Container gardening is a great way to make an otherwise barren deck, porch or patio colorful, inviting and productive. But there are so many potting mixes on the market that it's hard to choose the right one.
The good news is that most commercial potting mixes will be at least adequate -- and usually better than adequate -- for growing most plants. But there are differences among the mixes. Learning a little bit about how plants and soil interact can help you pick the right mix.
What roots want
When a plant's roots grow into the soil (or a potting mix), they're not just searching for water. They're seeking air, too. Without air, plant roots quickly die. That's why good gardeners value well drained potting mixes. Water flows quickly through well drained mixes, leaving behind pockets of air. And the best of mixes strike a balance between water retention and water drainage.
But while well-drained soil is a boon for most plants, not all plants thrive in it. Some plants, such as orchids, need an extremely well-drained potting mix. Others, such as willows, prefer to have their feet wet, so don't need good drainage. Before you select a potting soil, make sure you know what your container plants need.
In the mix
The soil from your garden rarely makes for a good potting mix for container planting. Although it can support plant life in your yard, when used in a container this same soil typically holds too much water. In addition, garden soil often contains diseases that can infect the roots of your plants, especially if the plant is already weakened because of poor drainage.
By using commercial potting mixes, you can avoid these problems. Most potting mixes contain a large quantity of organic (carbon-containing) material, which does a good job of both holding and draining water.
Today, most potting mixes are made up primarily of peat. Although peat is excellent for containers, some gardeners oppose its use because it's mined from ecologically sensitive areas, which may be damaged in the mining process.
If you're one of those people, look for peat-free potting mixes. Made primarily of compost, bark (usually pine bark) and coconut coir, these mixes perform just as well as peat, but tend to be slightly more expensive.
Other materials are often added to container mixes, usually to improve drainage. They include perlite (a type of volcanic ash) and vermiculite (a natural mineral that's heat-treated so it holds more air). Sand is sometimes used, as well. While it improves drainage only slightly, sand does add weight to a container, which can keep it from tipping over.
If you discover that a potting mix you bought doesn't drain as well as it should, mix in some perlite or vermiculite. (One part perlite to five parts potting mix is usually sufficient.)
Most potting mixes now contain some fertilizer. (If you're an organic gardener, you can find mixes that incorporate organic fertilizers instead of chemical-based ones. Look for mixes with an OMRI label, meaning they've been listed by the Organic Materials Review Institute.)
It's best to find a potting mix that includes a time-release fertilizer, which lasts longer. Even so, most time-release fertilizers last only two to three months, which means you'll need to feed after that.
If you don't use a potting mix with fertilizer in it, you should feed your container plants on a regular basis. (Nutrients are washed out of containers when they're watered.)
Some potting mixes boast that they contain "moisture control" ingredients, which supposedly allow you to water less frequently.
There are two ways that a mix could do this. First, it could include composted coconut coir, which holds more water than peat. Or it could contain crystals that hold water, then release it into the potting mix as the mix dries out. So far, these crystals have not been shown to work consistently. So it is unclear whether they can actually reduce the need to water.
Jeff Gillman, an associate professor of horticulture at the University of Minnesota, is the author of several gardening books.