How do plants hold dirt? –Gordon, Pullman, WA

Dear Gordon,

The other day, I wandered into a Washington State University greenhouse and ran into my friend Mechthild Tegeder, a professor and expert on plants.

She gently dug a small plant out of a pot so we could take a closer look. When she lifted it up, I pawed at the clumpy soil hanging from the bottom to reveal some stringy roots.

“They’re amazing, aren’t they?” Tegeder said. “The root system functions like a web, anchoring the plant and the soil.”

The plant had lots of short, fine roots growing near the surface. Tegeder explained that another kind of root is a taproot and it tends to grow straight down. You have eaten one of these before if you’ve ever had a carrot.

While some roots grow near the surface, other roots make a journey deeper into the Earth. In fact, scientists have found roots nearly 200 feet below the surface of huge trees.

These roots can grow wide, too. Like the underwater part of an iceberg, a plant’s underground web of roots can take up to about four times as much space as the plant itself.

Whether you look at the roots of a giant tree, a little dandelion, or a carrot, they each have a couple things in common. As you know, they anchor plants to the soil. But they also deliver water and nutrients, or food, to the above-ground part of the plant.

Plants actually do this using really tiny hairs that sprout out of their roots. These little root hairs absorb the water and nutrients from the dirt. They deliver them up the roots, to the stem, and the rest of the plant or tree.

And roots look for these important resources anywhere they can. That’s part of the reason they will grow out in different directions.

In fact, there is even a special part of these hairs that scientists believe helps the roots sense where they are going in the soil. It’s a bit like an obstacle course, or like using your hands to navigate through a dark room.

These roots will grow in any space they can find. For small plants, this might mean empty space between clumps of soil. For big trees, it might mean roots that start to grow up and over sidewalks or walls.

Not all roots grow down or underground, though. Roots can grow up and out of the soil to reach into the air for nutrients and water. Then there are plants that don’t have roots at all.

But roots are really helpful to plants that do use them. As the roots and soil hang onto each other, they keep the important top layer of soil—the part we use to grow food—from washing away in the rain or blowing away in strong winds. Roots don’t just help the plant, but also the soil itself.

As you can see, it really just takes a bit of digging to get to the bottom of it. Keep asking great science questions.

Sincerely,
Dr. Universe

Ask Dr. Universe is a science-education project from Washington State University. Send your own question to Dr.Universe@wsu.edu or vote in this week’s reader poll at askDrUniverse.wsu.edu.