What Is Soil and Why Is It Important?

The United Nations has designated December 5 World Soil Day, to bring awareness to the necessity of improving soil health for the good of the planet. Which may leave some folks wondering what, exactly, soil is, let alone why it matters enough to rate its own global day.

The answer to the first part of this question depends who you ask. For a lot of us, there’s no discernable difference between dirt and soil. It’s what we wash off our hands, freak about when it’s tracked in the house, and maybe what we stick the roots of a windowsill-bound basil plant into (that we’ll probably forget to water).

For scientists, farmers, and (real) gardeners, though, soil is a loftier thing. Unlike dirt, which is mostly broken-down rocks that contain minerals like calcium, magnesium, and iron, soil is dirt with the added bonus of bacteria, fungi, protozoa, worms, and insects. These aerate, build structure, and give vital nutrients to soil. Add plants to this mix, and what you get is a complex system called a “soil food web.” It’s alive, and it also supports our life on this planet.

There isn’t just one kind of soil, and the kind of soil you’ve got determines the kind of food you can grow well. Which is to say, if you care about eating, you care about soil. Healthy soil can also help us cope with the precarious effects of climate change on our food system—including by storing a whole lot of carbon. How does it all work? Here we get down—way down—to basics.

What You Can See

What You Can’t See

There are a mind-boggling number of microscopic organisms in your rhizosphere. If you have “good soil,” says soil biologist Elaine Ingham, one gram of it can contain 75,000 species of bacteria, 25,000 species of fungi, 1,000 species of protozoa, and several hundred species of nematodes (small worms).

If you remember food chains from elementary school, soil food webs will sound familiar. Plant roots secrete sugars and organic matter, which are eaten by bacteria and fungi. Nematodes and protozoa eat the bacteria and fungi, parts of which they excrete; then plant roots absorb those nutritious excretions.

Bacteria and fungi are especially important to plant growth, each in their way contributing to its health and its ability to fight off pests and disease. According to Dorn Cox, research director at the Wolfe’s Neck Center for Agriculture & Environment in, Freeport, ME, the fungi that create a network with a plant’s roots “provide feedback between what bacteria in the soil want and what the plants are producing. A huge amount of the energy of plants is not put into their own growth but into feeding their microbiome.”

There are (visible) insects is soil, too. Beetles, earthworms and spiders are eating and also being eaten. When these organisms die naturally, they, too become food for the organisms that remain.

Your Soil Food Web

A soil scientist will tell you that the kind of soil you’ve got depends on the kinds of rocks it started with — its “parent material.” It contains differing amounts of sand, silt, and clay and a variety of minerals. A good combination for food-growing purposes creates loam; the sand in it lets roots penetrate, the clay in it holds moisture.

Pretty much any soil can grow something. But if you want to grow tomatoes, or wheat, or peaches and not just weeds, says Ingham, you’re going to need a balance of organisms in your soil food web. So, if you had an underwhelming harvest of tomatoes this past summer, for example, it’s highly likely you had the wrong ratio of fungi and bacteria in your soil.

Write Jeff Lowenfels and Wayne Lewis in their book “Teaming With Microbes,” “Each soil environment has a different set of organisms and thus a different soil food web.” That’s because, as Mark Kopecky, a private soils agronomist in Florida, explains it, “Plants were designed to be members of a community and every type has its own mix of microbes and other organisms it would like to have growing with it, and they take care of each other,” including alerting each other when there’s a pest problem.