Soil Facts

Soil ['sɔĭ(-ə)l]

The mineral and organic matter that covers most of the Earth's surface.

Holding soil

Soil, at first glance, may not be your favorite topic. But maybe it should be. Soil is a critical natural resource that provides for our environment in so many ways. Soil is a complete ecosystem of its own. Soil is food for some creatures and home to many others. Soil helps to clean our water, recycles nutrients, provides a place for plants to grow, and provides gases to our atmosphere. Soil impacts our weather and provides materials for human industry and construction. So what is soil ‐ exactly? Let's dig in and find out…

What is soil?

Soil sits on the outer layer of the earth. It forms at the surface of land and can be thought of as the “living skin of the earth.” Soil is a complex combination of minerals, rock fragments, air, water, and organic matter -- both living and decaying remains of once-living plants, animals, and microorganisms. The recipe for soil varies depending upon the type of rock material, living organisms, climate, and surface slope. Soil contains small spaces, called pores, that hold air and water. In some places on Earth, a thin layer of soil only six inches (15 cm) thick lies on top of bedrock. In other places, the soil may be hundreds of feet deep.

Layers of soil

What is the difference between soil and dirt? We refer to “dirt” as what gets under our fingernails, in the corners of our garage, or on the bottoms of our shoes. Soil, on the other hand, is capable of supporting plant life and is essential to life on earth.

All soil consists of weathered rocks that are broken down over time by wind, water, and temperature changes. It takes a long time for soil to form. It can take between 500 and 1,000 years to form one inch of soil.

Potters soil mixture
Potters soil mixture

The tiny rock particles in soil are known as sand, silt, and clay. The mix of sand, silt, and clay is referred to as soil texture. Sand is the largest type of soil particle. You can actually see individual grains of sand. Sand does not hold water well and feels gritty when you touch it. Soil with smaller particles is called silt. It feels smooth between your toes, almost like powder or flour. Clay has the smallest particles and individual rock pieces cannot be seen without a microscope. Clay feels sticky when wet. Some soils contain more sand, some more silt, and some more clay. Different plants require different amounts of sand, silt, and clay to make ideal soil for growing.

Layers of Soil

Layers of soil diagram

Soil can actually be mapped for different features and qualities. There are over 25,000 different named soils in the United States alone. Even though each soil has specific conditions that give it different names, there are some consistent properties in soil. If you could dig far underground, you’d see that soil forms in layers, or soil horizons (O, A, B, C, R.) Each layer contains specific ingredients. The layers do not have defined borders, but can be gradient in nature due to gravity and water flow. Here is a general look at the common soil horizons.

Humus (O horizon)Humus is the topmost layer. It is the dark, nutrient-rich layer which can be very thick in some geologic areas and from thin to nonexistent in others. This layer consists of decayed plants and animals along with microorganisms. The microorganisms have the job of decomposing the dead plants and animals into soil. This is a matter of recycling. As a part of this decomposition, the dying plants and animals give off carbon which is also part of the humus layer.

(Don't confuse humus with hummus ‐ humus is soil, and hummus is ground garbanzo beans that you eat.)


Topsoil (A horizon)Topsoil contains the humus and can be about 5—10 inches thick. It can also contain earthworms, insects, and other small creatures, water, plant parts, oxygen, minerals, and chemicals from our environment and may be covered in living plants. The A horizon is the primary layer where plants and animals live.


Subsoil (B horizon) — Just below the topsoil is the subsoil layer which contains minerals and clay that have been weathered from rocks. These are mixed with a small amount of decayed matter. Water and wind grind the rocks into smaller and smaller pieces. Rain and gravity can help the small rock particles settle down to the subsoil layer. Plants' roots can reach to the subsoil layer as they grow looking for water. The roots absorb and take in iron, calcium, magnesium, and other known beneficial minerals.


Parent Material (C horizon) — This layer is just below the subsoil layer and is known as parent material, but is really just rock. It consists of small weathered particles, sand, clay, salts, and minerals with no living matter of any kind.

Parent Material

Bedrock (R) ‐ Below the parent material is the bedrock. Bedrock is solid rock. It will stay solid until some force of nature exposes it to the elements where it will be weathered and the process of soil creation will begin again.


The layers vary in depth depending on the geologic area in which they exist. Weather and temperature can have an impact on the layers' content and thickness, as well. These illustrations show variation in the depth of each layer.

What Soil Does

Soil for Plants — Plants need soil, and we need plants. Much of our food and clothing comes from plants or from plant-eating animals. About 95% of human food production depends directly or indirectly on soils. Soil provides plants with necessary minerals and other nutrients. The soil also holds water until the plant needs it. Soil anchors the roots of a plant to help give the plant a base ‐ the roots grab the soil and use it to keep them upright and strong. Plants also give back to the soil by adding nutrients when they die. Their roots can also prevent erosion and help the soil hold water by shading it from the sun.


Soil for Animals — Many living creatures make their homes in the soil. Insects, bacteria, earthworms, snails, fungi, and others help build soil by the waste they create as they eat dead plants and animals. They also dig and tunnel in the soil creating pockets of air and loosening the soil for water and roots. In one acre of cropland soil, there can be as many as 1 million earthworms, hard at work improving the soil. Larger animals make their home in the soil too. Badgers, moles, gophers, snakes, turtles, and other burrowing creatures can dig huge tunnel systems under the ground.


Soil Filters Our Water — Water can trickle down through the layers of soil, sand, and clay because of its ability to fit between the particles. Soil has a negative charge, while the contaminants tend to be positively charged. As the water moves down, the soil acts like a magnet, attracting the impurities and holding onto them. The water continues to move through the layers of soil, leaving the impurities behind. Clean water ends up in the groundwater. For more information about ground water visit Science Trek's water site.

Soil filtering water

Recycling System — Leaves, dead plants, dead insects, and other waste are recycled by microorganisms, bacteria, fungi, and other decomposers that actually feast off of these items and release nutrients back into the soil. In a teaspoon of good soil, there are more microorganisms than there are people on Earth! The nutrients from the dead life are recycled to the worm, the insect, the snail, or the oak tree that takes it from the soil. Other organisms in turn feed on them. Take a look at this soil food web. This is actually a great system. Without it, we would be knee-deep in dead stuff everywhere with no way to really dispose of it. We would have no fertile soil and there would be no home for plants and small creatures to live. Soils process recycled nutrients so that living things can use them over and over again.

Larva underground

Soil and Weather — Soil heats up when the sun shines on it, creating temperatures that can impact evaporation of the water within the soil and in nearby plants, lakes, or rivers. As part of the water cycle, this evaporation puts water into the atmosphere that later becomes rain, snow, sleet, or hail. In areas where there is a lot of soil being heated, such as a desert or a beach, the soil temperature can create areas of warm air. When cooler air moves into these areas of warm air, wind is created. So soil temperature has an impact on wind. During a wind storm, soil can be blown up into the air causing a dust storm. This blowing dust can impact the weather too. Soil also impacts our atmosphere by giving off and absorbing gases such as carbon dioxide and methane. Soil scientists believe that farming practices that keep carbon in the soil may help reduce climate change.

Dry Soil

Soil and Construction — People have been using soil as a construction material for thousands of years. Bricks, concrete, cement, rock, marble, and even lumber rely on the soil and the layers under the soil to provide these building materials. Even glass is fashioned from silica which is a type of sand. So while we think of soil being the home for small creatures like earthworms and ants, we see that soil helps provide people with homes, as well.

Brick makers

Surprises from Soil — Did you know that aluminum cans come from soil? Aluminum starts out locked inside the soil in an ore called bauxite. Penicillin, a medicine that fights disease-causing bacteria, is made from a mold that grows in soil. Ceramic pottery and bowls are made of a type of clay soil. Pigments found in soil are used in paints and dyes.


Erosion and Weathering

Soil does not stay put in one place. The wind blows soil around, water washes soil from one place to another, people put soil in the back of their pickup and take it to another location or they drive their motorbikes repeatedly over a trail. These are just some of the ways that soil moves. When soil is displaced by nature or man it is called erosion. Erosion can be good at times. When the nutrient-rich soil at the bottom of a river moves out onto the land following a flood, it provides good food for growing plants. This type of erosion might be considered good.

Soil erosion

However, erosion can be considered bad when it carries humus and topsoil down a mountainside after a heavy rainstorm and leaves plants and animals without its benefits. In this case, plants can become uprooted and animals can lose their shelter. Grooves form in the mountainside and the land becomes unstable. Rocks can slide down the mountain and become dangerous.

Weathering happens when rocks are broken down into smaller pieces. Repeated wind or water actions can chisel away at rock and dissolve it into smaller rocks or sand. We have all seen rock arches along a beach or in rock canyons. This is a result of weathering. Repeated wind and rock pummeling the once strong rock structure takes its toll and removes portions little by little until all that is left is the arch. Once the rock has been reduced to smaller pieces, erosion can then jump in and move the pieces
to new locations.

Weathering clay

Chemicals can also contribute to weathering. When water reacts chemically with a mineral or compound and drips repeatedly on a rocky surface, it has the potential to dissolve rock. Other agents of weathering include plants growing in holes or cracks of rocks, water repeatedly freezing and thawing in rocks, animals, earthquakes, and even people. Lots of evidence remains of the trek made across the plains by the pioneers. Grooves were sometimes made by the wheels of their many wagons that repeatedly rolled over and over rocks. The rock was eroded away and became part of the soil. The result is the weathered scar that can still be seen in some places today.

Topsoil is a valuable natural resource that provides the foundation for life on earth. Life as we know it could not exist without soil! It is important that we conserve and take care of our earth’s soil. In the past, careless building, mining, and farming practices have sometimes increased soil erosion and degraded the quality of soil on our earth. Today, farmers often reduce erosion by planting trees, grasses, and windbreaks to help keep the soil in place. You can help protect soil by planting vegetation or starting a compost pile. When you are hiking, stick to established trails, and don’t hike when it’s muddy – leave the soil on the trail, not on your shoes!

Agronomy and Soil Science


An agronomist studies soils to improve crops and growing technology. A soil scientist studies the physical, chemical, and biological components of soil. You might think about becoming one someday. Learn more about the work of agronomists and soil scientists.

Top 10 Questions

May 2015

Thanks to Carla Rebernak, soil scientist, Natural Resources Conservation Service, USDA; and David Huber, PhD candidate in Biology, Idaho State University for the answers.

  1. How is soil formed?

    Soil usually starts as a rock. The rock gets rained on, frozen, or falls and it breaks into small pieces where you have a lot of little rocks or grains. Matter, like a leaf or animal waste, gets added to that over thousands of years. Water and air become a part of the mixture, and this is what we call soil. (From Kaden at Kamiah Elementary School in Kamiah)

  2. Are there more creatures that live in the dirt or more that live on land onEarth?

    There is estimated to be about ten to the 19th different species of bacteria and other organisms living in the soil, which is about ten quintillion different species. While on land, there are close to ten to the seventh, which is around ten million different species. So there are quite a few more that live in the soil than live on land. (From Mason at Whittier Elementary School in Boise)

  3. Are there different kinds of soil?

    There are different types of soil across the world and countries use different systems for describing soils. In the United States, we have 12 main types of soil that we call orders. Then, if you describe those soils in detail down to their finer properties, there are about 15 to 20 thousand unique soils in the United States alone. (From Ryenne at Dalton Elementary School in Dalton Gardens)

  4. How deep does soil go?

    That can vary. Soil can be a few centimeters thick or meters thick. It depends on its position on the land form, how resistant to weathering the parent materials are, or the age and its exposure to weathering. (From Jahal at Owyhee Harbor Elementary School in Boise)

  5. How many layers of soil are there?

    When scientists talk about the layers, they are talking about the horizons. The horizons are unique layers, usually parallel to the surface, and they differ from one layer to the next. Most know of the three layers called topsoil, subsoil, and parent material. Those are just broader categories of the layers. The top soil is the part on the surface. It usually has more organic material in it and it's a little darker in color. It's also crumbly and soft and allows us to plant our gardens in it. Beneath that is the subsoil. It's lighter in color because it has less organic material. Then below the subsoil is the parent material. This layer is the unweathered sediment from which the soil is formed. (From Curtis at Dalton Elementary School in Dalton Gardens)

  6. How does soil give nutrients to plants?

    Soil stores a lot of the nitrogen, phosphorus, potassium, and other nutrients that plants need both in the solution and attached to the clay and organic particles in the soil. When water moves through the soil, it picks up some of those nutrients and moves them to the plant roots where they are able to take them up and use them for their biomass and the processes that they need. (From Aaron at Basin Elementary School in Idaho City)

  7. Is soil alive?

    Yes, there are parts of the soil that are very alive as the soil has many micro-organisms too tiny for the eye to see. In one teaspoon of soil, there are a billion bacteria. So, soil is definitely alive. (From Reanna at Hawthorne Elementary School in Boise)

  8. Is soil different from dirt?

    Soil is the scientific term we would use to describe that intact part of the Earth's crust that supports plant life. Dirt is usually a term used to describe soil out of its place of origin or when it has been removed from the Earth's crust. You may say you have dirt under your nails or on your boots. (From Evan at Owyhee Harbor Elementary School in Boise)

  9. How long does it take for something to break up and make soil?

    It depends on a few things. If the climate is warmer, has more moisture and is less resistant to weathering, it will become soil faster. If the climate is colder, has less moisture and is more resistant to weathering, it will take longer for soil to form. (From Alex at Basin Elementary School in Idaho City)

  10. Why is soil brown?

    Soil is usually brown because of organic input from dead plant or animal material that gets incorporated into the top layer. Soil, however, doesn't have to be brown. It can be red, blue, purple, pink, white, or all kinds of color. Iron oxide may make soil a reddish brown color. Certain materials in the parent matter may make soil, like sand, where all organic matter has been stripped away, appear white. It is the organic matter that typically makes it brown. (From Justin at Cynthia Mann Elementary School in Boise)