High School Earth Science/Soils

Lesson Objectives

Discuss why soil is an important resource.
Describe how soil forms from existing rocks.
Describe the different textures and components of soil.
Draw and describe a soil profile.
Define the three climate related soils: a pedalfer, pedocal and laterite soil.

Characteristics and Importance of Soil

Thank goodness for mechanical and chemical weathering, because without these forces working to breakdown rock we would not have any soil on Earth. It is unlikely that humans would have been able to live on Earth without soil! Your life and the lives of many organisms depend on soil. We get wood, paper, cotton, medicines and even pure water from soil. So soil is a very important resource. Even though it is actually only a very thin layer on Earth's surface over the solid rocks below, it is the place where our atmosphere, hydrosphere, biosphere and the rocks of the Earth meet. Within our soil layer, reactions between solid rock, liquid water and air take place. It is a mistake on our part to disregard this important resource, yet we say things like "soiled" or "dirty" when we talk about ruining something. Our precious soil resource needs to be carefully managed and cared for. If we neglect or abuse the soil we have, it will not remain the renewable resource that we have relied on throughout human existence.

We can think about soil as a living resource or an ecosystem all by itself. Within soil, there are many elements. It is a complex mixture of different materials. Some of them are inorganic, like the products of weathered rock, including pebbles, sand, silt and clay particles. There are also bits of organic materials, formed from the partial breakdown and decomposition of plants and animals. In general, the pieces of rock and minerals make up about half of the soil, with the other half made of organic materials. In between, in the spaces of soil, there are thousands or even millions of living organisms. Those organisms could be earthworms, ants, bacteria, or fungi, as well as many other types of organisms. In between the solid pieces, there are tiny spaces filled with air and water. In some soils, the organic portion could be entirely missing, such as desert sand. At the other extreme, a soil could be completely organic, such as the materials that make up peat in a bog or swamp. The organic materials are necessary for a soil to be fertile. The organic portion provides the nutrients, like nitrogen, needed for strong plant growth. We will learn about that organic portion in just a bit.

Soil Formation

How well soil forms and what type of soil forms depends on several different factors. Some of these factors are the climate, the original rock the soil formed from, the slope, the amount of time and biological activity. Climate is ultimately the most important factor and will determine the type of soil that forms in a particular region. The climate of a region is principally determined by temperature and the amount of precipitation. This also influences the type of vegetation that grows in the region. We can identify different climates by the types of plants that grow there. Depending on how closely you look, we can divide land areas into many different climate regions (Figure 9.6).

Figure 9.6: Climate is the most important factor in determining the type of soil that will form in a particular area.

Given enough time, even different rock types will produce a similar climate related soil. Climate is such an important factor that even the same type of rock in different climates will produce a different climate related soil. This is true because the rocks on Earth are predominantly composed of eight elements and as rock breaks down, there will mostly be just these eight elements. Surely, if an element is not present in the original rock, then it will also not be present in the soil that forms from it. The amount of precipitation in an area is important because it influences the rate of weathering. The more it rains, the more rainwater passes through the soil and the more it reacts chemically with the particles. Those reactions are most efficient in the top layers of the soil and become less effective as the water continues to percolate through lower layers of soil. The top layers of soil in contact with the freshest water react most. Increased rainfall in a region increases the amount of rock that is dissolved as well as the amount of material carried away by moving water. As materials are carried away, new surfaces are exposed and this also increases the rate of weathering.

The temperature for a region is important too. The rate of chemical reactions increases with higher temperatures. For every 10°C increase in temperature, the rate of chemical reactions doubles. Warmer regions also have more vegetation because plants and bacteria grow and multiply faster. This means that in tropical regions, where temperatures and amounts of rainfall are consistently high, thick soils form with no unstable minerals and therefore no nutrients. Conversely, arid regions produce thin soils, rich in unstable minerals. The rate of soil formation increases with greater amounts of time. The longer the amount of time that soil remains in a particular area, the greater the degree of alteration.

The original rock is the source of the inorganic portion of the soil. Chemical reactions from weathering break down the rock's original minerals into sand, silt and clays. A soil is called a residual soil when it forms in place, with the underlying rock breaking down to form the layers of soil that reside above it. Only about one third of the soils in the United States form this way. The rest of the soils form from materials that have been transported in from somewhere else. These soils are called transported soils. Glaciers bring bits of rock from far away, depositing the materials they carried as the ice of the glacier melts. Wind and rivers also transport materials from their places of origin. These soils form from the loose particles that have been transported in to a new location and deposited. For transported materials, the rate of soil formation is faster because the transported materials have already been weathered. The closer the materials are to their place of origin, the greater the influence of the original materials. The further those materials move from their origin, the greater the degree of weathering and the influence of the original materials becomes obscured.

Soils thicken as the amount of time available for weathering increases. The warmer the temperatures, the more rainfall and greater amount of time, the thicker the soils will become. Biological activity produces the organic material and nutrients in soil. The partial decay of plant material and animal remains also forms organic acids which in turn increase the rate of soil formation and the rate of weathering. The organic material increases the ability of the soil to hold water, create a soil’s structure and enhance its fertility and ability to be cultivated.

The decayed remains of plant and animal life are called humus. Humus is an extremely important part of the soil. It coats the mineral grains, binding them together into clumps that then hold the soil together. The humus in soil also increases the porosity and water holding capacity of a soil. Humus helps to buffer rapid changes in soil acidity and helps the soil to hold its nutrients. Decomposing organisms in the soil breakdown the complex organic molecules of plant matter and animal remains to form simpler inorganic molecules that are soluble in water. Bacteria in the soil change atmospheric nitrogen into nitrates.

One indicator of a soil's fertility is its color. Soils that are rich in nitrogen and contain a high percentage of organic materials are usually black or dark brown in color. Soils that are nitrogen poor and low in organic material might be gray or yellow or even red in color.

Soil Texture and Composition

The inorganic portion of soil is made of many different size particles. In addition to many particle sizes, there can be different proportions of each particle size. The combination of these two factors determines some of the properties of the soil. A soil will be very permeable, which means that water will flow through it easily, if the spaces between the inorganic particles are large enough and are well connected. Soils that have lots of very small spaces tend to be water holding soils. Clays are an example of a type of soil that holds water. When clay is present in a soil, the soil is heavier and holds together more tightly. Sandy or silty soils are considered 'light' soils because they are permeable, water draining types of soils. When a soil contains a mixture of grain sizes, the soil is called a loam. When soil scientists want to precisely determine the soil type, they measure the percentage of sand, silt and clay and plot this information on a triangular diagram, with each type of soil at one corner (Figure 9.7).

Figure 9.7: Soil texture triangular plot diagram.

Soil scientists use a diagram like this to plot the percentages of sand, silt, and clay in a soil. The soil type can then be determined from the location on the diagram. At the top, a soil would be a clay; at the left corner, it would be a sand and at the right corner it would be a silt. Regions in the lower middle with less than 50% clay are called loams.

Soil Horizons and Profiles

A residual soil forms from the underlying bedrock. This happens over many years, as mechanical and chemical weathering slowly change solid rock into soil. The more time available, the greater the degree of alteration that will occur. Perhaps the first changes to bare rock would be cracks or fractures due to mechanical weathering from ice wedging. Then plants like lichens or grasses become established. As more and more layers of material weather, the soil develops soil horizons, as each layer becomes progressively altered. The place where the greatest degree of weathering occurs is the top layer. Each successive, lower layer is altered just a little bit less. This is because the first place where water and air come in contact with the soil is at the top. As water moves down through the layers, it is able to do less work to change the soil. If you were able to dig a deep hole into the ground, you could see each of the different layers of soil. All together, these are called a soil profile. Each horizon has its own particular set of characteristics (Figure 9.8).

Figure 9.8: Each soil horizon is distinctly visible in this photograph.

Figure 9.9: A soil profile is the complete set of soil layers. Each layer is called a horizon.

In the simplest soil profile, a soil is considered to have three horizons. The first horizon is the top soil, which is called the A horizon. The topsoil will usually be the darkest layer of the soil, because this is the layer with the highest proportion of organic material. Remember that humus forms from all the plant and animal debris that falls to the ground. This includes branches and twigs, acorns and pine needles as well as waste from animals and fungi. The top soil is the region of most intense biological activity. Many living organisms live within this layer and plants stretch their roots down into this layer. In fact, plant roots are very important to this layer because vegetation helps to hold this layer of soil in place. The top soil layer is usually a layer where minerals that can dissolve and very small particles like clay are absent. This is because clay sized particles get carried to lower layers as water seeps down into the ground. Soluble minerals are missing because they readily dissolve in the fresh water that moves through this layer and are carried down to lower layers of the soil.

The next soil horizon is the subsoil, which is called the B horizon. This is the region where soluble minerals and clays accumulate. This layer will be lighter brown in color and more water holding than the top soil, due to the presence of iron and clay minerals. There will be less organic material in this layer. The next layer down, called the C horizon will be a layer of partially altered bedrock. There will be some evidence of weathering in this layer, but pieces of the original rock can still be seen and it would be possible to identify the original type of rock from which this soil formed (Figure 9.9).

Not all climate regions develop soils, and not all regions develop the same horizons. Some areas develop as many as five or six distinct layers, while others develop only very thin soils or perhaps soil doesn't form well at all.

Types of Soils

If we were to talk to soil scientists, you would learn that there are thousands of types of soil. Soil scientists study each of the many different characteristics of each soil and put them into very specific groups and have many different names for soils. Let's consider a much simpler model that considers just three types of soil. This will help you to understand some of the basic ideas about how the particular climate of an area produces a certain type of soil, but there are many exceptions to what we will learn right now.

Let's consider the type of soil that would form in a region of the world where there are forests of trees that lose their leaves each winter, called deciduous trees. In order for trees to grow here, there needs to be lots of rain, at least 65 cm of rainfall per year. Wherever there are trees, there is enough rain to help them grow! The type of soil that forms in a forested area is called a pedalfer and this type of soil is common in many areas of the temperate, eastern part of the United States (Figure 9.10). The word pedalfer comes from some of the elements that are commonly found in the soil. The element aluminum has the chemical symbol Al and the element iron has the chemical symbol Fe. These two symbols are combined '-al' and '-fe' to make the word ped –al –fe r. This type of soil is usually a very fertile, dark brown or black soil. It is rich in aluminum clays and iron oxides. Because it rains often in this type of climate, most of the soluble minerals dissolve and are carried away, leaving the less soluble clays and iron oxides behind.

Figure 9.10: A pedalfer is the dark, fertile type of soil that will form in a forested region.

Another type of climate related soil, called a pedocal, forms in drier temperate areas where grasslands and brush are the usual types of vegetation (Figure 9.11). It rains less than 65 cm per year in these areas, so there is less chemical weathering for these soils. With lower amounts of rainfall, there is less water to dissolve away soluble minerals, so more soluble minerals are present here but fewer clay minerals are produced. With lower rainfall, there is also less vegetation here, so the soils have lower amounts of organic material, making them slightly less fertile types of soils than a pedalfer. A pedocal is named for the calcite enriched layer that forms. Some water begins to move down through the soil layers, but before it gets very far, it begins to evaporate away. Soluble minerals, like calcium carbonate, concentrate in a layer that marks the lowest place that water was able to reach before it evaporated away. This layer is called caliche.

Figure 9.11: A pedocal is the alkaline type of soil that forms in grassland regions.

Figure 9.12: A laterite is the type of thick, nutrient poor soil that forms in the rainforest.

A third type of soil called a laterite forms in tropical areas, where rainfall is so intense that it literally rains every day. The tropical rainforest is an example of this type of region (Figure 9.12). In these hot, wet tropical regions nutrient poor soils form due to intense chemical weathering. So much weathering happens here that there is practically no humus. All soluble minerals are removed from the soil and all plant nutrients get leached or carried away. What are left behind are the least soluble materials like aluminum and iron oxides. These soils are often red in color from the iron oxides. These soils bake as hard as a brick if they are set out in the sun to dry.

You can probably very quickly name many climates that have not been mentioned here. Each climate will produce a distinctive soil that forms in the particular circumstances found there. Where there is less weathering, soils are thinner but soluble minerals may be present. Where there is intense weathering, soils may be thick but nutrient poor. In any case, soil development takes a very long time. It may take hundreds or even thousands of years to form a good fertile top 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!

Soil Conservation

Figure 9.13: Organic material can be added to soil to help increase its fertility.

Soil is only a renewable resource if we carefully manage the ways in which we use soil. There are natural cycles of unfortunate events like drought or insect plagues or outbreaks of disease that negatively impact ecosystems and also harm the soil. But there are also many ways in which humans neglect or abuse this important resource. One harmful practice is removing the vegetation that helps to hold soil in place. Sometimes just walking or riding your bike over the same place, will kill the grass that normally grows there. Other times land is deliberately cleared to make way for some other use. The 'lost' soils may be carried away by wind or running water. In many areas of the world, the rate of soil erosion is many times greater than the rate at which it is forming. Soils can also be contaminated if too much salt accumulates in the soil or where pollutants sink into the ground.

There are many ways that we can protect and preserve our precious resources of soil. There are many ways to help to keep soil in good condition. Adding organic material to the soil in the form of plant or animal waste, like manure or compost, increases the fertility of the soil as well as improving its ability to hold onto water and nutrients (Figure 9.13). Inorganic fertilizer can also temporarily increase the fertility of a soil and may be less expensive or time consuming, but won’t provide the same long term improvements as organic materials. Agricultural practices like rotating crops, alternating the types of crops planted in each row and planting nutrient rich cover crops all help to keep soil more fertile as it is used season after season. Planting trees as windbreaks, plowing along contours of the field or building terraces into steeper slopes will all help to hold soil in place (Figure 9.14). No till or low tillage farming helps to keep soil in place by disturbing the ground as little as possible when planting.

Figure 9.14: Steep slopes can be terraced to make level planting areas and decrease surface water runoff and erosion.

Lesson Summary

Soil is an important resource. Life on Earth could not exist as it does today without soil.
The type of soil that forms depends mostly on climate but to a lesser extent on original parent rock material.
Soil texture and composition plus the amount of organic material in a soil determine a soil's qualities and fertility.
Given enough time, existing rock will produce layers within the soil, called a soil profile.
Ultimately, the climate of a particular region will produce a unique type of soil for that climate.

Review Questions

Describe at least two ways in which soil is a living resource.
Name two factors that influence soil formation.
Which region of a soil profiles reacts the most?
Is the soil in your back yard most likely a residual soil or a transported soil? How could you check?
Name several advantages to adding humus to the soil.
What are three soil horizons? Describe the characteristics of each.
Name three climate related soils. Describe the climate and vegetation that occurs in the area where each forms.
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?

Vocabulary

deciduous trees: Trees that lose their leaves once a year.
humus: The partially decayed remains of plants and animals; forms the organic portion of soil.
inorganic: Parts of the soil which do not come from living organisms; the rock and mineral portion of the soil.
laterite: Nutrient poor, red, tropical soil which forms in a region with rainforest vegetation.
loam: Soil texture which forms from a roughly equal combination of sand, silt and clay.
organic: Generally considered to mean components of the soil which come from living organisms.
pedalfer: Fertile, dark soil which forms in mid latitude, forested regions.
pedocal: Slightly less fertile soils which forms in drier, grassland regions.
permeable: Describes a type of soil which allows water to move through it easily.
residual soil: Soil that forms from the bedrock upon which it resides or lies.
soil horizon: An individual layer of a complete soil profile; examples include A, B, and C horizons.
soil profile: The entire set of soil layers or horizons for a particular soil.
subsoil: The B horizon of a soil; the zone where iron oxides and clay minerals accumulate.
topsoil: The A horizon of a soil; most fertile layer of soil where humus, plant roots, and living organisms are found.
transported soil: A soil formed from weathered components transported by water, wind, or ice to a different area.

Points to Consider

Why is soil such an important resource?
Do you think a mature soil would form faster from unaltered bedrock or from transported materials?
If soil erosion is happening at a greater rate than new soil can form, what will eventually happen to the soil in that region?
Do you think there are pollutants that could not easily be removed from soil?

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