Weekend Argus (Saturday Edition)
The earth at work beneath our feet
Feed the soil not the plants, says study
THE gardener has a long, touchy-feely relationship with the soil. As every good cultivator knows, you assess the earth by holding it. Is it dark and crumbly, is there an earthworm or beetle in there, is it moist, and when you smell it, are you getting that pleasant earthy aroma?
All these signs are reassuring, and have been through the ages, but they are mere indicators of something much greater and infinitely mysterious: a hidden universe beneath our feet.
This cosmos is only now revealing itself as a result of scientific discoveries based on better microscopic imaging and DNA analysis.
There is much still to learn, but it boils down to this: Plants nurture a whole world of creatures in the soil that in return feed and protect the plants, including and especially trees. It is a subterranean community that includes worms, insects, mites, other arthropods you’ve never heard of, amoebas, and fellow protozoa. The dominant organisms are bacteria and fungi. All these players work together, sometimes by eating one another.
The awareness of this biosphere should change the way gardeners think about cultivating plants and heighten everyone’s understanding of the natural world. In other words, don’t ever call it “dirt” again.
The sheer vitality of it is mind-bending: A teaspoon of good loam may contain a billion bacteria, yards of fungal strands, several thousand protozoas and a few dozen nematodes, according to Jeff Lowenfels, co-author of Teaming With Microbes.
This is, basically, how it works: Plants manufacture carbohydrates through photosynthesis, but not just for themselves. They release some of their carbon sugars into the soil, which causes the bacteria and fungi to show up to feed. The bacteria crowd around the root zone, and the fungi form vast networks of interlocking strands that often link one plant to another. The bacteria convert nitrogen and other nutrients into forms the plants can use, often by getting devoured by other microbes.
The fungal strands, the mycelium, effectively increase the root mass of its host plant by as much as a thousand times and transport a bevy of goodies to the host plants, including phosphorus, copper, calcium and zinc. There is also evidence that trees use this network to send signals to one another if, say, leaf-eating pests have arrived.
In his Ted Talk, mycologist Paul Stamets referred to mycelium as “Earth’s natural Internet”.
Although some plant (and human) diseases are caused by soil-borne fungi and bacteria, most of these microbes are beneficial and keep the bad ones in check. The organisms assist in other ways, by increasing the size of soil particles, which improves the ability of the soil to hold water and air.
Even in the middle of a city, the subterranean world is thriving.
Scientists took almost 600 soil samples from across New York’s Central Park and discovered a surprising diversity and richness. They identified more than 120 000 types of bacteria and more than 40 000 species of fungi, protozoa and arthropods.
Among the unexpected findings: The microbial species were the same, more or less, as those found in parts of the world with dramatically different flora and climates from New York’s, including Antarctic cold deserts, tropical forests and grasslands.
There was a strong association between the diverse organisms in each sample. “Unravelling these relationships will be critical to building a more integrated understanding of below-ground ecology,” the researchers wrote in a paper published by the journal for the British Royal Society. “Our work highlights that most of the diversity found in soil remains undescribed.”
Enough is known, however, to create a 21st century subset of farming known as regenerative agriculture. The farmers have discovered that if you foster this biosphere, you don’t need expensive fertilizers because the microbes repay the plants with nutrients. They also, for obvious reasons, avoid pesticides that would kill this soil life.
The farmers do as little soil digging as possible because traditional tillage destroys the fungal networks and the desirable soil structure. Cover crops keep the soil life happy between growing seasons.
Advocates of this lowimpact farming say it can restore soil carbon lost by the historic conversion of forest and prairie to farmland and help to mitigate greenhouse gases. In the 1990s, Sara F. Wright, an agricultural research service scientist in Beltsville, Maryland, discovered a sticky coating to fungal threads named glomalin that, it turns out, is a major reservoir for carbon.
Lowenfels says it’s also time for gardeners to adopt practices that nurture the soil biosphere. To say he thinks deeply about this subterranean world is an understatement. In addition to Teaming With Microbes, he has written Teaming With Nutrients. His latest title is Teaming With Fungi, which dwells on the type of fungi that directly associate with plant roots. They are known as mycorrhizal fungi, and he’s a big fan of adding them to his plants when they are installed, either as a spray or in powdered form available from the garden centre. “It works. My tomato plants are bigger than the control, they’ve got more fruit on them, the plants are so healthy,” he said. “My carrots are unbelievable this year.”
Some gardeners turn to compost tea to build soil microbes. This is made by aerating sugars, compost and humic acids in non-chlorinated water and then spraying the brew on plants and soil. Others are not convinced that this is needed, though everyone agrees that the way to foster the soil food web is to topdress growing beds and lawns with organic matter such as shredded leaves or finished compost.
James Nardi, a biologist at the University of Illinois in Urbana, offers this advice: “Work with your fellow nonhuman gardeners. I never use synthetic fertilizers, and I never use pesticides.” Nardi’s 2007 book, Life in the Soil, remains an excellent introduction to the subject.
In autumn, he mixes horse manure with fallen leaves, shreds the mixture and applies it as a mulch to his growing beds. “In the spring, I have this lovely, spongy soil,” he said. Lowenfels shreds autumn leaves on his lawn and lets the biosphere use them over the winter.
The organic gardener’s mantra has never seemed more appropriate. Feed the soil, not the plant. – Washington Post