How to choose the best potting soil for the job
Q uestion: I keep reading that I’m supposed to be using a special potting soil to start my seeds that’s different from the kind of potting soil I’m supposed to use to fill my patio containers. What’s the difference between different potting soils and how do I know which one I should use?
Answer: Potting soil, also called potting mix, actually contains no real soil. Instead, it’s a soilless blend of ingredients that’s used to grow plants. Often containing a combination of peat moss, coir fiber, vermiculite, sand, perlite, pine bark, compost and other ingredients, there are dozens of brands of commercial plant potting mix on the market. The combination and ratio of each of the various ingredients is what determines both the quality and the type of potting soil.
All good-quality potting mixes should be easy-to-handle, well-draining and light-weight, but you’ll find that each brand has its own distinct features. There’s wide variation in the texture, nutritional content, water holding capacity and density of each different potting mix. This can make it difficult to know which brand is right for your needs.
When deciding which type of potting soil is best for the job, you have to start with the particular thing you’re going to be using it for.
Most commercial potting soils are formulated with a specific purpose in mind and the ratio of ingredients is determined to be the best way to accomplish that purpose.
If a potting mix was created specifically for seed-starting, for example, it will have a lighter weight and finer texture than potting soils intended for potting trees or shrubs. It’s also likely have a large percentage of sphagnum peat moss in the blend, because peat has natural fungicidal properties that help prevent damping off, a fungal disease that strikes young seedlings.
Seed-starting potting soil will not be too heavy for young seedlings to emerge through it, nor will it have large chunks of bark or other materials in it. And, seed-starting potting soil also has minimal fertilizer amounts in it, if it has any at all. This is because all the nutrition a new seedling needs is found in its endosperm, the fleshy material that’s inside the seed. Much like the yolk of an egg, the endosperm is a source of nutrition for the young seedling until it’s several weeks old. Having too much fertilizer in a seed-starting potting soil could cause fertilizer burn on tender young seedlings and seedling roots.
In contrast, a potting soil created for growing containerized trees and shrubs contains ingredients such as coarse sand, pine bark, and/or compost to add weight and make the texture coarser. It also often has higher amounts of fertilizer in it.
Potting mixes blended for succulents and cactus have a gravelly texture that drains quickly, a necessary trait for these plants that have evolved to thrive in well-draining, sandy desert soils. Orchid blends for repotting orchid plants consist mostly of bark chips since most orchids are epiphytes that grow in trees. There are commercial potting mixes formulated for just about any type of plant you can grow, from African violets and houseplants to shrubs and vegetable transplants.
But in addition to all of these specialized blends, there are also plenty of general, all-purpose potting mixes that are suitable for everything from container gardens to hanging baskets.
Start by considering what you’ll be growing and then choose your potting mix accordingly.
Horticulturist Jessica Walliser co-hosts “The Organic Gardeners” at 7 a.m. Sundays on KDKA Radio with Doug Oster. She is the author of several gardening books, including “Attracting Beneficial Bugs to Your Garden,” “Good Bug, Bad Bug,” and her newest title, “Container Gardening Complete.” Her website is jessicawalliser.com. Send your gardening or landscaping questions to [email protected] or The Good Earth, 622 Cabin Hill Drive, Greensburg, PA 15601.