How to make your own potting soil

CORVALLIS, Ore. – It may be too early to put seeds in the ground, but itchy gardeners can get ready to grow by mixing up a batch of clean potting soil for starting seeds. Mixing up your own is more economical than buying sterile potting mix at a garden store.

A good germinating medium is fine textured and free of pests, diseases and weed seeds. It should be low in fertility and soluble salts and capable of holding and moving moisture.

But beware: Soil straight from your backyard just won’t do the job, says Barb Fick, home horticulturist with the Oregon State University Extension Service.

Typical backyard soil is compacted and full of weed seeds. Native soil may not drain as well as potting mixes, and it can develop a crust that prevents seedlings from pushing though the surface. And if it's not pasteurized, it can cause diseases in seedlings.

Fick’s recipe for a good basic pasteurized soil for starting seedlings is a mixture of one-third pasteurized soil or finished compost, one-third sand or perlite, and one-third peat moss.

You can use your oven to pasteurize a small quantity of seedling soil. Put slightly moist garden soil or compost in a heat-resistant pan and cover with a lid or foil. Place in a 250-degree oven with a food thermometer, to ensure that the mix reaches a temperature of 180 degrees for a full half-hour. Avoid overheating it, as the structure of the soil may be damaged.

Sand, peat moss and perlite are available at most nurseries and garden stores, and a mixture of half peat moss and half perlite or sand also works well, according to Fick.

Another task is to clean your pots, trays and flats in preparation for planting. Scrape old dirt from containers, and then rinse them in a solution of one part chlorine bleach to 10 parts water to kill remaining plant disease microorganisms that could invade your tender young seedlings.

Was this page helpful?

Related Content from OSU Extension

Ask an Expert

Have a question? Ask an Expert!

Ask an Expert is a way for you to get answers from the Oregon State University Extension Service. We have experts in family and health, community development, food and agriculture, coastal issues, forestry, programs for young people, and gardening.