Contributed by Kerry Lake, Master Gardener
What is Potting Soil? Potting Soil is the potting material for growing plants in containers. The containers can be seed tray for starting seeds, small pots for transplanting seedlings, plastic or clay pots for house plants, or any size for plants to be grown in green houses or on a patio, deck, or porch. Potting soil can contain soil (minerals, sand, clay, loam, compost, aka organic material) or contain non-soil material:
Specific plants may need specific types of ‘potting soil’ to grow and prosper. Examples include seed starting which requires a fine textured sterile soil with the addition of a small amount of fertilizer, African violets which require a mix that has more peat moss, cactus and succulents which require a sandy mix, and plants to be grown on the porch or patio, such as herbs and vegetables that require a different potting mix than our house plants.
- Sphagnum peat moss from the US and Canada
- Coconut coir from coconut husks which like peat moss will hold water and then slowly release it back into the potting mix
- Perlite, a mineral derived from volcanic rock that has been crushed and heat treated and is used to increase the air space in the potting mix
- Vermiculite, a mineral that resembles mica. It is used in potting mixes to increase water and nutrient retention, and aerate the soil
- Bark ‘fines’, a byproduct from the lumber industry
- Commercial fertilizers or organic composted material
Commercial potting mixes are sterilized. Commercial non-organic potting mixes may also contain additives, such as limestone to balance the pH, chemicals to improve the absorption of water in the mix, and even water-retaining gel. These type of potting mixes were designed for large containers on a deck, window boxes, or hanging baskets as the mix is lightweight and may not dry out as quickly as other potting material.
Can you make your own Potting Mix? You can make your own basic potting non-soil by mixing one gallon of moist, coarse sphagnum peat moss with up to one gallon of coarse sand, perlite, or vermiculite. When using peat moss as the basic ingredient of your non-soil potting mix, be sure to water the mix prior to planting. Using very warm water allows the peat to absorb the water quickly.
David Culp in his book, The Layered Garden, suggests using a potting mix of coarse turkey or chicken grit, coarse sand, and then sterile soil, approximately 1/3 of each in the final mix. He reminds us that shade plants will require more soil in this mix for their roots.
In the Ultimate Container Gardener, author Stephanie Donaldson details her 5 different types of potting mix for different plants:
Peter McCoy keeps it simple in his book How to Garden. He recommends using the peat-based potting mixes for window boxes and hanging baskets, and loam (soil and compost) based mixes for most plants in containers due to the higher nutritional value.
- Standard Mix of peat-based with added fertilizers.
- Ericaceous Compost is peat-based with no added lime, which is important for heathers (Erica), camellias, rhododendrons and azaleas when grown in containers. These plants require an acidic planting medium.
- Container Compost is peat-based with added moisture-retaining granules and fertilizer, which she recommends for window boxes.
- Peat-Free Compost using renewable resources such as coir fiber instead of the peat moss.
- Loam-based Compost which has sterilized loam as the main ingredient, and added fertilizers to supplement the nutrients of the loam. This is heavier than the peat-based mixes but is ideal for long-term plantings, such as small shrubs.
What is Organic Potting Soil and how does it differ from other potting mixes? Organic Potting Soil does not have addition of chemicals and commercial fertilizers. Any additions to the organic potting mix should be produced from composted material. A good organic mix should include decaying plant material, microorganisms, worms, and other living soil organisms, in other words, like the good compost from your own compost bin. For an organic approach for your large containers, add 2 quarts of organic material (compost) instead of commercial fertilizers to the basic home-made potting mix described above for your herbs and vegetables that you will be growing on your patio or deck.
Which Potting Mix should you use? Know the requirements of your plants when choosing a potting material. Do you prefer an organic or non-organic mix? Would you like to make your own potting mix? Will the container for these plants be in the sun or the shade? If in the sun, are you able to water daily? If you use a potting mix with water-retaining features, be sure to use a water gauge prior to your daily watering. While the soil may feel dry at the top of the pot, the root level may already have just the right amount of moisture, and not require watering again for a day or two. A plant that is wilted may be suffering from not enough water, or too much water; use a meter to determine the true cause.
No matter what type of potting soil you use, it is recommended to start fresh each year whenever possible (Shrubs in containers would appreciate a little top dressing of new potting soil each spring). Put last year’s soil medium in your compost pile, clean the containers thoroughly and start with new potting mix in the spring.
The Layered Garden by David Culp, published by Timber Press
The Ultimate Container Gardener by Stephanie Donaldson, published by Hermes House
Container Gardening for All Seasons, published by the Reader’s Digest Association Limited
How to Garden by Peter McHoy. Published by Lorenz Books.
Low Maintenance Gardening by Sunset Publishing Corporation