ESR: Lesson - Soil Composition (Topic 4.3) 📖

⏳ Estimated Reading/Watching Time: 17 - 19 minutes

Describe the similarities and differences between properties of different soil types.

Soil Composition

Relative soil particle sizes. Gravel is the largest at approximately 2 mm, sand is the next largest at approximately 1 mm, silt is visible, but too small to see the size at this scale. There is a note that says clay is invisible at this scale. — **Soil is primarily made up of sand, silt, and clay. A soil can have only one of these particle types or some of each. You will learn how to identify different types of soil in this course.**

Soil scientists view soil through a variety of physical and chemical properties.

In Environmental Science, we focus on a few of these, but farmers, ranchers, and gardeners focus on all these properties.

Essential Knowledge

There are a variety of methods to test the chemical, physical, and biological properties of soil that can aid in a variety of decisions, such as irrigation and fertilizer requirements.

Physical Properties

The main physical properties of soil include soil texture, structure, color, porosity, permeability, and the consistency or workability of soil.

Explore each physical property of soil below. You will need to know these for the AP exam and for your Soil Lab.

Soil Texture

Two identical jars - one is full of tennis balls and one is full of small marbles. There is more empty space in the jar of tennis balls because the tennis balls are larger. — **Two full jars: one with tennis balls, one with marbles. There is more empty space (pore space) in the tennis ball jar.**

Soil texture is related to the ratio of different sized particles occurring in a soil.

Sand is the largest particle found in soil. Sand is composed of particles from 2.0 to 0.063 mm in size.
Silt is the middle-sized particle of soil. Silt particles are 0.063 to 0.004 mm in size.
Clay are the smallest particles in soil and consist of any particle smaller than 0.004 mm in size.

Larger soil particles have larger spaces separating them than smaller particles. This is known as pore space. Water tends to flow better through larger particles than smaller particles. This means that clayey soils will hold more water than sandy soils.

If a soil has too much clay, water will not drain well, and the surface can become waterlogged quickly. If a soil has too much sand, the water may flow too quickly through the soil and not give plants enough time to absorb the water they need to grow and thrive.

Soil particles will settle in water according to their weight, which allows you to determine the texture of a particular soil. Sand settles first, then silt, and clay settles last. Some particles of clay are even so small they NEVER settle out, which is why some clay-based lakes seem to always be murky.

A graduated cylinder showing sand that settled in minutes at the bottom of the cylinder, above the sand is a silt layer that formed after about 2 hours, and on top of the silt is a clay layer that formed after the water is clear. Above the sediment layers, the cylinder is filled the rest of the way with water. — **Soil particles will settle in water according to their weight, which allows you to determine the texture of a particular soil. Sand settles first, then silt, and clay settles last.**

Soil texture is determined by calculating the percentages of each of these three particles in a soil sample then locating the assigned soil texture on a soil texture triangle.

The consistency or workability of soil relates to how easily the soil responds to tilling. Farmers especially like soil that has a high workability because it is easier to plow and tends to better support crop growth. Workability is directly related to soil texture and soil structure. Loamy soils are highly desired by agriculturists due to their high workability.

The texture of a soil is the relative proportions of each type of particle in a given soil. Loamy soils are considered ideal. Luckily, they are also most common and consist of roughly 40% sand 40% silt, and 20% clay.

Watch the video below to learn how to use a soil texture triangle. You will need to know how to use a soil texture triangle on the AP exam.

Soil Structure

Soil structure refers to the types, amounts, and shapes of aggregates in a given soil sample. Imagine soil structure as the way that soil particles stick together in a space.

Factors like the weather in an area, the size of the soil particles, and how much natural material is in the soil all play a part in this determining the soil structure.

Different kinds of soil structure can help water and nutrients move through the soil more easily, which is crucial for helping plants grow big and strong.

Soil scientists recognize a variety of soil structures, such as granular (like tiny grains), blocky (like cubes), and platy (like flat layers). Each type of soil structure has its own "personality," affecting how well the soil can hold onto nutrients and let water flow through it.

You do not need to know each type of soil structure, but you should understand HOW soil structure affects how water, fertilizers, and contaminants are retained or pass through soil.

Soil Color

Soil color isn't just about aesthetics - it can actually tell us a lot about the composition of the soil itself. For example, darker soils often indicate a higher presence of organic matter, which is great for plant growth. On the other hand, red soils typically signal higher levels of iron. This valuable information can help farmers and gardeners understand the fertility of their soil.

The U.S. Department of Agriculture relies on the Munsell System of Color Notation to classify soil based on its color, adding a touch of color theory to the world of agriculture.

By simply observing the soil's hue, one can uncover hidden clues about its nutrient content and overall health. So, next time you're gardening or exploring the great outdoors, take a moment to appreciate the vibrant world beneath your feet - who knew soil could be so revealing?

Water-Holding Capacity

The first graphic is a box labeled "permeable" with a few pieces of sediment and abundant space between the particles. The next box is labeled "impermeable" and has very little space between the particles. The space between the particles in both boxes is labeled "pores". — **The more pore space in a sample of soil, the more permeable the soil. For example, sand is much more permeable than clay.**

Soil porosity is a measure of the amount of pore space in soil. Pore space can be occupied by air, water, and/or nutrients and directly relates to the bulk density of a soil and is affected by soil structure, soil texture, and soil compaction.

Pore spaces can be micro-pores among clay particles, macro-pores among sand and silt particles, or bio-pores caused by living organisms in the soil such as burrowing earthworms. Porosity has a direct effect on the permeability of a soil.

Soil permeability indicates the speed at which water or other liquids percolate through soil. Permeability depends on the type and size of soil particles as well as their arrangement. Sandy soils tend to have higher permeability than clay soils which is an important concept to understand when thinking about applying chemicals on the soil surface. Due to its high permeability, chemicals applied above a sandy soil can percolate quickly downward through the soil and end up in groundwater.

Essential Knowledge

The particle size and composition of each soil horizon can affect the porosity, permeability, and fertility of the soil.

A soil texture triangle is a diagram that allows for the identification and comparison of soil types based on their percentage of clay, silt, and sand.

Water-holding capacity - the total amount of water soil can hold - varies with different soil types. Water retention contributes to land productivity and fertility of soils.

Chemical Properties

The main chemical properties of soil include fertility, pH, nutrient-holding capacity, and cation-exchange capacity.

A satellite image of a large eutrophic lake. The left side of the lake is bright green due to an algal bloom. The right side of the lake is dark blue. — **Cultural eutrophication can cause algal blooms. The green areas in this lake are algal blooms caused by eutrophication.**

Soil provides access to important nutrients such as nitrogen, phosphorus, potassium, and a host of other elements. Soil fertility is a measure of how much of these necessary substances are found in a given soil sample. For farmers and ranchers, high amounts of fertility are necessary to support crop and pasture growth but fertility can be reduced by factors such as erosion, leaching, or simply overuse of the soil. To increase fertility, fertilizers are applied but overuse of fertilizers can lead to environmental impacts such as cultural eutrophication, a condition in which nutrients enter waterways and cause overgrowths of aquatic plants which, in turn, upsets the balance of the aquatic ecosystem.

Explore the chemical properties of soil in the tabs below. Which do you think is most important? Why?

pH

The pH scale ranges from 0-14. Zero is most acidic, 14 is most basic (or alkaline), and 7 is neutral. Because the pH scale is logarithmic, going up (or down) 1 number on the scale is equivalent to being 10 times more basic (or acidic).

The pH scale ranges from 0 (red) to 14 (dark purple). In the 0-4 range, acidic soil (pH < 5.5) can make aluminum and manganese toxic, while essential elements like nitrogen, calcium, and potassium become less available. Between pH 5 and 8.5, soil is slightly acidic to neutral, ideal for most plants. In the 8.5-14 range, alkaline soil (pH > 7) reduces the availability of elements like iron and zinc for plant absorption. — **Different plants thrive in different pH ranges.**

Soil pH is an important chemical property of soil. pH can affect the availability of nutrients as well as the release of certain ions that may become toxic in aquatic ecosystems. In areas where acid deposition occurs, metals such as aluminum can be released and runoff into streams, rivers, and lakes impacting the aquatic ecosystem. A pH between 6.0 and 7.0 is generally the most favorable for plant growth although different species may require a higher or lower pH.

Weathering, erosion, and the presence of chemicals all have an effect on pH.

Nutrient-Holding Capacity

a red tractor tilling a field of dry grasses. There are large trees surrounding the field — Farmers often add fertilizers to their soil to replace nutrients that might be missing from the soil. This can increase the fertility of the soil, but too much fertilizer can hurt plants and run off into waterways.

A soil’s nutrient-holding capacity refers to a soil’s ability to bind and hold nutrients until they are absorbed by plant roots. If nutrients are leached, or washed, from the soil too quickly, the plants living in that soil will not have time to take up the nutrients they need to grow and thrive. To replace lost nutrients, farmers and ranchers may need to apply fertilizer which can lead to environmental issues if not done properly.

The nutrient-holding capacity of a soil is directly related to its cation-exchange capacity (CEC) which is a measure of how strongly soil particles cling to the ions of different elements. A higher CEC typically results in the ability of the soil to retain higher amounts of ions necessary for plant growth. CEC is affected by soil texture, soil pH, and the amount of organic matter present.

Minerals and nutrients in the soil tend to become available due to weathering. A soil’s nutrient-holding capacity refers to a soil’s ability to bind and hold nutrients unit they are absorbed by plant roots. If nutrients are leached, or washed, from the soil too quickly, the plants living in that soil will not have time to take up the nutrients they need to grow and thrive.

Some soils are continually lacking the nutrients needed for the types of plants people try to grow in them. When this happens, we often add fertilizer to soils. Fertilizers contain one or more of the necessary nutrients missing from the soil, usually nitrogen or phosphate. Fertilizer can be inorganic, chemical formulations of these nutrients, or organic, formed by plant and/or animal waste.

Fertility

A colorful display of hydrangea flowers in blue and purple hues, surrounded by green leaves. — **Did you know that pH determines the color of hydrangea blooms? What do you think the pH of this soil might be?**

The fertility, or tilth of a soil, is the soil’s ability to support plant growth and pH and salinity are critical to a soil’s fertility. The pH of the soil affects the availability of nutrients to plants and the activity level of soil microorganisms.

For example, the pH of the soil affects the availability of aluminum, which is key to determining the color of hydrangea blooms. Aluminum is more available in acidic soils, which results in blue or purple blooms. Aluminum is less available in alkaline soils, which results in pink or red blooms.

To change the color of hydrangea blooms, you can alter the pH of the soil by adding lime (to make it more alkaline) or sulfur (to make it more acidic). However, this may take several months to have an effect, and some hydrangeas are less sensitive to pH changes than others. You can also use fertilizers that contain aluminum sulfate or iron sulfate to increase the aluminum availability in the soil.

Soil Classification

Global soil regions with colored areas corresponding to different soil types. Mollisols occur in central United States, Central Asia, and central South America. Oxisols are found near the equator in Africa and South America. Alfisols occur in temperate forest areas like the Pacific Northwest, north-central Asia, and parts of Australia, South America, and Africa. Aridisols are found in desert regions, including the western United States, North and South Africa, South-Central Asia, and Australia. — **The most common soil orders. Mollisols, oxisols, alfisols, and aridisols are the soil orders we will discuss the most.**

Soils have a taxonomy, just like plants and animals. Soil orders are the most inclusive and soil classes are more specific. There are hundreds of soil classes. We will just discuss the basics of soil orders to give you an idea of the diversity of soils across the globe.

Open each tab below to learn more about the four most important types of soil. Which soil is common where you live?

Mollisols are fertile, dark soils found in temperate grasslands. These soils have a deep A horizon and are rich in humus and minerals. These are known as the best soils for agriculture. The climate in this area does not receive enough rain to leach minerals downward.

🚜 Mollisols (click to reveal)

Oxisols are found in tropical and subtropical rainforests. These soils have a layer of iron and aluminum oxides in the B horizon. These soils have a small O horizon due to the rapid decomposition of plant matter in these ecosystems. These soils have little agricultural fertility. This is why when rainforests are cut down to make room for agricultural space, these farms do not last long and more rainforests must be cut down soon after because these soils are not suitable for agriculture and without the forest to protect the soil, what little O horizon that exists, tends to erode quickly.

🦜 Oxisols (click to reveal)

Alfisols are typical of moist, temperate forest biomes. These soils have well-developed, but shallow O, A, E, and B horizons.

🌲 Alfisols (click to reveal)

Aridisols are found in drylands and deserts. These soils are relatively unstructured vertically and are thin and light-colored. Irrigating these soils tends to lead to salinization because high rates of evaporation draw the salts present in these soils to the surface.

🏜️ Aridisols (click to reveal)

AP Exam Tip

You need to know how to use a soil texture triangle to determine a soil type and then understand what the soil type means in terms of its suitability for plant growth.

Porosity is the amount of pore space and permeability how quickly water flows through soil. In general, the higher the porosity, the higher the permeability.

You should know that acid deposition can lower soil pH, making the soil inhospitable to plant growth. We will learn more about the causes of acid deposition later.

You do not need to know the soil type names (e.g., Oxisol, Spodosol), but you do need to know which biomes have fertile soils (grasslands, deciduous forests) and why (moderate temperatures, ample precipitation, etc.).

You should be familiar with which biomes have poor soil (tropical rainforests, tundra, boreal forests, deserts) and which characteristics of those biomes result in their poor soils.

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