What Is Soil? Get the Dirt on Dirt

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The ground beneath your feet is essential for the survival of life on Earth, yet soil remains a complete mystery to most people.

What is soil? The answer is a lot more complicated than you might think. Soil is not just brownish-gray muck. It's alive, it's breathing, and you should probably get to know a little more about it.

But getting the facts on soil isn't just an academic exercise. Due to erosion, soil is disappearing at an alarming rate all over the world. Without it, life on the surface of this planet will be next to impossible.

The Facts on Soil

Soil is a complex mixture of four primary components: water, air, granular rocks and minerals, and the living creatures that thrive within soil.

The broken and eroded rocks and minerals in a particular area's soil are sometimes called the soil's "parent material." A soil's parent material plays a role in determining how acidic or alkaline it is (referred to as the soil's pH).

This can be an important consideration in farming and gardening because some plants (azaleas and rhododendrons, for instance) prefer acidic soil. Whereas others (e.g., cherries and maples) need a more alkaline soil.

Fun Fact

There are more microorganism in a teaspoon of healthy soil than there are people on Earth, according to the U.S. Department of Agriculture.

Sand, Silt, Clay, and Humus

When scientists talk about soil, they often refer to a "soil profile" or "series," the type of soil that's typical of a particular region. Their soil horizons or layers further describe soil series: The uppermost soil horizon is exposed at the surface and contains live plants and plant matter that hasn't decomposed yet.

Deeper soil horizons (there can be several in one region's soil profile) are generally dryer and lighter in color than the uppermost layer. Deeper horizons also have less biological activity than higher layers.

Soil series and soil horizons vary by how much sand, silt, and clay they contain. Except for occasional rocks and gravel, sand is the largest grain of solid matter that's in soil; silt particles are smaller. Clay is important because it helps soil retain water and nutrients. Soil that has a good balance of sand, silt, and clay is called "loam," and is often considered the best soil for farming.

Humus is the stable organic material in soil and is an important source of plant nutrients. Humus also helps soil retain water and suppress disease pathogens. Compost is one source of humus, as is the natural decay process of leaf litter, dead plants and animals and animal droppings.

Soil Is Alive

Perhaps the most intriguing component of soil is its living organisms. Scientists are just beginning to understand the mind-boggling complexity of the microbes, fungi, nematodes, mites, insects, and other animals that can only exist in soil.

"Scientists using new analytical techniques over the last decade have found that the world's ocean of soil is one of our largest reservoirs of biodiversity. It contains almost one-third of all living organisms," The New York Times reports, "but only about 1 percent of its micro-organisms have been identified, and the relationships among those myriad life-forms [are] poorly understood."

One small teaspoon of soil can contain billions of microbes―including some 5,000 different types―as well as thousands of species of other living things, from viruses and protozoa to earthworms and termites.

And all of it is threatened.

Why Soil Conservation Is Critical

Perhaps the leading soil scientist and soil conservation advocate alive today is Ronald Amundson, chair of the Department of Environmental Science at the University of California at Berkeley.

In 2003, Amundson was the lead author of a report detailing the disappearance of soil types across the North American continent. His research team's conclusion was unambiguous: 31 different soil types are now effectively extinct. The areas where they once existed have been completely converted to agricultural or urban/suburban land use.

An additional 508 endangered soil series are found across the United States in a variety of regions.

"Over the past two centuries, we have reconfigured part of a continent to the point where today's landscape is almost unrecognizable from its natural state," Amundson said. "The Great Plains used to be characterized by tall grasses and prairies. Crops and housing tracts have now replaced them."

And the degradation of soil may contribute to climate change: Digging up soil releases carbon dioxide, which contributes to the greenhouse gases in the atmosphere. "Soil has more carbon in the form of organic matter than all the plants in the world," said Amundson.

Other threats to soil include salination (increased salt content), acidification (lowered pH and reduced alkalinity), deforestation, overgrazing, construction activity and pollution from heavy metals and radiation.

The soil conservation message is clear: If we don't start taking better care of our soil, it won't be there to take care of us.