Formula for Mixing Starter Soil

Soil-less Mixes

• Soil-less mixes for germinating seeds are available commercially but can also be mixed at home using 4 qt. each of peat moss or shredded pine bark and vermiculite or perlite, 1 tbsp. of superphosphate and 2 tbsp. of dolomitic or ground limestone. This mixture alone has little fertility, so either water seedlings with a diluted fertilizer or include a few ounces of a complete fertilizer in the mixture. Peat moss mixes most easily with other ingredients if it is slightly moistened.

Vermiculite

• Vermiculite by itself is a suitable germination medium. This mica-like mineral is naturally-occurring and has been treated with heat. Vermiculite does not contain soil and therefore does not require sterilization as long as it is not contaminated during handling. A fine grade of vermiculite is best for seeding by itself or in soil mixtures. This material is good for germination because it can cover seeds but is not crust-forming and allows seedlings to emerge easily. Avoid over-watering when vermiculite is used.

Pasteurized Soil Mix

• Vegetable and flower seeds are started using a simple, three-ingredient medium that includes sterilized soil. One possible mixture consists of one-third sand, perlite or vermiculite, one-third peat moss and one-third pasteurized soil or finished compost. Before adding soil to a mixture and sowing seeds, the soil must first be sanitized using one of multiple possible methods.

Sanitizing Soil

• Soil from the garden or already-mixed media that contains ingredients like manure, peat moss or compost need sterilization. The soil should be moist but not wet before treatment in the oven. Spread soil no more than 4 in. deep in a nonplastic container and cover containers tightly with aluminum foil. Keep the oven temperature between 180 and 200 degrees Fahrenheit for 30 minutes. After this period of time, remove the containers, allow them to cool and leave the foil on the containers until the soil is ready to use. You can also use a microwave oven. Fill microwavable containers with very moist soil or medium and microwave on full power for 90 seconds per each 2.2 pounds. You can also use a pressure cooker or steam sterilization without pressure.