YOU might think it’s as common as muck. Dirt cheap, even. In fact, the soil beneath our feet is anything but. Good, fertile topsoil is crucial for 97 per cent of the world’s food supply, and without it parks and gardens would look more brown than green. So it’s a worry that soil has joined the long list of resources that are beginning to run out – and there is no natural way to replace it in our lifetime. But soon there might be. By mixing together a bit of animal, vegetable and mineral, researchers are turning waste into fertile ground.
Making soil is a complicated business. In nature it develops when weathered rock and decomposing plant and animal material are mixed and broken down by plant roots, soil fauna, microbes and fungi. Over hundreds of years, if the chemical and biological mix is right, the raw ingredients are transformed into a rich and complex substance with just the right balance of structure, nutrients and porosity to sustain plant life.
Understanding the process is one thing; making the stuff from scratch is quite another. Nevertheless, it looks like we will have to find a way. Natural topsoils are being scraped away by the truckload for use in landscaping and construction projects, putting pressure on natural supplies.
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Humankind has faced soil crises before, says David Montgomery, a geomorphologist at the University of Washington in Seattle and author of . Montgomery argues that the ancient civilisations of Greece and Rome declined along with their topsoil. He warns that the world’s soil is now being eroded at least 20 times as fast as it can regenerate.
“The world’s soil is being eroded at least 20 times times as fast as it can regenerate”
Preventing soil erosion and degradation is one way to slow this trend. Improving the quality of poor soil by adding organic matter and minerals, or altering soil acidity, is another. But while such tinkering can improve bad soil, sometimes you just need more dirt.
Faced with expensive and ever-scarcer topsoil supplies, researchers are trying to mimic the natural process of soil formation on vastly shortened timescales, using recycled road aggregate, coal shale or ash mixed with compost and other waste biomass.
Jody Tishmack started digging into soil-making at Purdue University in West Lafayette, Indiana, in the mid-1990s. The university was looking for ways to recycle the coal ash produced in the campus power plant and also needed large quantities of topsoil for landscaping. She had heard that fly ash – fine particles given off when coal is burned – could be used to improve soil. She wondered if she could she kill two birds with one stone.
To create her new soil, Tishmack needed a source of organic matter to mix with the coal ash. As luck would have it, pharmaceutical giant Eli Lilly had a production plant nearby with a ready supply of used fungal mats – an extremely rich organic by-product of antibiotics manufacture. The researchers mixed the two waste materials – one mineral, one organic – then threw in some wood chips and leaves to add porosity, and composted it all together. The result was an artificial soil that Purdue used around its campus. “Thousands of tonnes of this material were made,” says soil chemist Cliff Johnston, who worked with Tishmack on the project.
The endeavour led to Soilmaker, a spin-off company that Tishmack still runs. The company no longer uses Eli Lilly’s bio-sludge or the coal-derived fly ash: the sludge smelled too unpleasant as it composted and Purdue’s power plant switched to a coal whose fly ash contained too much arsenic to be safely put in soil. Now Soilmaker mixes leftovers from cornstarch production to supply organic material while clay dug up in construction projects provides the mineral component.
Others are trying similar approaches. Richard Haynes at the University of Queensland, Australia, is trialling a mix of fly ash and chicken litter, composted together with tree and garden waste, as part of a government and industry-sponsored effort to turn waste into new soil. As the ingredients react, the organic material binds to the minerals, forming aggregates that provide the soil with a good pore structure. “Topsoil is about 50 per cent pore space,” Haynes says. That’s why using compost with no mineral component is only a short-term solution. “It loses its volume over time,” he says.
Trying to turn industrial waste into soil can be problematic, with contamination a key concern. Biosolids from sewage treatment plants may be a good source of organic matter, for example, but they can contain a lot of potentially toxic heavy metals. Fly ash can also have high concentrations of boron, which is toxic to plants, and arsenic.
Kimberley Neville, formerly an environmental geologist at Imperial College London and now an environmental consultant at the URS Corporation, also in London, found that plants grown in soil made from shale left over from coal mining, iron-rich ochre from mine drainage and composted sewage sludge had levels of arsenic and zinc slightly in excess of UK safety limits. “You might not want to grow food in it,” she says, but she was looking at this material for use in urban landscaping.
Montgomery, for his part, is uneasy about using waste to make soil. “I could see how it could be done well, but the potential is there for it to be done poorly,” he says. “One of the silliest things we can do is mix toxic waste with soil. For materials that have high concentrations of heavy metals, that’s basically just poisoning the earth.”
Nevertheless, all of these projects demonstrate a proof of principle that, should all else fail, it will be possible to make fertile soil from the right mix of ingredients. And one day that might just save us from the fate of the ancient Greeks.
Soil in space
When the time comes to set up camp on the moon or Mars, we won’t be able to rely indefinitely on food shipments from Earth. Nor, of course, will we be able to just plant rows of crops in the Martian soil and expect them to grow. So frontier colonies will need another food solution.
One possibility would be to grow crops in nutrient solutions instead of soil, but that has drawbacks too. “Hydroponics is a great way to grow plants, but it’s cumbersome,” says Douglas Ming of NASA’s Johnson Space Center in Houston, Texas. “It’s tough to control water in the microgravity environment of space.”
So NASA scientists have developed an artificial soil that may be able to use moon or Martian materials as one of its ingredients.
The “space soil” resembles cat litter in both appearance and character. Both make use of zeolites – porous minerals that can absorb large quantities of water. There are reasons to believe they may exist on the moon or Mars in large quantities. Zeolites can also store positive ions, including key plant nutrients such as ammonium and potassium ions. Another component of the space soil is the phosphorus-containing mineral apatite, which NASA synthesised to incorporate essential plant nutrients and to dissolve at a desired rate.
The advantage of the artificial soil is that the nutrients remain bound up in it until the plants need them, so they can’t wash away, Ming says. NASA tested its soil on a number of plants that might make up a space diet – those with high ratios of edible to inedible biomass, such as sweet potatoes and wheat, plus some treats like strawberries. The plants took to it just fine. “By providing lots of light and optimal carbon dioxide levels, we were able to outdo field production by many times,” says Don Henninger, who led the project.
Until we’re ready to grow potatoes on the moon, space soil has found applications close to home. The Colorado company Zeoponix has licensed the technology as a low-leaching, well-draining soil improver for athletics fields and golf courses.
