The term potting soil has become something of a misnomer in today’s world of container gardening. Most bags of potting soil contain no field soil but are composed of a variety of organic and inorganic materials and are referred to as soilless mixes. As a commercial greenhouse operator and horticultural researcher, I’ve worked with all kinds of soilless mixes over the years and believe them to be far superior to soil-based mixes for a variety of reasons. Many excellent brands are readily available at chain stores and garden centers. If you have a clear understanding of the requirements for a good container medium and the various ingredients used in these products, choosing the right mix for your container plantings is in the bag.

Successful container gardening requires a potting medium that meets several of the plant’s needs. The medium must be a stable reservoir of moisture and nutrients and remain loose enough to allow for root and water movement and the exchange of gases in the root zone. A growing medium must also have a pH (a measure of the alkalinity or acidity of a medium) that can support adequate nutrient uptake, and it must be free of soil-borne diseases, weed seeds, and toxins. Finally, a container medium must provide adequate anchorage and support for the roots while still being heavy enough to provide sufficient ballast to prevent plants from tipping over. A well-blended soilless medium can easily satisfy all these requirements and do so without the inherent problems and variability frequently encountered when field, or native, soils are used in containers.

If you have a good mix, water will penetrate it quickly and drain freely from the bottom of the pot. When the excess water has drained away, air will fill the large pore spaces, but enough water will be retained in the smaller spaces to provide ample moisture for the plant. In a poor mix, water may be slow to penetrate, the medium will become heavy and waterlogged, and a crust from algae or accumulated salts may form on the surface. Under these conditions, the roots become starved for oxygen, plant growth slows, foliage may begin to yellow, and plants often succumb to root rot.

For the best results:

• Lightly moisten the mix before filling containers.
• Don’t pack the mix too tightly when planting.
• Water your pots thoroughly after planting.
• Begin fertilizing your pots two to three weeks after planting if you did not include a slow-release fertilizer at planting time.

Both organic and inorganic ingredients serve a purpose

Some organic ingredients, such as peat moss, provide needed water-holding capacity; others, like pine bark, can lend a porous structure to avoid compaction.

Peat moss: The physical and chemical properties of peat moss make it an ideal base for most soilless mixes because it can hold both water and air. It’s light, but its fibrous structure allows it to hold 15 to 20 times its weight in water. The peat fibers also give it a large amount of pore space (80 to 90 percent of its total volume). It holds nutrients well, and it readily shares them with the roots, thanks to its slightly acidic pH. Horticultural-grade peats come from the decomposed remains of sphagnum moss species that have accumulated over centuries in peat bogs. They are not a renewable resource, however, and concerns about the sustainability of harvesting this product is a common topic of discussion among gardeners. Another type of peat that is used in soilless mixes is known as reed-sedge peat, but this material is generally inferior to sphagnum peat.

Composted pine bark: This material is a renewable resource and is one of the most widely used components in commercial container media, although barks from many other species are also processed for this purpose. Bark lacks the moisture-holding capacity of peat moss, but it can dramatically increase the porosity of a mix. Bark particles used in container media generally range in size from dustlike to about 3/8 inch in diameter.

Coir: Another renewable organic material is coir, a derivative of coconut hulls that shows promise as a peat substitute. Coir has exceptional water-holding capacity, and when mixed with pine bark, it can eliminate or substantially reduce the need for peat moss in a mix. Other sources of organic matter that can be used in soilless mixes include composted manures, leaf mold, and crop residues such as rice hulls.

Inorganic ingredients improve drainage and add weight

Inorganic ingredients improve drainage and add weight. Inorganic ingredients like sand, vermiculite, and perlite generally lend porosity to a mix, but they can also help retain moisture and add weight or density.

Sand: This material can add needed weight to peat- and bark-based mixes and fill large pore spaces without impairing drainage. Coarse sand is preferred in most cases, and sand ground from granite is used in the best mixes. Fine sand with rounded grains like that found at the beach can actually reduce drainage when used in excessive amounts.

Vermiculite: A mineral that has been heated until it expands into small accordion-shaped particles, vermiculite holds large amounts of both air and water. But it can easily be compacted, so avoid packing down mixes containing large quantities of it. Vermiculite can also retain nutrients and help a mix resist changes in pH.

Perlite: One of the more common ingredients in commercial potting mixes, perlite is an inert ingredient manufactured by heating a volcanic material to produce lightweight white particles. It promotes good drainage while holding nearly as much water as vermiculite.

Other inorganic materials that are useful in potting media include polystyrene (plastic) beads and calcined clay, which is similar to kitty litter. Plastic beads are inert and serve only to promote drainage, but calcined-clay particles can actually improve the moisture- and nutrient-holding capacity of a mix.

Use crystal polymers to help retain moisture

Many soilless mixes have either liquid surfactants or gel-forming granules added to help them retain moisture. If you have trouble keeping containers well watered in hot weather or in sunny locations, you may want to consider adding one of these products to your mix before you plant. As with fertilizers, follow the label directions and don’t overapply. Soilless mixes that already have extra wetting agents typically indicate this on the label

Why don’t native soils belong in pots?

Field soils can be appropriate for growing plants in the garden, but these soils are unsuited for growing plants in containers. In most cases, the texture of field soils is simply too fine to ensure adequate aeration in containers, and pots or planters of any size are generally too shallow to permit proper drainage. Soilless media have larger particles, which form bigger spaces or pores to hold air in the medium, while still retaining enough water for plants to survive. Adding too much water-absorbing material, which expands greatly when moistened, can knock your plants out of their container.

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Jim Garner is associate professor of horticulture at Horry-Georgetown Technical College in Myrtle Beach, South Carolina.

Photos, except where noted: Steve Aitken

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