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What’s in the Bag?
by Beth Botts - posted 05/02/12

Potting soils are not all the same. It pays (literally) to pay attention to the contents.

It was a heartbreaking moment in August when I finally gave up on my container vegetables. In previous years I’d had a bounteous harvest of tomatoes, herbs and greens, but last year everything that didn’t rot was either stunted or sick.

The reason was obvious: The potting mix was as dense as a damp brick. The tomatoes had scarcely developed beyond the 4-inch root balls they had started with; new roots simply couldn’t find their way out. Water could not drain away and air could not penetrate the packed particles to reach the roots, inviting rot and fungal diseases. I had filled all the containers in the spring with new potting mix I had bought hastily on sale, and I’d chosen badly.

Potting mix isn’t at the top of our minds when we plan containers. We’d rather think about the plants. Just as in the ground, though, if we don’t get the soil right, the plants won’t thrive. The principle holds for both indoor and outdoor gardening.

But what is a potting mix, exactly? How is it different from garden soil?

And how can you make sure you get a good one?

What you put in a pot needs to be substantial enough to anchor the plant’s roots and support the weight of the stem, branches and leaves. It needs to absorb water to supply the plant’s needs, but it also needs to be extra porous so surplus water can drain away — especially outdoors, where it rains. The mix needs to have large particles that don’t fit together too tightly, leaving spaces where air can filter in and roots can reach out.

In earlier times, gardeners filled pots with their own mixtures of compost or manure and garden soil, according to Mark Highland, president of Organic Mechanics in Modena, Penn., which makes organic compost-based potting soils.

That changed in the 1950s when commercial growers switched to mixes based on sphagnum peat moss harvested from bogs. Peat held water well and could be heated and sterilized to stave off disease problems in greenhouses. To make the fine, dense peat more porous, it was mixed with bits of vermiculite, a fibrous mineral, and perlite, little pellets of clay heated until they pop like popcorn. The mixture had few nutrients, but that was fine with growers; they preferred synthetic fertilizers, which could be carefully measured and controlled. And the mix was consistent and reliable from batch to batch.

Peat-based mixes were soon marketed to gardeners and became the standard for container growing. Today, most potting mixes are still variations on the formula.

They’re called “mixes” or “growing media” rather than “soil” because they don’t contain mineral matter — the clay and sand that make up the bulk of garden soil. That’s much too dense for containers, where water can be trapped and drainage is so crucial. If you see a bag labeled “potting soil” with a cheap price, it’s a red flag; the bag is likely to contain heavy topsoil, often little better than construction debris, perhaps mixed with plastic foam pellets that can easily be mistaken for perlite.

This can be tricky, though. Organic Mechanics products, which contain no mineral matter, are nonetheless labeled “potting soil.” They are compost-based, rather than peat-based, part of a new wave of products that is moving away from the 1950s formula.

Even Scotts, the nation’s largest producer of garden products, is using less peat and more composted bark, landscape waste, agricultural byproducts and coir, a fiber derived from coconut shells.

Coir has a remarkable ability to absorb water. Dry, it can be packed in lightweight bricks that are compact and cheap to ship from the tropics. Wet, it swells to about three times its dry volume. Yet even water-filled particles of coir remain discrete, leaving spaces for air. “It’s a good balance,” says Kayeon Jeong, Scotts’ senior scientist for growing media.

One reason to use less peat is controversy over whether it is sustainable. Environmentalists say the peat harvest strips bogs and destroys natural ecosystems; Canadian peat producers say they replant as fast as they harvest; and scientists have reached no consensus.

There can also be problems with supply. Eastern hurricanes greatly disrupted the peat harvest this last year. But potting mix producers are also concerned about the fuel and emissions costs of trucking around the materials as well as the bags of finished potting mix. The trend is to make potting mixes locally from local materials.

Scotts makes potting mixes in 29 different manufacturing plants around the country, supplementing peat with coir and whatever is available locally — landscape waste and pine bark in the Midwest, rice hulls in the South and in California. The mixes are carefully tested to make sure they will all perform consistently no matter where you buy them, Jeong says.

Consistency is a big issue in potting mixes. Plants trapped in pots have few defenses against poor conditions or disease, and we gardeners don’t like surprises. That’s a big reason to choose a good commercial mix rather than using your own compost.

Organic Mechanics potting soil is also made in local facilities (around here it comes from Midwest Trading in West Chicago). The major ingredients are commercially made compost, which is much less likely to carry pathogens or weed seeds than home compost; worm castings for nutrients; coir to hold water; and ground bark particles to keep the mix porous, Highland says.

But it’s not sterile. Part of the philosophy of Organic Mechanics is to create a more natural environment in the pot, with plenty of beneficial bacteria, nematodes and other microorganisms, much like the ones that make good garden soil. “The beneficial biology, if it’s there, can outcompete many disease organisms,” Highland says. In a sterile mix, the plant has fewer defenses.

The one time you do want a sterile environment is when you’re starting seeds. Tiny sprouts are nearly defenseless against fungal diseases, so it’s always best to buy a sterile soilless seed-starting mix.

You can find potting mix for every purpose: outdoors, indoors, orchids (big bark chunks), African violets (mostly peat, for the acidity), cacti (almost gravel). Scotts’ Expand ’n Gro is mostly coir and comes in a dry, compacted form — easy to tote home, where you add water to fluff it up. Some garden centers, such as Pesche’s in Des Plaines and Chalet in Wilmette, sell proprietary mixes made to their specifications from mostly local ingredients.

Some mixes have polymer crystals that are supposed to hold water (though tests have shown they don’t do plants much good), and many have chemical fertilizers designed to release nutrients slowly over the six to nine months of a growing season.

I’m starting all over with my pots this year — again. I’ll be taking a close look at some of the peat-free mixes; maybe I’ll buy one bag first and test a mix for drainage before I commit. But I won’t look for bargains, remembering what it cost me in the long run last year. This time, I’ll be a careful shopper.

 

(From Chicagoland Gardening Volume XVIII Issue II. Photo © Ping Han - Fotolia.com)

 


I'm an award-winning garden writer, speaker and consultant in Chicago, where I was on staff at the Chicago Tribune for more than 20 years. Raised on the South Side by an organic gardener and environmentalist, I now garden in a leafy suburb on the edge of Chicago — in the deep shade on the north side of a four-story building, in the sunny strip by the garbage cans, in pots on the third-floor porch and on the windowsills in the winter.