Almost every tomato plant, pansy, or succulent you’ve ever purchased first sprouted in something called potting soil, a concoction that often contains neither soil nor compost. Instead, all those bags at the nearest nursery are dirt-less sterile blends of exotic mosses, fibers, and minerals, ingredients that hide lung disease, water waste, and a whopping carbon footprint.
So why use it at all? Demand for potting soil in the United States has exploded; the number of urban gardeners has risen 30 percent in the last 30 years, and nurseries and greenhouses are the two fastest growing agricultural industries. Some lucky gardeners can grow directly in the ground, but dirt is too heavy and becomes too compacted for raised beds and seed trays.
That rapidly growing demand has increased the need for potting soil’s key ingredients: vermiculite, sphagnum peat moss, and coconut coir, all three of which are risky for environmental and human health, though greater awareness of those risks is encouraging more sustainable solutions.
“Growing plants isn’t quick and easy,” says Linda Chalker-Scott, a professor at Washington State University who writes the blog, Horticulturalist Myths. “If you want to have a sustainable system you’ve got to do it the right way.”
These are the top three most problematic ingredients in potting soil.
Valued in horticulture for its popcorn-like texture, the mineral vermiculite is mined and then baked at over 1000 degrees Fahrenheit into the light crumbs we see in potting soil. But the deep, open pits, heavy machinery, and propane-reliant production plants wreak havoc on the environment, as can asbestos-contaminated vermiculite on human health.
The largest U.S. vermiculite mine, in Libby, Montana, was contaminated with asbestos. Shut down in 1990 because of asbestos poisoning in the community after 70 years of continuous operation, Libby’s mine once produced 80 percent of the world’s vermiculite. Its distribution contaminated hundreds of potting mixes, landscaping products, brake pads, chlorine filtration systems, popcorn ceilings, and the insulation of 35 million U.S. homes. Asbestos-contaminated mines still operate in Virginia, South Carolina, and South Africa.
Fortunately, not all vermiculite contains asbestos. “The fact that we don’t see more cases of mesothelioma in gardeners and farmers is a testament to the low percentage of contamination,” says Michelle Whitmer, an asbestos expert at The Mesothelioma Center, “but repeat exposure, to even a low amount, makes it dangerous.”
Consumer safety regulations are nonexistent for most horticultural materials sold in the United States. And a complete Environmental Protection Agency ban on asbestos is still in the proposal stage, 22 years after asbestos-containing vermiculite was discovered in gardening and lawncare products from almost 20 different retail brands.
Made from spongy, waterlogged layers of slowly decomposing plant matter, peat is the primary ingredient for the world’s largest potting mix manufacturers. Organic, plentiful, sterile, lightweight, and nontoxic, it can hold 20 times its weight in water.
But peatlands are also the world’s largest terrestrial storer of carbon, even more than forests. Though they cover only 3 percent of land and forests cover 30 percent, peatlands store twice as much carbon.
Wetlands, in which peatlands are included, are already in decline—35 percent since 1970. The current system of harvesting peat can remove one thousand years’ worth of sphagnum peat moss within just one or two decades. In North America alone, 3-5 million metric tons of Canadian peat head south into the U.S. horticultural market every year. That does not include the environmental footprint of processing, packaging, and transportation, says Justin Freiberg of the Yale Carbon Containment Lab.
In Europe, peat has been harvested for centuries, largely for fuel, but commercial peat harvesting is now banned in Ireland; the U.K. will ban all peat-based potting mix by 2024. Canada and the U.S. (where peat is mined in 11 different states) have no bans.
3. Coconut Coir
Today’s most popular alternative to peat is coconut coir, made from the fibrous shell left behind after harvesting a coconut’s milk and meat. Pure coir is neutral, absorptive, and renewable. Valued as a fiber product for over a century, 90 percent of the world’s coir is shipped from Sri Lanka and India where, despite a historic water crisis, it must be repeatedly soaked and rinsed during processing. Once the dehydrated coir bricks arrive at their destination, they again require large amounts of water for rehydration.
Beyond water waste, the coir industry is infamous for low wages, child labor, and dangerous work conditions: A recent study of Sri Lankan coir factories revealed a rate of 1,063 injuries per 1,000 workers per year.
We are moving into the “third paradigm of potting mix,” says soil scientist Charles Bethke, from heavy steam-sterilized garden soils to peat to the model of “recycled lignose cellulosic fiber.” Which of those fibers is the most promising? According to Brian Jackson, a professor and the current director of the North Carolina State Horticultural Substrates Lab, “wood fiber and wood products are believed by every professional organization in Europe and North America to be the highest-potential material to continue to fill the void left behind as peat is being used less.”
But wood is not the only option. From corn stalks to peanut hulls, nettles to yucca, beach grass to recycled cardboard, there are many things that can be recycled into growing media. This diversity of inputs also makes it easier to shift from long-distance shipping to local production hubs.
Chalker-Scott encourages focusing on remediating the soil beneath our feet. “One hundred years ago we didn’t have potting mix,” she says. “Plants have done just fine without it.”