The Dirt on Potting Soil

Summer 2006

The Dirt on Potting Soil

by Craig Idlebrook
Copyright 2006

Last year, I tried container gardening after grabbing the first bags of potting soil I found at the local big box store. If the bags had been any more generic looking, they would have said “ACME.” The packaging had almost no information about the content or source of the soil, but, I figured, dirt is dirt.

Within a week, my potted plants held water about as well as a brick. The soil pulled from the sides of the containers and formed tight, water-shedding balls. Only my hardiest seedlings survived.

Clearly, not all potting soils are created equal.

It’s What’s Inside That Counts

In a way, good potting soil tries to mimic nature. Healthy earth relies on critters, rocks and roots for aeration; but when put in a pot, the same soil quickly becomes a dense mass that water and oxygen can’t permeate.

“Soil alone is heavy and poorly aerated. It tends to become waterlogged and sticky when wet,” writes Eric Sideman, director of technical services for MOFGA, in his soil mix fact sheet.

Good potting soil tries to replicate nature by providing plant nutrients and materials that allow for drainage and aeration–but good potting soil, especially organically-approved, can be hard to come by.

Bruce Hoskins, assistant scientist and soil testing coordinator at the University of Maine, says of potting soil, let the buyer beware. “There’s some pretty questionable stuff that we’ve seen sold as potting soil.”

Lois Stack, ornamental horticulture specialist with University of Maine Cooperative Extension, says that just because something is labeled “potting soil” doesn’t mean it’ll work in a pot. “It’s really hard to define what is in that container,” Stack observes. “The word ‘soil’ has a very broad definition.”

Sideman suggests that an ideal mix should be dense enough to hold seedlings and water, but porous enough to shed excess water. It must be free of weed seeds and plant pathogens.

Peat and humus normally provide sponge-like water retention and drainage in organically-approved mixes, while sand provides plant support and drainage. The heat-expanded minerals perlite and vermiculite commonly provide pore space, improving drainage while minimizing the weight of the soil

Good potting soil must have plant nutrients, but getting enough nutrients into an organically-approved potting soil is not easy. In conventional potting soil, synthetic fertilizer can provide most of the food a plant will need for a while, but organic growers must rely on compost, which often doesn’t provide enough nutrients to last long in a pot. One way to overcome the shortage, Hoskins says, is to add fresh compost when transplanting to a larger container. Watering with manure tea or compost tea can also help.

Two Recipes for Success

Making successful compost-based potting soil is both art and science. While some hard and fast rules apply, soil recipes often differ from grower to grower.

Dennis King of King Hill Farm in Penobscot makes most of the potting soil for his vegetable seedlings rather than buying packaged, organically-approved soil, because, “It’s a whole lot cheaper.” He says good compost is the key to healthy potting soil, and heat is the key to good compost. Compost must get hot enough to kill pathogens and weed seeds, but not hot enough to kill beneficial organisms. One batch of improperly heated compost can impact a farmer for a long time. “One year of seeds, seven years of weeds,” he recites.

King keeps his compost at 140-150 degrees F. for at least two weeks in a self-propelled compost-turner that he shares with a neighbor. He then mixes his potting soil in a cement mixer using a recipe from Sideman: 5 gallons of compost, 5 gallons of peat, 2 ½ gallons of sand, a gallon of perlite, and a cup each of greensand, bone meal and lime.

His mix works better than material he used to buy. Store-bought soil caused problems with damping-off, a disease that weakens stems of young seedlings and usually kills them. “With good compost-based potting soil, I’ve forgotten all about damping-off,” says King.

Claudette Nadeau and her husband, Michael Beaudry, make at least 50% of the potting soil they need to grow seedlings at their Roots-n-Shoots Farm in Montville. The main ingredient for their compost is manure from the farm’s five sheep and four goats. “The manure from our girls is purely organic,” says Nadeau. “They are fed ‘Nature’s Best’ organic grain and fed organic hay in the winter, [they get] no antibiotics, and are out to pasture and woods in the spring, summer and fall.”

Like King, Nadeau makes sure her compost reaches over 130 degrees for 10 days, turning the pile three times during that period, to destroy pathogens. Her recipe is 21 quarts compost, 6 quarts perlite, 15 quarts peat moss, and 1/2 cup each of lime, rock phosphate and greensand.

To Buy or To Mix

When deciding whether to make or buy soil, consider labor, time, access to dry storage and the cost of materials and mixing equipment. Stack notes that making your own isn’t always cheaper. Homemade mixes “often are not dependably consistent,” writes Sideman, but finding organically approved potting soil that is effective and affordable can be just as tricky.

Prepackaged potting soil is a relatively new concept. Hoskins says that until 30 years ago, most greenhouse growers simply sifted field soil for potting soil.

The organic soil market is even younger. Hoskins says the first wave of organically-approved soils often had problems. Some relied too heavily on compost and drained poorly; others had insufficient nutrients. Stack adds that until recently, turnover in the compost-soil market was high. “Every time they came and went, a grower would have to get used to a new mix.”

While the market has stabilized, Hoskins says knowledge about organically approved mixes still lags far behind that of conventional soil. “We’re all still learning about organic media.”

It’s All in the Nose

Nadeau uses soil made by the Vermont Compost Company (VCC) for the rest of her soil mix needs. She says its light texture is good for plant roots.

Being one of the more established and consistent organic soils on the market, VCC has attracted a following among Maine farmers.
Karl Hammer, VCC founder and owner, says the key to making good compost is to recognize problems that arise before the composting process is finished. “It comes down to telling yourself the truth.”

As VCC compost ages, it changes daily, so employees constantly adjust conditions to create a good product. Hammer adds that a compost maker’s chief ally is his or her nose. “Each of the compounds that could give you trouble has an odor signature.”

Each day, VCC employees sniff each batch of compost. If they detect a problem, they adjust the mixture or the conditions. Hammer smells finished batches as well, because, he says, compost making is too important not to get right. “The cost of potting soil failure to a grower is…hard to exaggerate.”

In the ‘70s, Hammer lived in the same Vermont town as organic pioneer Eliot Coleman, and the two experimented with soil blocks, a Dutch technique of packing soil so that it stands on its own. They needed a peat-free mix for this technique, so Hammer began to make his own. Neighbors soon came knocking for his mix.

After running his own manure removal business in New York, Hammer founded VCC in 1992 and sold his first soil two years later. His compost typically takes a year to age.

Much of VCC’s compost comes from manure generated by large Vermont dairy farms, which must follow VCC guidelines on bedding and manure management. The company also receives 12 tons of food waste a week from the town of Montpelier, but Hammer says the company’s secret weapon is its own flock of chickens. “Chicken manure is the Holy Grail of compost making.”

Chickens defecate and urinate at the same time and often mix their waste into their bedding immediately. This keeps nitrogen from escaping from the manure and maintains a good pH, creating excellent compost material. Hammer says eggs are an added bonus. [Chicken manure must be used in moderation, however, because of its nitrogen and salt content.]

Ultimately, Hammer believes composting can be boiled down to a science only so far. The rest is accepting and working with nature’s variables—and demand for that work is big. “There’s a hell of a lot of new demand and not a lot of excellent product,” concludes Hammer.

About the author: Craig Idlebrook is an Ellsworth-based freelance writer whose daughter has cut her first tooth. He often wears earplugs. He can be reached at [email protected].


MOFGA Fact Sheet # 9. Soilless Mixes for Vegetable Seedling Production, by Eric Sideman, Ph.D.

Catalogs from Johnny’s Selected Seeds and Fedco Seeds offer soil mixes that are approved for organic production. The MOF&G editor has been pleased with NP Mix TM from Living Acres in New Sharon, Maine. If you have a favorite commercial mix that is approved for organic production, or your own favorite soil mix recipe that you’d like to share, please send it to [email protected].

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