden
You certainly could use soil right from your garden to start seedlings indoors, but garden soil comes with two major disadvantages:
- You don’t know what’s coming in with it: Disease spores, bacteria, plant-eating insects, weed seeds, and other unwanted materials can easily hitch a ride with your garden soil. There are all kinds of natural predators and weather phenomenon outdoors that help keep these things in check. To use this soil indoors, you’d need to sterilize it first, with some sort of heat treatment.
- Lack of 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. This compaction is especially hard on the tender roots of young seedlings just getting established.
A soilless mix gives you more control. Besides being free of disease and other contaminants, you can blend ingredients for preferred drainage, water retention, and airspace. It is also lighter in weight, which you’ll appreciate when you have to move the pots outdoors.
Soilless Potting Mix
Most soilless mixes are predominantly sphagnum peat moss. Sphagnum peat is lightweight and inexpensive. Just as importantly, it’s well draining yet water retentive. Granted, until you get the peat thoroughly moistened, the particles can be very unpleasant to work with. Peat is also on the acidic side, and most seed starting mixes have a soil pH around 5.8, which is fine for starting most seeds.
Peat takes hundreds of years to form, alternatives, like coir, are being sought. Expect to start seeing more potting mixes that omit peat altogether.
Amendments Added to Peat
- Bark: Bark is added to improve drainage and airspace within the mix. This means it will also decrease the water retention slightly. Bark mixes are better for use with mature plants that need to dry between waterings than for starting seeds.
- Coir: Coir is a coconut fiber by-product and works similarly to peat in providing good drainage while also retaining water. As mentioned above, coir is becoming a substitute for peat.
- Perlite: Perlite is that stuff that looks like pebbly Styrofoam. It’s a volcanic mineral, although it does not affect the nutrient quality or the pH of the mix. It does add in drainage and in air and water retention, that magical balance. In fact, it is sometimes used in outdoor gardens to prevent sandy soil from leaching nutrients.
- Vermiculite: Vermiculite is those silvery-gray flecks you see in potting soil. It’s a mica-type material that is heated up 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. Perlite is also good as a soil covering for seeds that need to remain consistently moist to germinate. You may see vermiculite for sale at home improvement stores, for use in insulation or plaster. This grade vermiculite is not really suitable for potting mixes since it does not absorb water easily.
Fertilizer and Trace Elements
- Seeds don’t require fertilizer for germination, so it is somewhat wasted if you are using it for seed starting. By the time the seedlings have grown true leaves and require supplemental food, whatever was 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. I don’t know of any certified organic wetting agents, perhaps because by nature, a soil wetting agent can’t be quickly bio-degradable or they’d be useless. I’ve always had good luck without a wetting agent but making sure my mix is well moistened before I put it into pots or cell packs and then, not letting it dry out. Watering containers from the bottom will help with this.
- You may also see pH adjusting amendments such as limestone or gypsum. 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 judge a potting mix is to see how well your seedlings do. If you get good germination and the seedlings start off a healthy green, all is well. Otherwise adjust your mix, starting with the pH.
A soilless potting mix is preferable to using outdoor garden soil for several reasons, but if you need a large quantity of mix or have a need for a special blend, it is often easier to simply create your own potting mix.
Basic Soilless Potting Mix
4 to 6 parts sphagnum peat moss or coir
1 part perlite
1 part vermiculite
Basic Mix with Compost
2 parts compost
2 to 4 parts sphagnum peat moss or coir
1 part perlite
1 part vermiculite
Basic Mix with the Addition of Nutrients
Add ½ cup each per every 8 gallons of the 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)