Almost every tomato, pansy, or succulent plant you’ve purchased has first germinated in what’s called potting soil, a concoction that often contains no soil or compost. Instead, all those bags from the nearest nursery are sterile, dirt-free mixes of exotic foams, fibers and minerals, ingredients that hide lung disease, water waste and a huge carbon footprint.
So why use it at all? The demand for soil in the United States has exploded; the number of urban gardeners has increased by 30% over the last 30 years, and nurseries and greenhouses are the two fastest growing agricultural industries. Some lucky gardeners can grow straight into the ground, but the soil is too heavy and becomes too compact for raised beds and seed trays.
This rapidly growing demand has increased the need for key potting soil ingredients: vermiculite, peat moss and coir, all of which are hazardous to the environment and human health, although greater awareness of these risk encourages more sustainable solutions.
“Growing plants isn’t quick or easy,” says Linda Chalker-Scott, a Washington State University professor who writes the blog, Horticulturist Myths. “If you want to have a sustainable system, you have to do it the right way.”
These are the three most problematic ingredients in potting soil.
Prized 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 deep open pits, heavy machinery and propane-dependent production plants take their toll on the environment, as does asbestos-contaminated vermiculite on human health.
The largest vermiculite mine in the United States, in Libby, Montana, was contaminated with asbestos. Closed in 1990 due to asbestos poisoning in the community after 70 years of continuous operation, the Libby mine once produced 80% of the world’s vermiculite. Its distribution has contaminated hundreds of potting soils, landscaping products, brake pads, chlorine filtration systems, popcorn ceilings and 35 million insulation American homes. Mines contaminated with asbestos are still in operation in Virginia, Caroline from the southand South Africa.
Fortunately, not all vermiculites contain asbestos. “The fact that we are not seeing more cases of mesothelioma among gardeners and farmers is a testament to the low percentage of contamination,” explains michelle whitmeran asbestos expert at the Mesothelioma Center, “but repeated exposure, even to a small amount, makes it dangerous.”
Consumer safety regulations are non-existent for most horticultural materials sold in the United States. And a total ban on asbestos by the Environmental Protection Agency is still at proposal stage22 years after the discovery of vermiculite containing asbestos in gardening and lawn care products from nearly 20 different retail brands.
Made from spongy, waterlogged layers of slowly decaying plant matter, peat is the main ingredient of the largest potting soil manufacturers. Organic, abundant, sterile, light and non-toxic, it can hold 20 times its weight in water.
But peatlands are also the largest terrestrial carbon store in the world, even more so than forests. Although they cover only 3 percent of the land and forests cover 30 percent, peatlands store twice as much carbon.
Wetlands, including peatlands, are already in decline – 35% since 1970. The current peat harvesting system can remove a thousand years of sphagnum peat moss in just one or two decades. In North America alone, 3 to 5 million metric tons of Canadian peat makes its way to the US horticultural market each year. This does not include the environmental footprint of processing, packaging and transportation, says Justin Freiberg from the Yale Carbon Containment Laboratory.
In Europe, peat has been harvested for centuries, mainly for fuel, but commercial peat harvesting is now banned in Ireland; the UK will ban all peat-based potting soils by 2024. Canada and the US (where the peat is extracted in 11 different states) have no bans.
3. Coco Coco
Today, the most popular alternative to peat is coir, made from the fibrous husk left after harvesting coconut milk and meat. Pure coconut fiber is neutral, absorbent and renewable. Considered a fibrous product for more than a century, 90% of the world’s coir fiber is shipped from Sri Lanka and India where, despite a historic past the water crisis, it must be soaked and rinsed several times during treatment. Once the dehydrated coconut bricks arrive at their destination, they again need large amounts of water to rehydrate.
Beyond wasted water, the coir industry is notorious for low wages, child labor and dangerous working conditions: A recent study of Sri Lankan coir factories found a rate of 1,063 injuries per 1,000 workers per year.
We are entering the “third paradigm of potting soil,” says soil scientist Charles Bethke, from heavy peat-steam sterilized garden soils to the model of “recycled lignose cellulosic fiber.” Which of these fibers is the most promising? According Brian Jacksonprofessor and current director of the North Carolina State Horticultural Substrates Lab, “wood fiber and wood products are seen by all professional organizations in Europe and North America as the most potential material to continue to fill the void left by peat is used less.
But wood is not the only option. From corn stalks to peanut shells, nettles to yucca, beach grass to recycled cardboard, there are many things that can be recycled into growing media. This diversity of inputs also facilitates the shift from long-distance shipping to local production hubs.
Chalker-Scott encourages focusing on cleaning up the ground beneath our feet. “A hundred years ago, we had no potting soil,” she says. “The plants did very well without it.”