6. Is Dirt Alive?
School gardens have introduced millions of students to the wonders of growing things in soil, dispelling the notion that soil is just dirt. It’s important to call attention to soil’s role in mitigating climate change. Beyond that, teaching about soil and climate creates opportunities to explore the incredible communities living beneath our feet. And soil ecology is a relatively young, emerging field, providing students with a window into how science at the frontiers of knowledge develops.1
“To see a World in a Grain of Sand …” wrote William Blake in his poem “Auguries of Innocence.” That could also be an invitation to appreciate the world of soil. It’s often said that a handful of soil contains more living organisms than the number of people on Earth (currently 7.4 billion); some put the number of soil organisms considerably higher.2
Researchers estimate that we’ve only identified 10 percent of the species living in soil.3 Different ecosystems (prairies, forests, or agricultural fields, for example) contain very different communities of living beings. Here’s one partial census from the Soil and Water Conservation Society of types of organisms in a teaspoon (one gram) of healthy agricultural soil:4
Bacteria: 100 million to 1 billion
Fungi: Several yards in length
Protozoa: Several thousand flagellates and amoebae; 100 to several hundred ciliates
Nematodes: Ten to 20 bacterial-feeders; a few fungal-feeders; a few predatory nematodes
In addition, a square foot of agricultural soil is typically home to up to 100 arthropods (insects, sow bugs, spiders, etc.) and five to 30 earthworms. They’re often joined by mice, voles, shrews, and other larger animals, as well as plants’ roots and shoots. In addition to living organisms, the soil ecosystem includes decomposed organic matter, minerals, water, and air.
Systems Perspective: Those organisms aren’t just living near each other, of course. They make up an intricate soil food web whose members feed, and feed on, each other. They are an interdependent community sharing responsibilities analogous to many of those in a human city: generation and distribution of energy and resources, infrastructure construction and maintenance, health and safety, civil defense, communications, waste treatment, and so on. And as with other complex systems, the well-being of the organisms within the soil community depends on the well-being of the whole—its members and the relationships among them.
Some of the most important emerging scientific news is a growing understanding of the role of microbes, thanks to the development of new tools such as gene sequencing. University of Washington geomorphologist David. R. Montgomery and biologist Anne Biklé describe many of these discoveries in their 2016 book The Hidden Half of Nature: The Microbial Roots of Life and Health. “Until very recently,” they report, “the field of soil ecology was much like ancient astronomy, when our view was limited to the stars we could see with the naked eye.”5
Scientists once thought that the carbohydrate-rich exudates on which microbes around plant roots feed had passively leaked from plants’ roots. Scientists now see, David Montgomery writes, that special cells “help manufacture exudates and push them out of the roots and into the rhizosphere”6 where they can attract microbes, which bring nutrients that the plants require. “I was trained to think of roots as straws, things that draw material out of the soil,” says Montgomery. “But it turns out they’re two-way streets.”7
Along with releasing nutrient-rich exudates that feed microbes, plants also broadcast information through the soil network. According to writer Kristin Ohlson, “Plants…can change the formula of their exudates and send chemical messages—kind of like lighting a flare—to attract specific players with specific services. For instance, scientists have discovered that when corn rootworm larvae attack some older varieties of corn…the plant sends out a chemical signal to beckon a nematode that feasts on this pest.”8
Living Soil and Climate Change
Soil’s ability to store carbon (thereby reducing the level of greenhouse gases in the atmosphere) depends on activities throughout the soil community. Through photosynthesis, plants use energy from the sun to convert CO2 from the atmosphere and water into the simple sugar glucose, and release oxygen as a by-product:
6CO2 + 6H2O —> C6H12O6 + 6O2
Glucose is then resynthesized into more complex carbon compounds which provide energy and become the building blocks for cell walls and other plant structures.
At the same time, as described by David Montgomery, plants pump 30 to 40 percent of their photosynthetically produced carbohydrates through their roots into the soil.9 They travel via what Australian soil ecologist Christine Jones calls the “liquid carbon pathway,”10 where they are consumed by microbes. Some of their carbon is released back into the air as CO2 through respiration, but some becomes incorporated into the microbes’ bodies. Writes Kristin Ohlson:
Fungal hyphae snake that carbon throughout the soil as if they were railroad tracks; when they die that far-reaching network of carbon stays in the soil to be nibbled at by other creatures…. The carbon keeps cycling through the soil food web, and each time it’s eaten and excreted it emerges in a more concentrated form.11
The total amount of carbon currently stored in soil is estimated to be three to four times as much carbon as is in the atmosphere. But the soil once held much more, and many believe that it has the potential to hold much more again. The Intergovernmental Panel on Climate Change estimates that cultivated soils have lost 50 to 70 percent of their original organic carbon.12 Much of that loss can be attributed to overuse of practices—some recent, some dating almost to the origins of agriculture—which disturb relationships within soil ecosystem.
Plowing exposes soil organisms to much more oxygen than in their underground environment and stimulates rapid bacterial oxidation and the release of CO2. Heavy equipment compacts soil and interferes with the movement of water and air. Overgrazing compromises the plants and their root systems at the head of the liquid carbon pathway. Intensive use of nitrogen fertilizers degrades soil organic matter, and may interfere with the signals that trigger plants’ release of nutrients required by microbes. Monocropping inhibits microbial diversity, as does indiscriminate application of biocides. (Recent research suggests that the negative effects of some herbicides such as glyphosate may be due not to their toxicity, but to the disruptions they cause to the relationships within microbial communities.)13
Yes, “dirt” is alive, and wondrously so. And our health, as well as our ability to respond to climate change, depends on its health.
(The Center for Ecoliteracy, working with the Whole Kids Foundation, has developed “Starting with Soil,” an interactive tablet-based introduction to living soil.14)