What is dirt? There’s a whole wriggling world alive in the ground beneath our feet, as a soil scientist explains

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Curious Kids is a series for children of all ages. If you have a question you’d like an expert to answer, send it to [email protected].

What is dirt? – Belle and Ryatt, ages 7 and 5, Keystone, South Dakota

When you think about dirt, you might picture the rock dust that gets on your pants. But there’s so much more going on in the ground under our feet.

When I began studying soil, I was amazed at how much of it is actually alive. Soil is teeming with life, and not just the earthworms that you see on rainy days.

Keeping this vibrant world healthy is crucial for food, forests and flowers to grow and for the animals that live in the ground to thrive. Here’s a closer look at what’s down there and how it all works together.

Soil is a vibrant ecosystem.
Gabriel Jimenez via Unsplash, CC BY-SA

The rocky part of soils

If you scoop up a handful of dry soil, the basic dirt that you feel in your hand is actually very small pieces of weathered rock. These tiny bits eroded from larger rocks over millions of years.

The balance of these particles is important for how well soil can hold water and nutrients that plants need to thrive.

For example, sandy soil has larger rock grains, so it will be loose and can easily wash away. It won’t hold very much water. Soil with mostly clay is finer and more compact, making it difficult for plants to access its moisture. In between the two in size is silt, a mix of rock dust and minerals often found in fertile flood plains.

Some of the most productive soils have a good balance of sand, clay and silt. That combination, along with the remnants of plants and animals that have died, helps the soil to retain water, allows plants to access that water and minimizes erosion from wind or rain.

Loamy soil, ideal for gardens, is a mix of sand, clay and silt.
NOAA

The wriggling, munching parts of soil

Among all those rock particles is a whole world of living things, each busy doing its job.

To get a sense of just how many creatures are there, picture this: The zoo in Omaha, Nebraska, boasts over 1,000 animal species. But if you scooped up a small spoonful of soil in your backyard, it would likely contain at least 10,000 species and around a billion living microscopic cells.

Most of those species are still largely a mystery. Scientists don’t know much about them or what they do in soil. In fact, most species in soil don’t even have a formal scientific name. But each plays some kind of role in the vast soil ecosystem, including generating the nutrients that plants need to grow.

Lifting a rock reveals a symphylan, or garden centipede, left, and a poduromorph, or plump springtail, munching through the soil.
Marshal Hedin via Wikimedia, CC BY

Imagine a leaf falling from a tree in late autumn.

Inside that leaf are a lot of nutrients that plants need, such as nitrogen, potassium and phosphorus. There is also a lot of carbon in that leaf, which holds energy that can be used by other organisms such as bacteria and fungi.

The leaf itself is too large for a plant to take up through its roots, of course. But that leaf can be broken down into smaller and smaller pieces. This process of breaking down plant and animal tissue is known as decomposition.

When the leaf first falls to the ground, arthropods – such as insects, mites and collembolans – break the leaf down into smaller chunks by shredding the tissue. Then, an earthworm might come along and eat one of the smaller chunks and break it down even more in its digestive tract.

PBS explores how earthworms help turn dead plants into fertile soil.

Now the broken-up leaf is small enough for microbes to come in. Bacteria and fungi secrete enzymes into the soil that further break down organic material into even smaller pieces. If enough microbes are active, eventually this organic material will be broken down enough that it can dissolve in water and be taken up by plants that need it.

To aid in this process, there are many small animals, such as nematodes and amoebae, that consume bacteria and fungi. There are also predatory nematodes that feed on other nematodes to make sure they don’t become too abundant, so everything remains in balance as much as possible.

It’s quite a complicated food web of interacting species in a delicate balance.

A time-lapse video filmed about 4 inches underground shows a leaf decomposing over 21 days in July. At the end, radish roots make their way down into the soil. Video by Josh Williams.

While some fungi and bacteria can harm plants, there are many species that are considered beneficial. In fact, they may be the key to figuring out how to grow enough crops to feed everyone without degrading and overburdening the soil.

Figuring out your soil type

Scientists have named over 20,000 different types of unique soils. If you’re curious about the soil and dirt in your area, the University of California, Davis has a website where you can learn more about local soils and their chemical and physical attributes.

Caring for soil to promote its living creatures’ benefits and minimize their harm takes work, but it’s essential for keeping the land healthy and growing food for the future.

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