Soil is a precious resource. It allows us to grow food and the materials we use to make everything from the shirt you have on to the medicine you took this morning. Soil is made up of small pieces of rock that have broken down over hundreds, if not thousands, of years. Soil is also partly made up of the remains of plants and animals, and is home to many organisms, from earthworms to ants. But soil can be damaged by unsustainable farming practices and clear-cut logging. In this chapter, you will learn how soil forms, what it contains, and how to protect it.
Weathering breaks rocks apart. Some types of weathering alter some minerals. Erosion moves the broken pieces.
Weathering changes solid rock into sediments. Sediments are different sizes of rock particles. Boulders are sediments; so is gravel. At the other end, silt and clay are also sediments. Weathering causes rocks at the Earth’s surface to change form. The new minerals that form are stable at the Earth’s surface.
It takes a long time for a rock or mountain to weather. But a road can do so much more quickly. If you live in a part of the world that has cold winters, you may only have to wait one year to see a new road start to weather (Figure below).
A hard winter has damaged this road.
Mechanical weathering breaks rock into smaller pieces. These smaller pieces are just like the bigger rock; they are just smaller! The rock has broken without changing its composition. The smaller pieces have the same minerals in the same proportions. You could use the expression “a chip off the old block“ to describe mechanical weathering! The main agents of mechanical weathering are water, ice, and wind.
Rocks can break apart into smaller pieces in many ways. Ice wedging is common where water goes above and below its freezing point (Figure below). This can happen in winter in the mid-latitudes or in colder climates in summer. Ice wedging is common in mountainous regions.
(A) Diagram showing ice wedging. (B) Ice wedging along the joints in this rock helped to break it apart.
This is how ice wedging works. When liquid water changes into solid ice, it increases in volume. You see this when you fill an ice cube tray with water and put it in the freezer. The ice cubes go to a higher level in the tray than the water. You also may have seen this if you put a can of soda into the freezer so that it cools down quickly. If you leave the can in the freezer too long, the liquid expands so much that it bends or pops the can. (For the record, water is very unusual. Most substances get smaller when they change from a liquid to a solid.)
Ice wedging happens because water expands as it goes from liquid to solid. When the temperature is warm, water works its way into cracks in rock. When the temperature cools below freezing, the water turns to ice and expands. The ice takes up more space. Over time, this wedges the rock apart. Ice wedging is very effective at weathering. You can find large piles of broken rock at the base of a slope. These rocks were broken up by ice wedging. Once loose, they tumbled down the slope.
Abrasion is another type of mechanical weathering. With abrasion, one rock bumps against another rock. Gravity causes abrasion as a rock tumbles down a slope. Moving water causes abrasion it moves rocks so that they bump against one another (Figure below). Strong winds cause abrasion by blasting sand against rock surfaces. Finally, the ice in glaciers cause abrasion. Pieces of rock embedded in ice at the bottom of a glacier scrape against the rock below. If you have ever collected beach glass or pebbles from a stream, you have witnessed the work of abrasion.
Rocks on a beach are worn down by abrasion as passing waves cause them to strike each other.
Sometimes biological elements cause mechanical weathering. This can happen slowly. A plant’s roots grow into a crack in rock. As the roots grow larger, they wedge open the crack. Burrowing animals can also cause weathering. By digging for food or creating a hole to live in the animal may break apart rock. Today, human beings do a lot of mechanical weathering whenever we dig or blast into rock. This is common when we build homes, roads, and subways, or quarry stone for construction or other uses.
Mechanical weathering increases the rate of chemical weathering. As rock breaks into smaller pieces, the surface area of the pieces increases. With more surfaces exposed, there are more places for chemical weathering to occur. Let’s say you wanted to make some hot chocolate on a cold day. It would be hard to get a big chunk of chocolate to dissolve in your milk or hot water. Maybe you could make hot chocolate from some smaller pieces like chocolate chips, but it is much easier to add a powder to your milk. This is because the smaller the pieces are, the more surface area they have. Smaller pieces dissolve more easily.
Chemical weathering is different than mechanical weathering. The minerals in the rock change. The rock changes composition and becomes a different type of rock. Most minerals form at high pressure or high temperatures deep within Earth. But at Earth's surface, temperatures and pressures are much lower. Minerals that were stable deeper in the crust are not stable at the surface. That’s why chemical weathering happens. Minerals that formed at higher temperature and pressure change into minerals that are stable at the surface. Chemical weathering is important. It starts the process of changing solid rock into soil. We need soil to grow food and create other materials we need. Chemical weathering works through chemical reactions that change the rock.
There are many agents of chemical weathering. Remember that water was a main agent of mechanical weathering. Well, water is also an agent of chemical weathering. That makes it a double agent! Carbon dioxide and oxygen are also agents of chemical weathering. Each of these is discussed below.
Water is an amazing molecule. It has a very simple chemical formula, H2O. It is made of just two hydrogen atoms bonded to one oxygen atom. Water is remarkable in terms of all the things it can do. Lots of things dissolve easily in water. Some types of rock can even completely dissolve in water! Other minerals change by adding water into their structure.
Carbon dioxide (CO2) combines with water as raindrops fall through the air. This makes a weak acid, called carbonic acid. This happens so often that carbonic acid is a common, weak acid found in nature. This acid works to dissolve rock. It eats away at sculptures and monuments. While this is normal, more acids are made when we add pollutants to the air. Any time we burn any fossil fuel, it adds nitrous oxide to the air. When we burn coal rich in sulfur, it adds sulfur dioxide to the air. As nitrous oxide and sulfur dioxide react with water, they form nitric acid and sulfuric acid. These are the two main components of acid rain. Acid rain accelerates chemical weathering.
Oxygen strongly reacts with elements at the Earth’s surface. You are probably most familiar with the rust that forms when iron reacts with oxygen (Figure below). Many minerals are rich in iron. They break down as the iron changes into iron oxide. This makes the red color in soils.
Iron ore oxidizes readily.
Plants and animals also cause chemical weathering. As plant roots take in nutrients, elements are exchanged.
Each type of rock weathers in its own way. Certain types of rock are very resistant to weathering. Igneous rocks tend to weather slowly because they are hard. Water cannot easily penetrate them. Granite is a very stable igneous rock. Other types of rock are easily weathered because they dissolve easily in weak acids. Limestone is a sedimentary rock that dissolves easily. When softer rocks wear away, the more resistant rocks form ridges or hills.
Devil’s Tower in Wyoming shows how different types of rock weather at different rates (Figure below). The softer materials of the surrounding rocks were worn away. The resistant center of the volcano remains behind.
Minerals also weather differently. Some minerals completely dissolve in water. As less resistant minerals dissolve away, a rock’s surface becomes pitted and rough. When a less resistant mineral dissolves, more resistant mineral grains are released from the rock.
1. Name two types of mechanical weathering. Explain how each works to break apart rock.
2. What are three agents of chemical weathering? Give an example of each.
3. How do acids form in the atmosphere? What increases the acidity of rainfall?
4. What are the effects of acid rain?
5. Describe what you think weathering would be like in an arid region. What would weathering be like in a tropical region?
6. What type of surface weathers faster: a smooth surface or a jagged surface?
Without weathering, we would not have any soil on Earth. People could not live on Earth without soil! Your life and the lives of most organisms depend on soil. Soil is only a very thin layer over solid rock. Yet, it is the place where reactions between solid rock, liquid water and air take place. We get wood, paper, cotton, medicines, and even pure water from soil. So soil is a very important resource. Our precious soil needs to be carefully managed and cared for. If we don’t take care of the soil we have, we may not be able to use it in the future.
We can think about soil as a living resource. Soil is an ecosystem all by itself! Soil is a complex mixture of different materials. Some of them are inorganic. Inorganic materials are made from non-living substances like pebbles and sand. Soil also contains bits of organic materials from plants and animals. In general, about half of the soil is made of pieces of rock and minerals. The other half is organic materials. In the spaces of soil are millions of living organisms. These include earthworms, ants, bacteria, and fungi. In some soils, the organic portion is entirely missing. This is true of desert sand. At the other extreme, a soil may be completely organic. Peat, found in a bog or swamp, is totally organic soil. Organic materials are necessary for a soil to be fertile. The organic portion provides the nutrients needed for strong plant growth.
Soil formation requires weathering. Where there is less weathering, soils are thinner. However, soluble minerals may be present. Where there is intense weathering, soils may be thick. Minerals and nutrients would have been washed out. Soil development takes a very long time. It may take hundreds or even thousands of years to form the fertile upper layer of soil. Soil scientists estimate that in the very best soil forming conditions, soil forms at a rate of about 1mm/year. In poor conditions, it may take thousands of years!
How well soil forms and what type of soil forms depends on many factors. These include climate, the original rock type, the slope, the amount of time, and biological activity.
Climate is the most important factor in soil formation. The climate of a region is the result of its temperature and rainfall. We can identify different climates by the plants that grow there (Figure below).
Climate is the most important factor in determining the type of soil that forms in a particular area.
Given enough time, a climate will produce a particular type of soil. The original rock type does not matter. The same rock type will form a different soil type in each different climate.
Rainfall in an area is important because it influences the rate of weathering. More rain means that more rainwater passes through the soil. The rainwater reacts chemically with the particles. The top layers of soil are in contact with the freshest water, so reactions are greatest there. High rainfall increases the amount of rock that experiences chemical reactions. High rainfall may also carry material away. This means that new surfaces are exposed. This increases the rate of weathering.
In tropical regions with high temperatures and lots of rain, thick soils form with no unstable minerals or nutrients. Conversely, dry regions produce thin soils, rich in unstable minerals.
The temperature of a region is the other important part of climate. The rate of chemical reactions increases with higher temperatures. The rate doubles for every 10oC increase in temperature. Plants and bacteria grow and multiply faster in warmer areas.
Soil formation increases with time. The longer the amount of time that soil remains in a particular area, the greater the degree of alteration. The warmer the temperatures, the more rainfall, and the greater the amount of time, the thicker the soils will become.
The original rock is the source of the inorganic portion of the soil. Mechanical weathering breaks rock into smaller pieces. Chemical reactions change the rock's minerals. A transported soil forms from materials brought in from somewhere else. These soils form from sediments that were transported into the area and deposited. The rate of soil formation is faster for transported materials because they have already been weathered.
A soil is a residual soil when it forms in place. Only about one third of the soils in the United States form this way. The material comes from the underlying bedrock. Residual soils form over many years since it takes a long time for solid rock to become soil. First, cracks break up the bedrock. This may happen due to ice wedging. Weathering breaks up the rock even more. Then plants, such as lichens or grasses, become established. They cause further weathering. As more time passes and more layers of material weather, the soil develops.
Biological activity produces the organic material in soil. Humus forms from the remains of plants and animals. It is an extremely important part of the soil. Humus coats the mineral grains. It binds them together into clumps that hold the soil together. This gives the soil its structure. Soils with high humus are better able to hold water. Soils rich with organic materials hold nutrients better and are more fertile. These soils are more easily farmed.
The color of soil indicates its fertility. Black or dark brown soils are rich in nitrogen and contain a high percentage of organic materials. Soils that are nitrogen poor and low in organic material might be gray, yellow, or red.
The inorganic part of soil is made of different amounts of different size particles. This affects the characteristics of a soil. Water flows through soil more easily if the spaces between the particles are large enough and well connected. Sandy or silty soils are light soils because they drain water. Soils rich in clay are heavier. Clay particles allow only very small spaces between them, so clay-rich soils tend to hold water. Clay-rich soils are heavier and hold together more tightly. A soil that contains a mixture of grain sizes is called a loam.
Soil scientists measure the percentage of sand, silt, and clay in soil. They plot this information on a triangular diagram, with each type of particle at one corner (Figure below).
This diagram plots soil types by particle size.
The soil type is determined by where the soil falls on the diagram. At the top, the soil is clay rich. On the left corner, the soil is sandy. On the right corner, the soil is silty.
Soil develops over time and forms soil horizons. Soil horizons are different layers of soil with depth. The most weathering occurs in the top layer. This layer is most exposed to weather! It is where fresh water comes into contact with the soil. Each layer lower is weathered just a little bit less than the layer above. As water moves down through the layers, it is able to do less work to change the soil.
If you dig a deep hole in the ground, you may see each of the different layers of soil. All together, the layers are a soil profile. Each horizon has its own set of characteristics (Figure below). In the simplest soil profile, a soil has three horizons.
In this diagram, a cut through soil shows different soil layers.
The first horizon is the “A“ horizon. It is more commonly called the topsoil. The topsoil is usually the darkest layer of the soil. It is the layer with the most organic material. Humus forms from all the plant and animal debris that falls to or grows on the ground. The topsoil is also the region with the most biological activity. Many organisms live within this layer. Plant roots stretch down into this layer. The roots help to hold the topsoil in place.
Topsoil usually does not have very small particles like clay. Clay-sized particles are carried to lower layers as water seeps down into the ground. Many minerals dissolve in the fresh water that moves through the topsoil. These minerals are carried down to the lower layers of soil.
Below the topsoil is the “B“ horizon. This is also called the subsoil. Soluble minerals and clays accumulate in the subsoil. Because it has less organic material, this layer is lighter brown in color than topsoil. It also holds more water due to the presence of iron and clay. There is less organic material in this layer.
The next layer down is the “C“ horizon. This layer is made of partially altered bedrock. There is evidence of weathering in this layer. Still, it is possible to identify the original rock type from which this soil formed (Figure below).
This image shows the various soil horizons.
Not all climate regions develop soils. Arid regions are poor at soil development. Not all regions develop the same soil horizons. Some areas develop as many as five or six distinct layers. Others develop only a few.
For soil scientists, there are thousands of types of soil! Soil scientists put soils into very specific groups with certain characteristics. Each soil type has its own name. Let’s consider a much simpler model, with just three types of soil. These types are based on climate. Just remember that there are many more than just these three types.
One important type of soil forms in a deciduous forest. In these forests, trees lose their leaves each winter. Deciduous trees need lots of rain — at least 65 cm of rainfall per year. Deciduous forests are common in the temperate, eastern United States. The type of soil found in a deciduous forest is a pedalfer (Figure below). This type of soil is usually dark brown or black in color and very fertile.
Pedalfer soils support temperate forests, such as in the eastern United States.
Pedocal soil forms where grasses and brush are common (Figure below). The climate is drier, with less than 65 cm of rain per year. With less rain, there is less chemical weathering. There is less organic material and the soils are slightly less fertile.
Grasslands grow on pedocal soils.
A third important type of soil is laterite. Laterite forms in tropical areas. Temperatures are warm and rain falls every day (Figure below). So much rain falls that chemical weathering is intense. All soluble minerals are washed from the soil. Plant nutrients get leached or carried away. There is practically no humus. Laterite soils are often red in color from the iron oxides. If laterites are exposed to the sun, they bake as hard as a brick.
The Amazon Rainforest grows on laterite soils.
Soil is a renewable resource. But it is only renewable if we take care of it. Natural events can degrade soil. These events include droughts, floods, insect plagues, or diseases that damage soil ecosystems. Human activities can also degrade soil. There are many ways in which people neglect or abuse this important resource.
People remove a lot of vegetation. They log forests or prepare the land for farming or construction. Even just walking or riding your bike over the same place can kill the grass. But plants help to hold the soil in place (Figure below). Without plants to protect it, soil may be carried away by wind or running water. In many areas, soil is eroding faster than it is forming. In these locations, soil is a nonrenewable resource.
Material that is not held down can blow in the wind. Topsoil is lost this way.
Soils may also remain in place but become degraded. Soil is contaminated if too much salt accumulates. Soil can also be contaminated by pollutants.
There are many ways to protect soil. We can add organic material like manure or compost. This increases the soil's fertility. Increased fertility improves the soil's ability to hold water and nutrients. Inorganic fertilizers also increase fertility. These fertilizers are less expensive than natural fertilizers, but they do not provide the same long term benefits.
Careful farming helps to keep up soil quality each season. One way is to plant different crops each year. Another is to alternate the crops planted in each row of the field. These techniques preserve and replenish soil nutrients. Planting nutrient rich cover crops helps the soil. Planting trees as windbreaks, plowing along contours of a field, or building terraces into steeper slopes all help to hold soil in place (Figure below). No-till or low-till farming disturbs the ground as little as possible during planting.
Trees form a windbreak at the edge of this wheat field.
1. What is the role of climate in soil formation?
2. What is the role of the parent rock in the creation of a soil?
3. Compare and contrast residual soils and transported soils.
3. Describe the characteristics of topsoil.
4. Describe two ways in which soil is a living resource.
5. Why do people add fertilizers to soil?
6. How does the C-horizon of a residual soil differ from the C-horizon of a transported soil?
7. Where would you choose to buy land for a farm if you wanted fertile soil and did not want to have to irrigate your crops?
Opening Image Copyright R Mcintyre, 2011. Used under license from Shutterstock.com. http://www.shutterstock.com.