Have you ever found yourself at the nursery or big box store standing before shelves of bagged potting soil wondering what to buy? There are so many choices. It all depends on what you are using it for.
Bagged garden soil and topsoil belong outside, in the ground. They are too dense for use in containers.
Potting soil probably won’t even have the word “soil” in the name because there is no actual soil included. Instead, it will say potting mix or potting medium. That tells you it is formulated for containers.
Peat comprises the majority ingredient, by volume, in potting mixes. The remainder of the mixture can include one or more of a variety of ingredients. Perlite, vermiculite, sand or grit, lime, bark, charcoal, wood fibers, wetting agents, and fertilizers are the usual additives.
Good potting mixes provide an open structure allowing for the passage of oxygen to the roots, while also retaining water so it is available to those same roots. Peat is ideal for that.
If the potting mix is formulated for plants needing fast drainage like cacti and succulents, it will have bark and sand included. Conversely, if you are growing plants that need a lot of water, vermiculite, a good moisture retainer, will be among the ingredients.
Peat is slightly acidic, so to balance the pH lime may be added.
If you are starting seeds in containers, which we will in a month or two, look for a mixture labeled “seed starter” mix. It is sterile to help keep pathogens from harming delicate seedlings. It is also formulated to be conducive to easy root formation.
There are specialty mixes for growing specific plants. I’ve noticed bags of potting medium for African violets, cacti, and orchids.
Until recently, peat was the only choice as the anchor ingredient in potting mixes. Peat is formed over hundreds of years from layers of partially degraded organic matter. In some countries, peat is becoming rare as we continue to harvest it. Also, it is feared the act of harvesting contributes to the release of carbon into the atmosphere, thought to be one cause of climate change.
What is an eco-friendly gardener to do? Enter coir (pronounced koy-er), a product made from the outer husks of coconuts. It used to be a waste product but is rapidly becoming a replacement for peat. Mexico, India, Sri Lanka, and Costa Rica are the major exporters to the U.S. and Europe.
To turn coir into a usable product, it is first washed to remove natural salts. Next, it is dried and compressed. All the gardener needs to do is soak it to turn it into a water-retaining medium to replace peat. However, at present coir is about double the price of peat so it is not yet readily available in potting mixes. You could always try mixing up your own potting medium as many nurseries do.
University of Illinois Extension Master Gardeners of Edgar County is open for questions at 217-465-8585.