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Soil: Facts


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 or the “crust.” It is a complex combination of decaying plants and animals, rock, clay, sand, minerals, water, oxygen, carbon, and microorganisms. This recipe for soil varies depending upon location and the raw ingredients available. Moisture, weather and temperature also impacts soil production.


Layers of Soil

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. Soil forms in layers and each layer contains specific ingredients. The layers do not have defined borders, but can be gradient in nature due to gravity and water flow. Recognizing one layer from another can be very difficult ‐ in some cases only identifiable by experts. Here is a general look at the common layers of soil.



HumusHumus 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, hummus is ground garbanzo beans that you eat.)



TopsoilTopsoil contains the humus and can be about 6 inches thick. It can also contain earthworms, insects and other small creatures, water, garbage, plant parts, oxygen, chemicals from our environment and may even be covered in living plants. Animals live in this layer.



Subsoil — 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 — 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.



Bedrock ‐ 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. Here is another illustration of soil layers.

What Soil Does


Soil for plants — Plants need soil to provide them with minerals and other nutrients. The soil also holds water until the plant needs it. The roots of a plant also use the soil to help give the plant a base ‐ they 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. 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 on to them. The water continues to move through the layers of soil, leaving the impurities behind. Clean water ends up in the ground water. For more information about ground water visit Science Trek's water site.



Recycling System — Trash, leaves, dead plants, dead animals and other waste is recycled by microorganisms, earthworms and other creatures that actually feast off of these items and turn them into soil. The nutrients from the dead life are recycled to the worm, the insect, the snail or possibly the oak tree that takes it from the soil. Take a look at this soil food web here. 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 soil at all and there would be no home for plants and small creatures to live. One helps the other.



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. These areas of warm air have what is called low pressure ‐ meaning the air pushes down against the earth with less force than over a cooler area. When cooler air ‐ which has high pressure ‐ moves into this warm land area, wind is created. So soil temperature has an impact on wind. Under extreme situations, these air pressure areas of warm and cool can create tornadoes, too. During a wind storm, soil can be blown up into the air causing a dust storm. This blowing dust can impact weather too.

Soil also puts off a number of gases into our atmosphere. Carbon dioxide, methane, and other gases change the temperature of the atmosphere and its ability to hold oxygen. This too, impacts our weather over longer periods of time.



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. Watch how glass is made here. 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.

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. Unless you built a house right next to that river. Then you might not feel the same way.

Rock Arch

But, 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.


Chemicals can also contribute to weathering. When water reacts chemically with a mineral or a compound and drips repeatedly on a rocky surface it has the potential to dissolve rock. This happens in caves, for example, where this constant dripping can dissolve the rock and cause it to reform on the bottom of the cave floor. This results in a stalagmite. Or if it reforms as it drips, it can cling to the roof of the cave and form a stalactite.

Other forms of weathering include: plants growing in holes or cracks of rocks, water repeatedly freezing and thawing in rocks, animals, volcanoes, 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.

Grooves formed by wagon wheels in a rock along the Oregon Trail



An agronomist studies soils and works to improve crops and growing technology. You might think about becoming an agronomist someday. To learn more about what an agronomist does, click here.

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