by Ellen R. Braaf
Dirt's gotten a bad rap. How many times have you been warned “Don't get dirty” or been scolded for tracking dirt into the house? No one wants to be called “as dull as dirt” or to be treated “like dirt.” Even the word “dirt” has lowly origins. It comes from the Old English word “drit,” meaning manure or excrement (poop).
But if you've ever felt oozy mud squishing between your toes or planted seeds in soil and watched them bloom into beautiful flowers, you know that dirt is amazing stuff!
Most of the things you see around you can be traced back to dirt—from the pizza you ate for lunch to the microchips in your computer. Dirt holds the roots of plants and supplies them with nutrients so the plants can grow. (And without green plants, we'd have no food to eat and no oxygen to breathe.) Dirt is the home for countless living organisms. Just a handful contains more living things than all the men, women, and children on the face of the earth. And dirt is nature's great recycler. When plants and animals die, organisms in the dirt feed on them, releasing their chemicals and energy into the soil for use by future generations.
So dirt is really one of the most important things there is! We need dirt as much as we need water and air and sunshine.
But there isn't as much dirt around as it might seem. The layer of soil covering the earth is only a few feet deep. It's like the thin skin on an apple—except this skin doesn't cover the whole apple. Remember that 75 percent of the earth's surface is water. And mountains and deserts and ice-covered polar regions take up their share of the remaining land. Scientists estimate that only about 10 percent of the earth is covered with soil that is useful for farming. People, however, use lots of that valuable land not for farms but for their homes, highways, schools, and shopping malls. Far from being “as common as dirt,” soil is precious. Some types are even so rare as to be listed as endangered.
A Rocky Start
There hasn't always been dirt on earth. Like plants and animals, dirt had to evolve.
Scientists believe that life on earth first appeared in the oceans about two billion years ago, but living things didn't begin to make their way to dry land for another billion and a half years or more. During all that time, most dry land was very rocky. There may have been windblown sand dunes in desert regions and some gravelly rock particles in mountain valleys, but no fine dirt and rich soil as we know it today. Why not?
Because fertile soil—soil in which plants can grow—is made up of two kinds of components: finely crumbled rocks, which are rich in minerals, and organic material, from rotting plants and animals, which is rich in energy and nutrition. These components had to work together over millions and millions of years to make today's dirt.
Life Depends on Dirt and Dirt Depends on Life
How is dirt made? When rock is exposed to the forces of nature, it breaks down into smaller and smaller pieces in a process called “weathering.” During the heat of the day, a rock expands. At night, when the temperature cools, the rock contracts. This cycle stresses the rock and eventually cracks it apart. Or water can get into rock crevices and freeze. As the ice expands, the rock fractures. Wind and rushing water can wear down rocks, too. The growth of plants, which send their roots into the soil, helps complete this weathering process. Over time, the roots break up the rocks into fine particles of dirt.
When, millions of years ago, primitive plants and animals moved out of the oceans to the land nearby, soil that could support an abundance of life began to form. The first land plants were small and simple. They could survive on the minerals in rocks along with air, sunlight, and rain. When they died and rotted, they left behind organic material, called humus, that would help feed other plants. Over time, as plants began to spread further inland, they became larger and more complex. Their roots broke up more rocks to help create more soil. Slowly, the world was covered with plants and soil.
Animals also began to inhabit the land, living off the plants. When they died, their bodies added to the organic components of the soil, making it richer and able to support a greater variety of life. Eventually, the land became as full of life as the oceans.
It's a Zoo Down There
People talk about being “dead as dirt,” but actually soil is teeming with life. With a little patience, you might see some of the bigger and furrier guys, such as ground squirrels and moles. But there are also less furry things down there. Most insects spend at least part of their lives in the soil. And there are worms. Lots and lots of worms. Under a meadow the size of a football field, there might be more than a million worms wiggling around. Sounds yucky, maybe, but you should be glad those worms are there. As they dig their little burrows, they break up the dirt so that it can hold more water and air and grow more plants. And worm poop fertilizes the soil. Worms swallow dirt and digest the bits of leaves and plants in it. Their waste, called castings, is filled with nutrients that plants need. Worms aren't the only wee beasts feasting underground. When something dies—a leaf, a tree, or an animal—nature's recyclers in the dirt get to work. It's like a science-fiction thriller where everything is eating everything else.
All kinds and shapes of bacteria feed on dead organisms. In turn, microscopic creatures called protozoa feed on the bacteria. The most common tiny animals in the soil are nematodes, or roundworms (although they are not really worms). Some roundworms dine on bacteria, some prey on protozoa, and some eat other roundworms. Fungi, such as mushrooms, grow and feed on rotting things. Some fungi also eat nematodes. And then there are armies of worms, millipedes, and ants munching away. They all work together to break down and recycle the chemicals and energy stored in dead plants and animals.
With dirt being so important, you might think people would take good care of it. But every year we lose untold tons of valuable soil to erosion and waste. Too often, people treat dirt, well, “like dirt.”
People need to understand how dirt works in order to preserve it. When farmers clear the land for crops, they interrupt dirt's delicate recycling process. Unless they leave organic material on the land to decompose and enrich the soil, the soil can become worn out. That's one reason farmers have to use artificial fertilizers.
Ancient Egypt was called “the gift of the Nile.” Every summer, as the monsoon winds from the Indian Ocean blew into the highlands of central Africa, a steady torrent of rain fell. Then rich, brown water flowed down the mountains into the Nile to Egypt, 1,500 miles away. The Nile was muddy with mineral-rich silt and nutritious humus. It flooded Egypt every year, leaving behind a thin skin of new soil only about 1/20th of an inch thick. But this little bit of good dirt meant the difference between life and death and allowed a thriving civilization to grow in Egypt. In modern times, however, dams have been built on the Nile to prevent flooding, and now artificial fertilizers are needed for the crops to grow. While we might take dirt for granted, some ancient people did understand the power of destroying the soil. When the Romans conquered Carthage, their chief rival, in 146 BC, they razed the city and sold the people into slavery. They also poured salt over the earth. Then they knew the city was dead, because the land could never again be used to grow crops.
Playing in the Dirt
It takes an amazingly long time for the weathering of rocks and the activities of animals and plants to produce dirt—as much as 1,000 years to form just one inch of fertile soil. So, give dirt some respect. Remember, the whole history of humanity is wrapped up in the history of dirt. But sometimes the best thing to know about dirt is just that it's so much fun to play in.
- abundance: An amount that is large or more than enough.
- decomposed: The breaking down of plant or animal matter.
- endangered: Threatened.
- nutrient: A substance that living things need in order to survive and grow.
- organic: Relating to living things.
- primitive: Of or in an early stage of development.
- raze: To level to the ground; demolish.
- torrent: A violent flow or downpour.
- What are the invertebrates mentioned in this article?
- What clue does the article give to support the fact that there are many more invertebrates than vertebrates on Earth?
- Why would it be good to introduce earthworms into a garden?