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• Types Of Soil

Types of Soil

Table of Contents

Overview of Soil

Important questions and answers about soil.

From a general perspective, “soil” is a very broad term and refers to the loose layer of earth that covers the surface of the planet. The soil is the part of the earth’s surface, which includes disintegrated rock, humus, inorganic and organic materials. For soil to form from rocks, it takes an average of 500 years or more. The soil is usually formed when rocks break up into their constituent parts. When a range of different forces acts on the rocks, they break into smaller parts to form the soil. These forces also include the impact of wind, water, and salts’ reaction.

There are three stages of soil:

• Soil with air in the pores
• Soil with water in the pores

Various types of soil undergo diverse environmental pressures. Soil is mainly classified by its texture, proportions and different forms of organic and mineral compositions.

Types of soils

Soil is classified into four types:

• Sandy soil.
• Loamy Soil.

The first type of soil is sand. It consists of small particles of weathered rock. Sandy soils are one of the poorest types of soil for growing plants because it has very low nutrients and poor water holding capacity, which makes it hard for the plant’s roots to absorb water. This type of soil is very good for the drainage system. Sandy soil is usually formed by the breakdown or fragmentation of rocks like granite, limestone and quartz.

Silt, which is known to have much smaller particles compared to sandy soil and is made up of rock and other mineral particles, which are smaller than sand and larger than clay. It is the smooth and fine quality of the soil that holds water better than sand.  Silt is easily transported by moving currents and it is mainly found near the river, lakes and other water bodies. The silt soil is more fertile compared to the other three types of soil. Therefore, it is also used in agricultural practices to improve soil fertility.

Clay is the smallest particle among the other two types of soil. The particles in this soil are tightly packed together with each other with very little or no airspace. This soil has very good water storage qualities and makes it hard for moisture and air to penetrate into it. It is very sticky to the touch when wet but smooth when dried.  Clay is the densest and heaviest type of soil which does not drain well or provide space for plant roots to flourish.

Loam is the fourth type of soil. It is a combination of sand, silt and clay such that the beneficial properties of each are included. For instance, it has the ability to retain moisture and nutrients; hence, it is more suitable for farming. This soil is also referred to as  agricultural soil as it includes an equilibrium of all three types of soil materials, being sandy, clay, and silt, and it also happens to have humus.  Apart from these, it also has higher calcium and pH levels because of its inorganic origins.

Related Links

• Soil Profile
• Photosynthesis
• Soil Pollution
• What Is Soil

The ground on which we walk is never quite the same; it keeps on changing. Sometimes, it is made up of millions of tiny sand granules and other times; it is a hard, rocky surface. Other places have the ground covered with moss and grass. When humans came along, the landscape slowly changed with the introduction of roads and rails.

1. State the classifications of soil.

Soil can be classified into three primary types based on its texture – sand, silt and clay. However, the percentage of these can vary, resulting in more compound types of soil such as loamy sand, sandy clay, silty clay, etc.

2. State the characteristics of sandy soil.

Sandy soil essentially consists of small particles formed by weathering rocks. It is also very low in nutrients and poor in holding water, which makes it one of the poorest types of soil for agriculture.

3. Explain the significant features of silty soil.

Silt has smaller particles compared to sand. It is also made up of rock and other mineral particles. Furthermore, its fine quality holds water better than sand. Due to the above-mentioned features, it is also beneficial for agriculture.

4. Explain the characteristic of Clay soil.

Clay contains the smallest particles among the other two types of soil. Particles are so densely packed that there is very little or no airspace. Consequently, this property effectively retains water. However, it also becomes hard for moisture and air to penetrate into it, thereby impeding the growth of plants.

Stay tuned with BYJU’S to know more about soil, its types and other interesting topics at  BYJU’S Biology .

Frequently Asked Questions on Types of Soil

What is soil.

Soil is usually referred to as the naturally occurring organic materials found on the earth’s surface. It is mainly composed of minerals, nutrients, water, other inorganic particles and some residues of plants and animals.

What are the different types of Soil?

There are different types of soil, and they are categorized mainly based on the size of the particles and the percentage of particles present in them—the three primary types of soil based on their texture are Sand, Loamy and Clay.

Other types of soil are based on the percentage of particles, resulting in more compound types of soil: loamy sand, sandy clay, silty clay, etc. Apart from these, soils are also classified based on their colour- Red soil, Black soil and Brown Soil.

Which soil is called the gardener’s best friend?

Loam or Loamy soil is called the gardener’s best friend.

Which crop can be grown in Loamy soil?

Loamy soil is suitable and the best soil for growing crops such as cotton, oilseeds, sugarcane, wheat, pulses, jute and other vegetables.

What is Sandy Soil?

Sand or sandy soil is formed by the smallest or fine particles of weathering rocks. This soil is known as the poorest type of soil for agriculture and growing plants as they have very low nutritional value and poor water holding capacity.

What is Clay Soil?

Clay or clay soil is mainly composed of the smallest particles of soil, which are densely packed with very little or no airspace and they effectively retain water. This soil is not suitable for growing plants as it is harder for moisture and air to penetrate into the soil.

What is Loamy Soil?

Loamy Soil is the mixture of clay, sand and silt soil which consists of additional organic matter and is very fertile compared to other types of soil. It is well suited for cultivation as the plant roots get a sufficient amount of water and nutrients for their growth and development.

Which soil is preferable to grow coconut and melon?

Sandy soil is the preferable soil to grow coconut and melon.

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13.9: Types of Soils

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Where would you rather grow crops?

The soil on the left is in Tuscany in Italy. Tuscany is known best for growing grapes for wine and olives for olive oil. The soil on the right is in China. It is the soil that remained when rainforest trees were removed. Do you know which soil would be better for crops?

Types of Soils

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.

The soil beneath a deciduous forest is a pedalfer. These soils are very fertile.

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 and organic material, and the soils are slightly less fertile.

Grassland soils are less rich than soils in more humid regions.

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 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.

Laterite soils, which are found beneath rainforests, are not good for growing crops.

• Pedalfer is the soil common in deciduous forests. Pedalfer is dark brown and fertile.
• Pedocal is the soil common in grasslands. The more arid climate increases calcium in the soil. Pedocal is not as fertile.
• Laterite forms in tropical rain forests. Chemical weathering strips the soils of their nutrients. When the forest is removed the soil is not very fertile.
• What is pedocal? How much chemical weathering leads to this soil type and why?
• What is the plant life found with pedocal soils? How much organic material do they contain?
• What is pedalfer? How much chemical weathering leads to this soil type and why?
• What is the plant life found with pedalfer soils?
• What is laterite? How much chemical weathering leads to this soil type and why?
• What is the plant life found with laterite soils?
• If a forest is leveled so that a laterite soil is exposed to the Sun, what happens to it?

Explore More

Use the resources below to answer the questions that follow.

• What is laterite?
• Where is it found?
• What zones are in laterite? What process or lack of that process produces each zone?
• How does an ore body with a high amount of metals form in these soils?

This page has been archived and is no longer updated

What Are Soils?

Soils are dynamic and diverse natural systems that lie at the interface between earth, air, water, and life. They are critical ecosystem service providers for the sustenance of humanity. The improved conservation and management of soils is among the great challenges and opportunities we face in the 21st century.

Soil is... a Recipe with Five Ingredients

Soil is a material composed of five ingredients — minerals, soil organic matter, living organisms, gas, and water. Soil minerals are divided into three size classes — clay , silt , and sand (Figure 1); the percentages of particles in these size classes is called soil texture . The mineralogy of soils is diverse. For example, a clay mineral called smectite can shrink and swell so much upon wetting and drying (Figure 2) that it can knock over buildings. The most common mineral in soils is quartz; it makes beautiful crystals but it is not very reactive. Soil organic matter is plant, animal, and microbial residues in various states of decomposition; it is a critical ingredient — in fact the percentage of soil organic matter in a soil is among the best indicators of agricultural soil quality (http://soils.usda.gov/sqi/) (Figure 3). Soil colors range from the common browns, yellows, reds, grays, whites, and blacks to rare soil colors such as greens and blues.

Soils are... Big

You may be surprised to hear " dirt " described as "big". However, in the late 1800's soil scientists began to recognize that soils are natural bodies with size, form, and history (Figure 4). Just like a water body has water, fish, plants, and other parts, a soil body is an integrated system containing soil, rocks, roots, animals, and other parts. And just like other bodies, soil systems provide integrated functions that are greater than the sum of their parts.

Soils are... Young to Very, Very Old

Soils are... diverse.

• Plinthite — which hardens irreversibly upon repeated wetting and drying (Figure 8a).
• Sulfidic — a horizon containing pyrite which, upon exposure to oxygen, can produce so much sulfuric acid that it kills plants and can cause fish kills (Figure 8b).
• Petrocalcic — in which so much calcium carbonate is accumulated that it literally forms a rock-like layer in the middle of a soil (Figure 8c).

Soils... Communicate

• O - Horizon containing a high percentage of soil organic matter.
• A - Horizon darkened by the accumulation of organic matter.
• E - Horizon formed through the removal ( eluviation ) of clays, organic matter, iron, or aluminum. Usually lightened in color due to these removals.
• B - Broad class used for subsurface horizons that have been transformed substantially by a soil formation process such as color and structure development; the deposition ( illuviation ) of materials such as clays, organic matter, iron, aluminum, carbonates, or gypsum; carbonate or gypsum loss; brittleness and high density; or intense weathering leading to the accumulation of weathering-resistant minerals.
• C - A horizon minimally affected or unaffected by the soil formation processes.
• R - Bedrock.

These master horizons may then be further annotated to give additional information about the horizon. Lower case letters can be placed as suffixes following the master horizon letter to give additional information about soil characteristics or soil formation processes. For example, the lower case "t" on the B horizon in Figure 9 indicates that the horizon is characterized by illuvial clay accumulation. Multiple letters can be used — Figure 8c depicts a Bkm horizon meaning that it is cemented (m) by illuvial carbonates (k). Numbers placed before the master horizon name (e.g., 2Bt) indicate a difference in parent material; numbers placed at the end of a horizon name are used to subdivide horizons that have the same designation but are different in some way (e.g., a red Bt1 over a yellow Bt2).

Soils are... Biological Bliss

Soils are... fertile.

Soils are the primary provider of nutrients and water for much of the plant life on earth. There are 18 elements considered essential for plant growth, most of which are made available to plants through root uptake from soils (Brady & Weil 2007). Soils retain nutrients by several mechanisms. Most nutrients are dissolved in soil water as either positively or negatively charged ions; soil particles are also charged and thereby are able to electrically hold these ions. Soils also hold nutrients by retaining the soil water itself.

Arguably the greatest of all the ecosystem services provided by soils is the retention of water — without soils our land would be little but rocky deserts. Plants use much more water than one might think because they are constantly releasing water into the atmosphere as a result of transpiration, which is a component of the process of photosynthesis. Clay and silt particles are the primary mineral components in soils that retain water — these small particles slow the drainage of water and, like a sponge, physically hold water through capillary forces. Clay provides such strong force that plants can't pull all the water away from it, which makes silt particles the ultimate ingredient for plant-available water storage — they hold large quantities of water but also release it to plant roots (Figure 3).

Soils are... Clay Factories

Soils are... service providers, soils are... degrading and polluted, soils are... home, soils are... a profession.

activity - A general term used to describe how chemically reactive a particle is with ions, water, and other particles.

clay - A mineral particle smaller than 0.002 mm.

clay synthesis - Clays are formed in soils through the transformation of existing clays or through the generation of entirely new clay particles from ions precipitating from solution.

desertification - The transformation of a non-arid landscape to an arid landscape, usually through a combination of climate changes and human-induced soil degradation.

dirt - 1. synonym for soil material; 2. soil out of place; 3. unclean material of any composition.

eluviation - The removal of materials such as clays, organic matter, iron, or aluminum from a horizon.

erosion - The surface removal of soil material from soils by the action of water or wind.

eutrophication - A process of excess algal growth that leads to oxygen depletion; often caused by excess nutrient inputs.

factors of soil formation - Factors from which soil scientists are able to predict the end result of soil formation processes: climate, organisms, topography, parent material, and time.

gas regulation - The absorption and release of gases that mediates the levels of these gases in the atmosphere.

illuviation - The deposition of materials such as clays, organic matter, iron, or aluminum into a horizon; generally the materials come from an upper horizon in the soil body.

leaching - The removal of dissolved ions from a soil.

natural bodies - Systems that form in nature with size, form, and history that act as in an integrated fashion to provide functions that differ from the sum of their parts.

remediate - To transform a chemical from a toxic form or state to a non-toxic form or state.

salinization - A build up of salts in soils to the point that they destroy the soil's physical and chemical properties and plants are not able to take up water due to the high salt concentration; often associated with improper irrigation.

sand - A mineral particle ranging in size from 0.02 to 2 mm.

silt - A mineral particle ranging in size from 0.002 to 0.02 mm.

soil - 1. A material composed of minerals, living organisms, soil organic matter, gas, and water. 2. A body composed of soil and other parts such as rocks, roots, and animals that has size, form, and history and provides integrated functions that are greater than the sum of its parts.

soil horizon - Layer present within soil bodies that are distinguishable from other layers; often generated through soil formation processes.

soil organic matter - Plant, animal, and microbial residues, in various states of decomposition.

soil texture - The percentages of sand, silt, and clay particles in a soil.

soil quality - The capacity of a soil to provide desirable ecosystem services.

transpiration - Evaporation of water from openings in plant tissues called stomata; associated with photosynthesis.

weathering - Physical, chemical, and biological processes that breakdown and transform rocks and minerals.

References and Recommended Reading

Ahrens, R. J. & Arnold, R. W. "Soil taxonomy," in Handbook of Soil Science , ed. M. Summer (CRC Press, 2000) E117-E135.

Brady, N. C. & Weil, R. R. T he Nature and Properties of Soils, 14th ed. Upper Saddle River, NJ: Prentice Hall, 2008.

Food and Agriculture Organization of the United Nations (FAO). Guidelines for Soil Description, 4th ed. FAO, Rome, 2006. ftp://ftp.fao.org/docrep/fao/009/a0541e/a0541e00.pdf

Haygarth P. M. & Ritz, K. The future of soils and land use in the UK: Soil systems for the provision of land-based ecosystem services. Land Use Policy 26S:S187-S197, 2009.

Jenny, H. The Factors of Soil Formation: A System of Quantitative Pedology . New York, NY: Dover Press, 1941.

Soil Survey Division Staff. Soil Survey Manual . Soil Conservation Service, United States Department of Agriculture, Handbook 18, 1993. http://soils.usda.gov/technical/manual/

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7 Soil Texture and Structure

Soil texture and structure are considered “master variables”, meaning that texture and structure directly influence a large number of other soil properties. For example, in comparing a clayey soil and a sandy soil, one would expect the clayey soil to have larger specific surface area, more cation exchange capacity, more total porosity, less macroporosity, and more organic matter than the sandy soil. Thus, by simply knowing the texture of the soil, inferences can be made in regard to many soil properties. Here, soil texture will be determined quantitatively using the hydrometer method, and estimated using the texture by feel method. Different soil structure types will also be observed. What you learn about texture and structure in these activities will be used later during the soil pit field trips to describe soil profiles in the field.

Learning Objectives

• Differentiate the three soil separates (sand, silt, and clay) based on their particle size diameters.
• Determine the percentages of sand, silt, and clay in selected soil samples using data collected from the hydrometer method of particle size analysis.
• Estimate the textural class using the texture-by-feel method on selected soil samples.
• Use a textural triangle to determine the textural class of a soil.
• Understand the relationship between particle size and specific surface area.
• Three soils of known textures
• Sodium hexametaphosphate
• Squirt bottles filled with tap water
• Lab balances accurate to the nearest 0.01 g
• Soil test cylinders, 1L volume (Item #200231000, Kimble ™ Kimax™ Soil Test Cylinders)
• Hydrometers calibrated in units of g/L (Item #13-202-133 Fisherbrand™ Soil Analysis ASTM Hydrometer)
• Digital thermometers accurate to the nearest 0.1°C
• Milkshake mixers and stainless steel milkshake mixer cups (Hamilton Beach HMD400 120V Triple Spindle Commercial Drink Mixer, Hamilton Beach, Glen Allen, Virginia, U.S.)

Recommended Reading

• Eye on Agriculture Today: Soil Texture by Feel (KSREVideos, 2010)
• Soil Profiling: Structure (KSREVideos, 2011c)
• Estimating Soil Texture by Feel (Presley and Thien, 2008)

Prelab Assignment

Using the recommended reading and viewing resources and the introduction to this lab, consider the questions listed below. These definitions/questions will provide a concise summary of the major concepts to be addressed in the lab. They will also serve as the basis for the post-lab quiz and are useful study notes for exams.

• Define and explain the difference between soil texture and soil structure.
• List the sizes for sand, silt, and clay particles using USDA criteria.
• What is a textural triangle?
• What is Stoke’s Law and how is it used in soil science?
• Define specific surface area and explain its relationship to size of soil particles.

Introduction

Soil texture refers to the proportions of sand (2.0 – 0.05 mm in diameter), silt (0.05 – 0.002 mm), and clay (less than 0.002 mm). The relative proportions determine the textural class. Soil texture influences nearly every aspect of soil use and management. Many of the physical and chemical properties of the soil depend on how fine (clayey) or coarse (sandy) a soil is. Soil texture is a permanent feature unless soils are subjected to rapid erosion, deposition, or removal.

Moreover, much of the reactivity of soils is related to the amount of surface area available. As the average particle size decreases, the surface area per unit weight increases (see Table 7.1).

Table 7.1. Surface areas of soil particle sizes.

Table from King et al. (2003).

Nearly any type of land management will be influenced by texture. Table 7.2 provides a summary of soil management factors related to texture.

Table 7.2. Summary of soil texture relationships to various soil physical and chemical properties.

Activity 1: Textural Triangle

Soils with similar distributions of sand, silt, and clay have similar properties and are therefore grouped into the same soil textural class. Twelve textural classes are recognized, and their compositions are designated on a textural triangle (Figure 7.1). Study the arrangement of the triangle. Each corner represents 100% of either sand, silt, or clay, and each represents 0 to 100% of a given fraction. The proportions of sand, silt, and clay define the twelve classes. Figure 7.1 depicts a soil with 20% sand, 25% silt, and 55% clay. Those three lines intersect within the boundaries of the “Clay” textural class, so the soil is a clay soil.

Table 7.3. Soil Texture Class Activity

Activity 2: Estimating Soil Texture by Feel

A soil scientist often needs to estimate soil texture while in the field or when laboratory data on the amounts of sand, silt, and clay are not available. With practice, you can learn to estimate texture by simply feeling or manipulating a moist sample.

To learn this technique, consider a simplified and generalized version of the textural triangle (Fig. 5.3). This modified triangle consists of three tiers based on approximate clay content. Clays are very cohesive, plastic, and can be easily molded. Clay loams are intermediate in clay content, cohesiveness, and ease of molding. Loams are soils low enough in clay content to possess little cohesiveness and are more difficult to mold. Sands do not form stable forms when molded.

Using the procedure outlined in Figure 7.4, determine the texture on the samples provided in the laboratory. Working the sample at the proper moisture content is very important. The sample must be moistened throughout. Achieving the correct moisture condition may take several minutes. After moistening and mixing soil to the proper consistency, perform the ribbon test, the grittiness test, and the smoothness test as described in the diagram. Try to estimate the texture of the samples, then check the answers provided. After you have calibrated your fingers on the practice samples, determine the texture for the unidentified samples provided, and enter your estimate in Table 7.3.

Table 7.4. Results for estimation of soil texture by feel.

Activity 3: Particle Size Analysis by Hydrometer

Particle size analysis is based on the principle that different size particles fall through a fluid at different rates.

A particle falling in a fluid is subjected to 3 forces: gravity, buoyancy, and friction (Figure 7.5). The gravity and buoyancy forces are constant, but the frictional force increases as the velocity increases (like the drag force on an airplane increases as it goes faster). Because of this increasing frictional force, the particle eventually reaches a constant velocity (terminal velocity). Constant velocity occurs when the sum of the forces acting on the particle is zero, or acceleration is zero.

Stoke’s Law is derived by setting up an equation containing the three forces acting on the particle when acceleration is zero:

[latex]\text{Force of gravity }=\text{ force of buoyancy }+\text{ force of friction}[/latex]

These forces are determined from the following relationships:

Force of Gravity

[latex]\text{Force of gravity }=\text{ mass of particle }\times\text{ acceleration of gravity}[/latex]

[latex]\text{Mass of particle }=\text{ Volume of particle }\times\text{ density of particle}[/latex]

[latex]\text{Force of gravity }=\text{ volume of particle }\times\text{ density of particle}\times\text{ acceleration of gravity}[/latex]

Force of Buoyancy

[latex]\text{Force of buoyancy }=\text{ mass of water displaced by particle }\times\text{ acceleration of gravity}[/latex]

[latex]\text{Mass of displaced water }=\text{ volume of particle }\times\text{ density of water}[/latex]

[latex]\text{Force of buoyancy }=\text{ volume of particle }\times\text{ density of water}\times\text{ acceleration of gravity}[/latex]

Friction Force

Friction force is a function of the size of the particle, the velocity of the particle, and the viscosity of the water.

[latex]\text{Friction force }=6π\times\text{ viscosity }\times\text{ radius of particles}\times\text{ velocity}[/latex]

After substituting the appropriate components of the forces into the equation, it can be solved for the terminal velocity:

[latex]\text{Velocity }=\frac{\text{ particle diameter}^2\times\text{ acceleration due to gravity}\times(\text{ particle density}-\text{ liquid density})}{18\times\text{viscosity of liquid}}[/latex]

Note that the larger the diameter of the particle, the faster it settles (sand grains will settle faster than silt particles, which will settle faster than clay particles). Also, the density and viscosity of water vary with temperature, so the velocity of settling will be influenced by the temperature of the water (Figure 7.6).

Stoke’s Law can be simplified by specifying the temperature of the water:

[latex]\text{Velocity }(\text{cm s}^2)=\text{K}\times(\text{diameter of particle in cm})^2[/latex]

Where K (cm -1 s -1 ) is a constant incorporating water density and viscosity and acceleration due to gravity. Because velocity is distance/time, this equation can be solved for time required for a particle of a specified diameter to fall a given distance.

[latex]\text{Velocity }=\frac{\text{distance}}{\text{time}}=\text{K}\times(\text{diameter})^2[/latex]

[latex]\text{Time}=\frac{\text{distance}}{\text{K}\times(\text{diameter})^2}[/latex]

Thus, for a particle of a given diameter, the time required for the particle to fall a specified distance can be calculated.

Consider this example: How long will it take a 0.05-mm particle to fall 10 cm in water at 25°C?

At 25°C, K = 10,000 cm -1 s -1 . Substituting this value and the diameter (0.005 cm) into the above equation yields:

[latex]\text{Time}=\frac{10\text{ cm}}{10000\text{ cm}^{-1}\text{ s}^{-1}\times(0.005\text{ cm})^2}=\frac{10}{0.25\text{ cm s}^{-1}}=40\text{s}[/latex]

Thus, after 40 seconds, the upper 10 cm of a soil-water suspension is completely free of all particles 0.05 mm or larger, so it is free of sand and contains only silt and clay.

Let’s do the same calculation for a 0.002-mm particle, which is the upper limit of the clay range.

[latex]\text{Time}=\frac{10\text{ cm}}{10000\text{ cm}^{-1}\text{ s}^{-1}\times(0.002\text{ cm})^2}=\frac{10}{0.0004\text{ cm s}^{-1}}=6.94\text{hr}[/latex]

Thus, after 6 hours and 56 minutes, the upper 10 cm of a soil-water suspension is free of all particles 0.002 mm or larger (sand and silt), so it contains only clay particles.

As we can see, Stokes Law can be used to determine when a volume of a soil-water suspension will be devoid of soil particles larger than a given size. Then we can measure the concentration of soil remaining in that volume. For example, after 40 seconds, we can measure the concentration of soil in suspension in the upper 10 cm of a suspension and thus determine how much clay + silt are present.

Under natural conditions, sand, silt, and clay particles are bound together in aggregates. These aggregates must be broken down so soil particles act independently of each other. For example, an aggregate of clay particles would behave as a silt particle, a phenomenon we want to avoid.

Dispersion is a 2-step, chemical/mechanical process. First, sodium hexametaphosphate (like the dishwashing detergent “Calgon®”) is added to a soil-water suspension to increase electronegativity of soil clays; it causes a repulsive force between clay particles. Then the suspension is stirred vigorously (milkshake mixer or blender) to assure complete dispersion. The repulsive forces generated by the chemical treatment tend to stabilize the dispersed condition. Dispersion thus assures that aggregated clay particles do not behave like silt-sized or sand-sized particles.

Soil organic matter is an important binding agent, so it first must be removed by oxidation (using hydrogen peroxide, for example). In soils with very low in organic matter, this step is often omitted. It will not be used in this exercise.

After proper dispersion, sand, silt, and clay can be separated and quantified by allowing the particles to settle in water. (NOTE: a sieve is commonly used to quantify sand content of a soil sample, and then a hydrometer to quantify silt and clay content of the remaining particles. We will use a hydrometer for all three particle sizes in this lab activity.)

One method of determining the concentration of soil in suspension is using a hydrometer to measure the density of the suspension. The hydrometer is commonly used in field labs. In this exercise, we’ll use a hydrometer calibrated to read directly in g/L of suspension.

Simplified Hydrometer Procedure

Chemical dispersion (performed in the previous lab)

• Weigh out 30.0 g of dry soil (assume oven-dry) into a 250-ml Erlenmeyer flask.
• Wash sides of flask with distilled water from a wash bottle.
• Add 100 ml of distilled water using a graduated cylinder, and add 10 ml of sodium hexametaphosphate solution (500 g/L) from the dispenser on the sodium hexametaphosphate bottle.
• Swirl to mix.
• Cover the flask with Parafilm and label the flask with your lab section and table number as well as soil type. Store the flasks in the location specified by your instructor until next laboratory period.

After chemical dispersion

• Quantitatively transfer the dispersed sample into a metal dispersion cup (a milkshake mixer cup), fill to half full with distilled water, and mix for 5 minutes as directed by your instructor. (“Quantitatively transfer” means to transfer all the sample.)
• Quantitatively transfer the sample into a 1-liter sedimentation cylinder.
• Fill cylinder to the 1000-ml mark with distilled water.
• Plunger method: Carefully insert stirring plunger and move up and down the full length of the cylinder for 30 seconds, ensuring that all particles are thoroughly mixed. Hold base of cylinder firmly with other hand.
• Stopper method: Place a rubber stopper (one properly sized for the sedimentation cylinders) into the top of the cylinder, and holding both the bottom of the cylinder and the top of the stopper, mix the solution vigorously by inverting the cylinder and turning it right side up repeatedly for 30 seconds.
• Record time when stirring is stopped and plunger removed, or when the stoppered cylinder is returned (quickly but gently) right-side-up to the laboratory bench.
• Immediately insert the hydrometer slowly and carefully. Read the hydrometer (top of the meniscus) at exactly 40 seconds after stirring was stopped or the cylinder was returned to the bench. (Note: if a soil is high in organic matter that was not removed before beginning this experiment, bubbles will form at the surface. To disperse the bubbles, add three to five drops of alcohol to the surface of the solution immediately after inserting the hydrometer.
• Repeat steps 4 through 6 until readings are within 0.5 units of each other. Record the reading on the data sheet.
• For each degree above 20°C, add 0.36 to the hydrometer reading.
• For each degree below 20°C, subtract 0.36 from the hydrometer reading.
• Because the soil suspension also contains sodium hexametaphosphate, each hydrometer reading must be corrected to account for the effect of sodium hexametaphosphate on density. A “blank” cylinder has been set up containing only water and sodium hexametaphosphate. Record the hydrometer reading from this cylinder and subtract that value from all hydrometer readings.
• Your lab instructor has conducted measurements for the 7-hour reading and will provide the data to you to complete your soil texture determination.
• Calculations:

Remember that these hydrometer readings are in g/L. Because the volume in the cylinder is one liter, the readings gives the amount of soil in the cylinder. For example, the 7-hour reading indicates the amount of clay (g) in the cylinder. Therefore, the percent clay is

[latex]\text{Percent clay}=\frac{\text{corrected 7 hr reading}}{\text{mass of dry sample}}\times100%[/latex]

The 40-second reading is used to calculate percent silt + clay:

[latex]\text{Percent silt }+\text{ clay}=\frac{\text{corrected 40 s reading}}{\text{mass of dry sample}}\times100%[/latex]

Then the silt and sand percentages can be determined.

[latex]\text{Percent silt }=(\text{percent silt}+\text{percent clay})-\text{percent clay}[/latex]

[latex]\text{Percent sand }=100-(\text{percent silt}+\text{percent clay})[/latex]

Table 7.5. Soil Texture by Hydrometer Method Data.

Table adapted from King et al. (2003).

Soil Texture by Hydrometer Method Calculations

Activity 4: Soil Structure

Table 7.6. Summary of soil structure types described in mineral soils.

Carefully examine the different structure types listed in Table 7.6 and shown in Figure 7.7, then answer the following questions.

Assignment: Online Quiz

A quiz will be available online. Please access it as directed by your instructor.

Soils Laboratory Manual Copyright © 2017, 2019, 2021 by Colby J. Moorberg & David A. Crouse is licensed under a Creative Commons Attribution 4.0 International License , except where otherwise noted.

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Soil Basics

What is soil?

In short, soil is a mixture of minerals, dead and living organisms (organic materials), air, and water. These four ingredients react with one another in amazing ways, making soil one of our planet’s most dynamic and important natural resources.

Soil is used by people in numerous ways. Because of this, it has many definitions. An engineer may view soils as a material upon which infrastructure is built, while a diplomat may refer to “soil” as a nation’s territory. From a soil scientist’s perspective, soil is:

The surface mineral and/or organic layer of the earth that has experienced some degree of physical, biological and chemical weathering.

Soils are limited natural resources. They are considered renewable because they are constantly forming. Though this is true, their formation occurs at extremely slow rates. In fact, one inch of topsoil can take several hundred years or more to develop. Soil formation rates vary across the planet: the slowest rates occur in cold, dry regions (1000+ years), and the fastest rates are in hot, wet regions (several hundred years).  Read more about how long it takes for soil to form.

How do soils form?

Soil Profiles - Dig down deep into any soil, and you’ll see that it is made of layers, or horizons. Put the horizons together, and they form a soil profile. Like a biography, each profile tells a story about the life of a soil.

Soil Changes with Age - As a soil ages, it gradually starts to look different from its parent material. That’s because soil is dynamic. Its components—minerals, water, air, organic matter, and organisms—constantly change. Some components are added. Some are lost. Some move from place to place within the soil. And some components are transformed into others.

CLORPT - Soils differ from one part of the world to another, and even from one part of a backyard to another. They differ because of where and how they formed. Over time, five major factors control how a soil forms. They are climate, organisms, relief (landscape), parent material, and time--or CLORPT, for short. Read more about CLORPT. 

What are the soil types?

To identify, understand, and manage soils, soil scientists have developed a soil classification or taxonomy system. Like the classification systems for plants and animals, the soil classification system contains several levels of detail, from the most general to the most specific. The most general level of classification in the United States system is the soil order ,  of which there are 12.

Each order is based on one or two dominant physical, chemical, or biological properties that differentiate it clearly from the other orders. Perhaps the easiest way to understand why certain properties were chosen over others is to consider how the soil (i.e., land) will be used. That is, the property that will most affect land use is given precedence over one that has a relatively small impact.

The 12 soil orders all end in "sol" which is derived from the Latin word "solum" meaning soil or ground. Most of the orders also have roots that tell you something about that particular soil. For example, "molisol" is from the Latin "mollis" meaning soft. Explore more about each soil order.

**Each state and territory in the United States has a representative soil, like a state flower or bird. Find your state soil ! 

What makes soil, soil?

Texture - The particles that make up soil are categorized into three groups by size:  sand, silt, and clay . Sand particles are the largest and clay particles the smallest. Although a soil could be all sand, all clay, or all silt, that's rare. Instead most soils are a combination of the three.

The relative percentages of sand, silt, and clay are what give soil its texture. A loamy texture soil, for example, has nearly equal parts of sand, silt, and clay.

Structure - Soil structure is the arrangement of soil particles into small clumps, called "peds". Much like the ingredients in cake batter bind together to form a cake, soil particles (sand, silt, clay, and organic matter) bind together to form peds. Peds have various shapes depending on their “ingredients” and the conditions under which the peds formed: getting wet and drying out, freezing and thawing--even people walking on or farming the soil affects the shapes of peds.

Ped shapes roughly resemble balls, blocks, columns, and plates. Between the peds are spaces, or pores, in which air, water, and organisms move. The sizes of the pores and their shapes vary from soil structure to soil structure.

A soil’s texture and structure tells us a lot about how a soil will behave. Granular soils with a loamy texture make the best farmland, for example, because they hold water and nutrients well. Single-grained soils with a sandy texture don’t make good farmland, because water drains out too fast. Platy soils, regardless of texture, cause water to pond on the soil surface.

Color - Color can tell us about the soil’s mineral content. Soils high in iron are deep orange-brown to yellowish-brown. Those with lots of organic material are dark brown or black; in fact, organic matter masks all other coloring agents.

Color can also tell us how a soil behaves. A soil that drains well is brightly colored. One that is often wet and soggy has an uneven (mottled) pattern of grays, reds, and yellows.

What do soils do?

Soils are amazing! Life as we know it would not exist without them, as they provide countless services that benefit all humans. Clean air and water, the clothes on our backs, habitat, and food for plants and animals are just a few things we can thank soils for. These 'goods and services' provided by soils are called ecosystem services . 

ENCYCLOPEDIC ENTRY

Soil composition.

Soil is one of the most important elements of an ecosystem, and it contains both biotic and abiotic factors. The composition of abiotic factors is particularly important as it can impact the biotic factors, such as what kinds of plants can grow in an ecosystem.

Biology, Ecology, Chemistry, Earth Science, Geography, Physical Geography

Soil Layers

Soil is composed of both biotic—living and once-living things, like plants and insects—and abiotic materials—nonliving factors, like minerals, water, and air.

Photo from Getty Images

Soil contains air, water, and minerals as well as plant and animal matter, both living and dead. These soil components fall into two categories. In the first category are biotic factors—all the living and once-living things in soil , such as plants and insects. The second category consists of abiotic factors, which include all nonliving things—for example, minerals , water, and air. The most common minerals found in soil that support plant growth are phosphorus, and potassium and also, nitrogen gas. Other, less common minerals include calcium, magnesium, and sulfur. The biotic and a biotic factors in the soil are what make up the soil ’s composition. Soil composition is a mix of soil ingredients that varies from place to place. The Natural Resources Conservation Service (NRCS)—part of the U.S. Department of Agriculture—has compiled soil maps and data for 95 percent of the United States. The NRCS has found that each state has a “state soil ” with a unique soil “recipe” that is specific to that state. These differing soils are the reason why there is such a wide variety of crops grown in the United States. Consider the soils of three states: Hawai'i, Iowa, and Maine. Hawai'i’s deep, well-drained state soil contains volcanic ash that makes it perfect for growing sugar cane, as well as ginger roots, papaya, and macadamia nuts. Iowa, which is in Midwest region of the United States, has a state soil that is good for farming because it is made up of a thick layer of organic matter from the decomposition of prairie grasses. Corn and soybeans are the primary crops grown in these soils . The state soil of Maine, located in the northeastern part of the country, is made from materials left behind after local glaciers melted. This soil is perfect for growing trees—specifically, red spruce and balsam fir. Many of the trees being grown today in Maine are harvested for timber or for making paper. Soil scientists conduct various tests on soils to learn about their composition. Soil testing can identify the amounts of biotic and a biotic factors in the soil . The results of these tests can also reveal if the soil has too much of a specific mineral or if it needs more nutrients to support plants. Scientists also measure other factors, such as the amount of water in the soil and how it varies over time—for instance, is the soil unusually wet or dry? The tests can also identify contaminants and heavy metal in the soil and determine the soil ’s nitrogen content and pH level (acidity or alkalinity). All of these measurements can be used to determine the soil ’s health.

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Related Resources

What is Soil?

Soils are complex mixtures of minerals, water, air, organic matter, and countless organisms that are the decaying remains of once-living things. It forms at the surface of land – it is the “skin of the earth.” Soil is capable of supporting plant life and is vital to life on earth. Soil, as formally defined in the Soil Science Society of America Glossary of Soil Science Terms, is:

• The unconsolidated mineral or organic material on the immediate surface of the earth that serves as a natural medium for the growth of land plants.
• The unconsolidated mineral or organic matter on the surface of the earth that has been subjected to and shows effects of genetic and environmental factors of: climate (including water and temperature effects), and macro- and microorganisms, conditioned by relief, acting on parent material over a period of time.

So then, what is dirt? Dirt is what gets on our clothes or under our fingernails. It is soil that is out of place in our world – whether tracked inside by shoes or on our clothes. Dirt is also soil that has lost the characteristics that give it the ability to support life – it is “dead.” Soil performs many critical functions in almost any ecosystem (whether a farm, forest, prairie, marsh, or suburban watershed). There are seven general roles that soils play:

• Soils serve as media for growth of all kinds of plants.
• Soils modify the atmosphere by emitting and absorbing gases (carbon dioxide, methane, water vapor, and the like) and dust.
• Soils provide habitat for animals that live in the soil (such as groundhogs and mice) to organisms (such as bacteria and fungi), that account for most of the living things on Earth.
• Soils absorb, hold, release, alter, and purify most of the water in terrestrial systems.
• Soils process recycled nutrients, including carbon, so that living things can use them over and over again.
• Soils serve as engineering media for construction of foundations, roadbeds, dams and buildings, and preserve or destroy artifacts of human endeavors.
•  Soils act as a living filter to clean water before it moves into an aquifer.

Soil Profile There are different types of soil, each with its own set of characteristics. Dig down deep into any soil, and you’ll see that it is made of layers, or horizons (O, A, E, B, C, R). Put the horizons together, and they form a soil profile. Like a biography, each profile tells a story about the life of a soil. Most soils have three major horizons (A, B, C) and some have an organic horizon (O).

O – (humus or organic) Mostly organic matter such as decomposing leaves. The O horizon is thin in some soils, thick in others, and not present at all in others. A - (topsoil) Mostly minerals from parent material with organic matter incorporated. A good material for plants and other organisms to live. E – (eluviated) Leached of clay, minerals, and organic matter, leaving a concentration of sand and silt particles of quartz or other resistant materials – missing in some soils but often found in older soils and forest soils. B – (subsoil) Rich in minerals that leached (moved down) from the A or E horizons and accumulated here. C – (parent material) The deposit at Earth’s surface from which the soil developed. R – (bedrock) A mass of rock such as granite, basalt, quartzite, limestone or sandstone that forms the parent material for some soils – if the bedrock is close enough to the surface to weather. This is not soil and is located under the C horizon.

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Soil formation

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As stated at the beginning of this article, soils evolve under the action of biological, climatic, geologic, and topographic influences. The evolution of soils and their properties is called soil formation, and pedologists have identified five fundamental soil formation processes that influence soil properties. These five “state factors” are parent material, topography , climate , organisms, and time.

Parent material is the initial state of the solid matter making up a soil. It can consist of consolidated rocks, and it can also include unconsolidated deposits such as river alluvium , lake or marine sediments, glacial tills , loess (silt-sized, wind-deposited particles), volcanic ash, and organic matter (such as accumulations in swamps or bogs ). Parent materials influence soil formation through their mineralogical composition , their texture, and their stratification (occurrence in layers). Dark-coloured ferromagnesian (iron- and magnesium-containing) rocks, for example, can produce soils with a high content of iron compounds and of clay minerals in the kaolin or smectite groups, whereas light-coloured siliceous (silica-containing) rocks tend to produce soils that are low in iron compounds and that contain clay minerals in the illite or vermiculite groups. The coarse texture of granitic rocks leads to a coarse, loamy soil texture and promotes the development of E horizons (the leached lower regions of the topmost soil layer). The fine texture of basaltic rocks, on the other hand, yields soils with a loam or clay-loam texture and hinders the development of E horizons. Because water percolates to greater depths and drains more easily through soils with coarse texture, clearly defined E horizons tend to develop more fully on coarse parent material.

In theory, parent material is either freshly exposed solid matter (for example, volcanic ash immediately after ejection) or deep-lying geologic material that is isolated from atmospheric water and organisms. In practice, parent materials can be deposited continually by wind, water, or volcanoes and can be altered from their initial, isolated state, thereby making identification difficult. If a single parent material can be established for an entire soil profile, the soil is termed monogenetic; otherwise, it is polygenetic. An example of polygenetic soils are soils that form on sedimentary rocks or unconsolidated water- or wind-deposited materials. These so-called stratified parent materials can yield soils with intermixed geologic layering and soil horizons—as occurs in southeastern England , where soils forming atop chalk bedrock layers are themselves overlain by soil layers formed on both loess and clay materials that have been modified by dissolution of the chalk below.

Adjacent soils frequently exhibit different profile characteristics because of differing parent materials. These differing soil areas are called lithosequences, and they fall into two general types. Continuous lithosequences have parent materials whose properties vary gradually along a transect, the prototypical example being soils formed on loess deposits at increasing distances downwind from their alluvial source. Areas of such deposits in the central United States or China show systematic decreases in particle size and rate of deposition with increasing distance from the source. As a result, they also show increases in clay content and in the extent of profile development from weathering of the loess particles.

By contrast, discontinuous lithosequences arise from abrupt changes in parent material. A simple example might be one soil formed on schist (a silicate-containing metamorphic rock rich in mica ) juxtaposed with a soil formed on serpentine (a ferromagnesian metamorphic rock rich in olivine ). More subtle discontinuous lithosequences, such as those on glacial tills, show systematic variation of mineralogical composition or of texture in unconsolidated parent materials.

Topography, when considered as a soil-forming factor, includes the following: the geologic structural characteristics of elevation above mean sea level , aspect (the compass orientation of a landform), slope configuration (i.e., either convex or concave), and relative position on a slope (that is, from the toe to the summit). Topography influences the way the hydrologic cycle affects earth material, principally with respect to runoff processes and evapotranspiration . Precipitation may run off the land surface, causing soil erosion , or it may percolate into soil profiles and become part of subsurface runoff, which eventually makes its way into the stream system. Erosive runoff is most likely on a convex slope just below the summit, whereas lateral subsurface runoff tends to cause an accumulation of soluble or suspended matter near the toeslope. The conversion of precipitation into evapotranspiration is favoured by lower elevation and an equatorially facing aspect.

Adjacent soils that show differing profile characteristics reflecting the influence of local topography are called toposequences. As a general rule, soil profiles on the convex upper slopes in a toposequence are more shallow and have less distinct subsurface horizons than soils at the summit or on lower, concave-upward slopes. Organic matter content tends to increase from the summit down to the toeslope, as do clay content and the concentrations of soluble compounds.

Often the dominant effect of topography is on subsurface runoff (or drainage ). In humid temperate regions, well-drained soil profiles near a summit can have thick E horizons (the leached layers) overlying well-developed clay-rich Bt horizons, while poorly drained profiles near a toeslope can have thick A horizons overlying extensive Bg horizons (lower layers whose pale colour signals stagnation under water-saturated conditions). In humid tropical regions with dry seasons, these profile characteristics give way to less distinct horizons, with accumulation of silica, manganese , and iron near the toeslope, whereas in semiarid regions soils near the toeslope have accumulations of the soluble salts sodium chloride or calcium sulfate .

These general conclusions are tempered by the fact that topography is susceptible to great changes over time. Soil erosion by water or wind removes A horizons and exposes B horizons to weathering. Major portions of entire soil profiles can move downslope suddenly by the combined action of water and gravity . Catastrophic natural events, such as volcanic eruptions, earthquakes, and devastating storms, can have obvious consequences for the instability of geomorphologic patterns.