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The scoop on potting “soil”

by Timber Press on May 25, 2016

in Gardening

Pansy orchid (aka milmiltoniopsis)

Pansy orchid (aka milmiltoniopsis)

What’s Wrong With My Houseplant? authors David Deardorff and Kathryn Wadsworth explain what’s found in common potting media and offer several ways to create your own organic alternative.

When you purchase a bag of “potting soil” from your local garden center what you’re actually getting is not soil. It’s an artificial mixture of ingredients specifically designed to meet the needs of plants in containers and usually contains no actual soil at all. Garden soil, plain old dirt to some, does not promote good growth of container plants because it becomes more and more dense and airless over time. Garden soil is perfectly adequate—in the garden—where it benefits from the activities of worms, insects, fungi, and bacteria. But a pot is a special environment, not a garden, and any plant in a pot has special needs that cannot be met by ordinary garden soil alone.

Roots in a pot, like roots in a garden, must have access to oxygen or they die. Roots of most plants (other than orchids) are not green and are not capable of making food through photosynthesis. In order to stay alive and grow, roots have to metabolize the sugar manufactured by the leaves. This process, called respiration in plants, burns sugar to release energy and consumes oxygen in order to do so. Whenever the air (oxygen) in the medium surrounding the root system of your houseplant is depleted, whether due to long-term saturation by water or tight density of the medium, it is stressful and life-threatening to your plant.

The combination of primary ingredients in artificial “potting soil” includes inorganic mineral items like perlite and vermiculite that create air spaces in the medium and some sharp horticultural sand to promote drainage. It also contains organic material like bark fines to hold and retain moisture. Such a mix never becomes as dense and airless as garden soil in a pot. As the organic fraction of the medium breaks down and decomposes over time, however, the medium will tend to hold more moisture than is good for the plant. Then it’s time to re-pot with fresh medium.

Tiger jaws (aka cat chap, cat jaws, shark jaws, wolf mouth)

Tiger jaws (aka cat chap, cat jaws, shark jaws, wolf mouth)

Potting mixes were invented in a time when seemingly inexhaustible quantities of sphagnum peat moss were available as a primary ingredient. Unfortunately those days are over and the sphagnum peat moss deposits have mostly been mined out. More responsible potting soil producers are now using the renewable and sustainable resource of bark fines obtained from tree farm timber harvesting, or coconut coir from coconut plantations instead of peat moss for the moistureretaining organic component of their mixes.

Organic potting media. Aside from the primary ingredients just mentioned, most really good general-purpose organic potting soils contain other ingredients such as nutrients derived from organic fertilizers (manures, compost, worm castings), spores of mycorrhizal fungi, and CFUs (colony-forming units) of beneficial bacteria. The organic fertilizers in these mixes release their nutrients slowly and do not burn plant roots. Non-organic potting soils, which often have processed, water-soluble fertilizers included in the mix, are not recommended. For one thing, non-organic processed fertilizers are more concentrated than organic fertilizers and you run the risk of burning the roots of your plants. For another, these non-organic commercially prepared potting soils take a one-size-fits-all approach, and they’re primarily made for annuals such as petunias, tomatoes, and the like, not for houseplants.

African violet

African violet

Special commercially prepared potting media are widely available for plants with special needs. Most of these are not organic, but you can make your own organic mix easily. Three kinds of houseplants benefit from special media that you can buy or make yourself.

Cactus and succulents. Mix 1 part sharp horticultural sand, 1 part perlite, and 1 part general-purpose organic potting medium. This
well-drained mix works very well for cactus and succulents of all kinds.

African violets. Commercially prepared African violet potting media are often too dense and hold too much moisture for really good African violet growth. You can make your own by adding 1 part commercial African violet mix to 1 part general-purpose organic potting medium.

Orchids. Most orchids commonly grown as houseplants are potted in chunks of straight Douglas fir bark, or other kinds of tree bark with no bark fines or other moisture-holding components. Many orchids grow perfectly well in bark with nothing else added but weak fertilizers. They will die if potted in potting soil. Orchid bark comes in three grades: coarse, medium, and fine. Coarse and medium bark are appropriate for most adult orchids. Use fine orchid bark for seedlings, miniatures, and other small plants. Bark media are extremely well drained, with very large pore spaces, and abundant oxygen around the roots. Two kinds of orchids, however, cymbidiums and lady’s slippers, do not fare well in straight orchid bark. You can mix 1 part fine orchid bark with 1 part general-purpose organic potting medium to make a well-drained medium appropriate for these semi-terrestrial orchids.

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deardorff_dDavid Deardorff, botanist and expert plant pathologist, loves to write and lecture about how to grow healthier plants. As a research biologist David has lived and gardened in many environments, from the desert southwest to the maritime northwest to the tropics. David earned his Ph.D. in botany from the University of Washington. He coordinated plant pathology research at the University of Hawaii and served as faculty advisor to the Master Gardener Program at Washington State University. He has served as Research Director at Island Biotropix, an orchid nursery and tissue culture laboratory which he co-owned with partner and co-author Kathryn Wadsworth.

wadsworth_kKathryn Wadsworth, writer, photographer, and naturalist, enjoys sharing the wonders of the natural world with others. While leading eco-tours around the world she has studied plant life and explored natural history from Australia to Alaska. In graduate school Kathryn studied film-making and communications at the University of New Mexico, where she made documentary films on a wide variety of topics ranging from the California Gray Whale to the impact of mining on the Navajo Nation. She has owned and operated a film production company, an orchid nursery, and a tissue culture laboratory.

You may also be interested in the authors’ own Web site, kathrynanddavid.com.

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Click on image for a look inside this book.

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Save your indoor plants with 100% organic solutions.

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