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Chapter 2: Soil origin and development.

OBJECTIVES

After completing this chapter, you should be able to:

* define a soil body

* list examples of the five soil-forming factors

* describe how soils develop

* describe the horizons of the soil profile

TERMS TO KNOW

alluvial fan

alluvial soil

chemical weathering

colluvium

delta

eluviation

eolian deposit

floodplains

frost wedging

glacial drift

glacial outwash

glacial till

hydrolysis

igneousrock

illuviation

lacustrine

leaching

levee

loess

marine sediment

master horizon

metamorphic rock

mineral soil

organic soil

oxidation-reduction

parent material

pedology

pedon

physical weathering

plow layer

polypedon

residual soil

river terrace

root wedging

sedimentary rock

soil genesis

soil horizon

soil profile

solum

solution

subsoil

talus

topsoil

transported soil

weathering

Soil is a very slowly renewable resource. Although many of our soils originated long ago, the forces that created them continue to operate. Some soils are probably hundreds of thousands of years old, while others may have just begun to make their appearance yesterday. Soils grow, change, and develop. This chapter describes the soil-formation processes.

Pedology is the study of soil formation, also known as soil genesis, and soil classification and mapping. We cover the latter subjects in the next chapter. Modern pedology dates to the eighteenth and nineteenth centuries in Germany, the United States, and especially Russia. These early researchers developed concepts of the soil as an evolving body arising from weathered rocks of the crust under a variety of influences. V. V. Dokuchaev (1846-1903), a Russian often credited with laying the foundation of modern pedology, published a careful study of Russian soils in 1883 that applied these concepts. He identified soil-forming factors, developed an early soil classification system, and began naming the soil horizons we use today. Dokuchaev's and other Russian publications remained unknown in the United States until into the twentieth century.

In the United States, Hans Jenny's 1941 publication Factors of Soil Formation further developed the five factors of soil formation detailed in this text. Jenny continued to develop and quantify these factors throughout his career and connected them with ecological principle. Much of the information in this chapter comes from the work of Jenny and other soil scientists practicing in the United States in the twentieth century.

Before describing how a body of soil forms, let's define what we mean by a soil body.

The Soil Body

The soil is a collection of natural bodies of the earth's surface containing living matter that is able to support the growth of plants. It ends at the top where the atmosphere or shallow water begins. It ends at the bottom at the farthest reach of the deepest rooted plants. The soil varies across the landscape: in one area it may be mostly made of decayed plant parts, in another place it may be mostly sand.

It is not possible to learn everything about a soil just by standing on the surface. One must dig a hole to see what it looks like below the surface. Because a soil scientist cannot dig up acres of ground to study a whole body of soil, the soil is broken up into small parts that can be easily studied. This small body is called the pedon (figure 2-1). A pedon is a section of soil, extending from the surface to the depth of root penetration of the deepest rooted plants, but generally examined to a depth of five feet. Generally a pedon has dimensions of about one meter by one meter, and about one and a half meters deep (about 3 ft. x 3 ft. x 5 ft.). Soil scientists use the pedon as a unit of soil easily studied by digging a pit in the ground (figure 2-2).

The traits of a pedon are set by the combination of factors that formed it. In the landscape near the pedon being studied are other pedons that are probably very similar. As one moves across the landscape, however, one will reach a pedon that is different, because the combination of factors that formed it were different. A collection of pedons that are much the same is called a polypedon. Later in this text we will learn how these polypedons are mapped into units called a soil series.

How does a soil pedon form? Picture a section of bare rock that will someday become a soil pedon. In the process of soil formation, this rock is changed into a layer of small, broken rock particles with some organic matter mixed in. Weather and plants are the major agents responsible for forming soil from rock, and the process is called weathering.

Physical weathering is the disintegration of rock by temperature, water, wind, and other factors. For instance, in cold climates frost wedging occurs when water freezes and expands in rocks or in cracks in the rock, causing it to break apart. The alternate expansion and contraction of rock caused by heating and cooling cycles also stresses the fabric of rock. Both cause rock to fracture or outer layers to peel away. Rain, running water, and windblown dust also wear away at rock surfaces.

Chemical weathering changes the chemical makeup of rock and breaks it down. The simplest process is solution. Rainwater is mildly acidic and can slowly dissolve many soil minerals, as in the solution of lime, calcium carbonate:

CaC[O.sub.3] + 2[H.sup.+] [right arrow] [H.sub.2]C[O.sub.3] + [Ca.sup.+2]

[FIGURE 2-1 OMITTED]

[FIGURE 2-2 OMITTED]

In this reaction, lime dissolves in acidic water by reacting with hydrogen ions to form carbonic acid and calcium ions. In hydrolysis, water reacts with minerals to produce new, softer compounds, as in the hydrolysis of the feldspar mineral orthoclase to a softer feldspar mineral:

NaAl[Si.sub.3][O.sub.8] + [H.sub.2]O [right arrow] HAl[Si.sub.3][O.sub.8] + [Na.sup.+] + [OH.sup.-]

Oxidation-reduction and other reactions are also important in chemical weathering. Refer to appendix 1 if you need help understanding these reactions.

Plants also play an important role in rock crumbling. Roots can exert up to 150 pounds per square inch of pressure when growing into a crack in rock. Root wedging from the pressure pries apart stone.

Lichens growing on bare rock (figure 2-3) form mild acids that slowly dissolve rock. When lichen dies, its dry matter is added to the slowly growing mixture of mineral particles and organic matter. When a small bit of soil forms in a rock crevice, plants begin to grow from seed that has blown into the crevice, continuing the cycle.

[FIGURE 2-3 OMITTED]

Soil formation does not stop when a layer of young soil covers the surface. The new soil continues to slowly age and develop over thousands of years. Soil scientists state that five factors operate during the process of soil formation and development: parent material, climate, life, topography, and time. One could say that over time, climate and living things act on parent materials with a certain topography to create soil. Some have suggested that human activities might be named a sixth factor, because most soils have been modified to some degree by humans.

Soil formation begins with rock, which supplies the parent materials for most soils. Before studying the five factors, let's look at rocks of earth's crust.

Rocks and Minerals

The original source of most soils is rock--the solid, un-weathered material of the earth's crust. Solid rock breaks into smaller particles, which are the parent materials of soil. Rock is a mixture of minerals that, when broken down, supply plant nutrients. Geologists classify rock into three broad types: igneous, sedimentary, and metamorphic. Refer to figure 2-4 for help in understanding the following paragraphs.

Igneous Rock. The basic material of the earth's crust is igneous rock, created by the cooling and solidification of molten materials from deep in the earth (figure 1-3). Igneous rocks, such as granite, contain minerals that supply fourteen of the seventeen required plant nutrients (listed in chapter 10 of this text). Granite, which is mined for monuments and building material, is a hard, coarse-grained rock made of feldspar, quartz, and other minerals. Feldspar, a fairly soft mineral containing potassium and calcium, weathers easily to clay. Quartz, a very hard and resistant mineral, weathers slowly to sand. Figure 2-5 lists the nutrient content of two sample igneous rocks: a granite and a basalt. Granite tends to weather slowly to create acidic parent materials high in sand, while basalt, a softer, finer-grained rock, weathers more quickly to less acidic materials low in sand.

Sedimentary Rock. Igneous rock comprises only about one-quarter of the earth's actual surface, even if most of the crust is igneous. This is because sedimentary rock overlays about three-quarters of the igneous crust. Sedimentary rock forms when loose materials like mud or sand are deposited by water, wind, or other agents, slowly cemented by chemicals and/or pressure into rock. Much of the sedimentary rock covering North America was deposited in prehistoric seas.

The parent materials of many American soils derive from sandstone and limestone. Sandstone, which consists of cemented quartz grains, weathers to sandy soils. Generally, these soils are infertile and droughty. Limestone is high in calcium and weathers easily to soils high in pH, calcium, and magnesium. Figure 2-5 lists contents of a typical sandstone and limestone.

Metamorphic Rock. If igneous and sedimentary rocks are subjected to great heat and pressure, they change to form metamorphic rock. For instance, limestone is a fairly soft, gritty rock. When subjected to heat and pressure, it changes to marble, which is harder and can be cut and polished. Soils arising from metamorphic parent materials resemble soils from the original sedimentary or igneous rock. Figure 2-6 shows relationships between common sedimentary, igneous, and metamorphic rock.

[FIGURE 2-4 OMITTED]

Parent Material

The description of soil origin at the beginning of this chapter was of soil formed directly from bedrock. These residual soils, as they are called, are actually less common than soils of parent materials carried from elsewhere by wind, water, ice, or gravity. Residual soils (figure 2-7) form very slowly, as solid rock must be weathered first. Transported soils, however, developed from already weathered material, form more quickly. Figure 2-8 shows parent materials of the United States.

Glacial Ice. Glacial ice carried parent materials over the northern part of North America (figure 2-9) during numerous glacial periods over the past two million years. The last four left the most evidence, and the most recent glacier, that of the Wisconsin period, reached its peak expanse about 18,000 years ago and melted back out the United States about 10,000 to 12,000 years ago. Most of the glacial deposits that cover the northern states come from the Wisconsin ice sheet. Glaciers expanded out of several centers in Canada carving and grinding the earth, picking up and transporting soil, gravel, rocks, and other debris. As the glaciers melted and shrunk between glacial periods, transported material remained in deposits called glacial drift. In the process, they left behind a very distinctive landscape over much of the northern United States and Canada.

Glaciers deposited materials in many ways, so there are several kinds of glacial drift. During the melting process, some debris simply dropped in place to form deposits called glacial till. Because there was no sorting action in the deposition, glacial till is extremely variable, and so are the soils derived from it. Till soils often contain pebbles, stones, and even boulders (figure 2-10).

Other materials carried by the glacier washed away in meltwater to form sediments in streams and lakes. During the process, the materials were sorted by size.

[FIGURE 2-7 OMITTED]

[FIGURE 2-8 OMITTED]

Coarser material, being larger and heavier, was deposited near the glacier and in streams and rivers to form glacial outwash. Outwash soils tend to be sandy. Smaller particles reached glacial lakes to form lacustrine deposits on the lake bottoms.

Wind. Some parent materials were carried by wind, leaving eolian deposits. For example, some soils in Nebraska formed from sand dunes, deposits of sand carried by rolling in the wind. Most eolian soils in the United States are actually a result of the last glacial period.

After the last glaciers melted and meltwaters subsided, large expanses of land were exposed to a dry climate with strong westerly winds. Winds picked up silt-size (medium) particles and deposited them in the Mississippi and Missouri River valleys and elsewhere. These loess soils--wind-deposited silt--are important agricultural soils in much of Iowa, Illinois, and neighboring states.

Water. Alluvial soils are soils whose parent materials were carried and deposited in moving fresh water to form sediments (figure 2-11). Alluvial materials can be deposited in several ways. Alluvial fans form below hills and mountain ranges where streams flowing down the slope deposit material in a fan shape at the base. As water speed slows abruptly at the foot of the slope, large particles drop out first. As a result, alluvial fans are generally sandy or gravelly. Finer materials are carried away in rivers.

[FIGURE 2-9 OMITTED]

Flooding rivers also leave deposits behind. Often coarser materials are deposited in low ridges, or levees, along the river bank. Away from the river, floodwaters spread over large flat areas called floodplains. Here the water will be shallow and slowmoving; fine particles will settle out (figure 2-12). Floodplains tend to be fertile because new soil is added at each flood, but the soils tend to stay wet. Levees, being coarser and elevated, dry more quickly. Floodplain soils are especially important along the Mississippi and its tributaries and along rivers that flow into the ocean on the East and Gulf coasts. Many important soils of California are from river alluvium.

[FIGURE 2-10 OMITTED]

Sometimes a river will cut deeply into its floodplain to flow at a lower elevation. This establishes a new riverbed and floodplain, while the old floodplain is left higher as a river terrace. An example of river terrace soils is some soil of the San Joaquin Valley of California.

Lacustrine deposits form under still, fresh water. Most of our lacustrine soils remain from giant glacial lakes that have since dried up. Examples include Glacial Lake Agassiz of northern Minnesota, North Dakota, and Canada, and Glacial Lake Bonneville of Utah. When glacial runoff water ran into the lake, the heaviest materials were left near the shore, while the smallest particles were carried to the center of the lake. Thus, lacustrine soils are sandy near the old shoreline and grade to soils with smaller particles toward the old lake center.

[FIGURE 2-11 OMITTED]

[FIGURE 2-12 OMITTED]

Marine sediments form in the ocean. Many scattered soils of the Great Plains and the Imperial Valley of California are beaches of prehistoric seas that once covered the United States. Other beach soils are common along the Atlantic coastline and the Gulf of Mexico. These all tend to be sandy soils. Deltas, in contrast, have very small particles and tend to be wet. Deltas form when rivers flowing into an ocean deposit sediments at the mouth of the river. The Mississippi River Delta of Louisiana is a prime example, as is the Rio Grande Valley of Texas and Mexico.

Gravity. Some parent materials move simply by sliding or rolling down a slope. This material, called colluvium, is scattered in hilly or mountainous areas. An example of a colluvial material is a talus--sand and rocks that collect at the foot of a slope. Avalanches, mudslides, and landslides are other examples.

Volcanic Deposits. The ash blown out of a volcano and deposited nearby or carried some distance by wind forms a chemically distinct, dark, and lightweight parent material. The Pacific Northwest, Hawaii, and Alaska are areas of the United States where such deposits are common.

Organic Deposits. Characteristics of the soils formed from parent materials described so far are set by mineral particles in the soil. Mineral soils contain less than 20 percent organic matter, except for a surface layer of plant debris. Organic soils, containing 20 percent or more organic matter, form under water as aquatic plants die. Low oxygen conditions under water retard decay of these dead plants, so they tend to pile up on the lake bottom. Eventually the lake fills in and is replaced by an organic soil. Organic soils are extensive in Minnesota, Wisconsin, and Florida.

Climate

Climate first affects soils by causing physical and chemical weathering of rock. However, climate continues to affect soil development long beyond this initial stage. The main effects are due to temperature and rainfall.

Temperature affects the speed of chemical reactions in the soil--the higher the temperature, the faster a reaction. Chemical weathering in soils occurs mostly when the soil is warmer than 60[degrees]F. Thus, in cold areas, like tundra, soils develop slowly. In warm areas, like the tropics, soils develop more rapidly.

Another result of temperature is its effect on organic matter. Warmth promotes greater vegetation, so more organic matter is added to the soil. However, warm temperatures also speed the decay and loss of organic matter. Thus, soils of warm climates tend to be low in organic matter.

Rainfall affects soil development mainly by leaching. Leaching moves materials deeper into the soil via water moving downward through the soil. Leached materials include lime, clay, plant nutrients, and other chemicals. These materials are then deposited in lower parts of the soil.

High rainfall areas also tend to grow more vegetation, so the soils of humid areas tend to have more organic-matter than soils of drier regions. To summarize, rainfall tends to cause leaching and the accumulation of organic matter.

The United States is a good example of the effects of climate on soil (figure 2-13). The climate of the United States cools from south to north. This is reflected in an increase in average organic matter content from south to north. Also, the most weathered soils in the United States are in the South. The average rainfall of the United States increases from west to east. As a result, the organic matter content of the United States' soils also tends to increase from west to east.

Soil color also follows north-south trends. Because organic matter is black, soils tend to appear darker as one moves from warmer to cooler climates. Because of changes in chemical reactions involving iron, soils tend to appear redder as one moves from cooler to warmer climates.

[FIGURE 2-13 OMITTED]

Organisms

Organisms that live in soil--like plants, insects, and microbes--actively affect soil formation. The actual properties of a developing soil are influenced especially by the type of plants growing on it. Figure 2-14 shows the parent vegetation of soils of the United States.

Mineral soils having the highest organic matter content form under grasslands. Grasses usually have a dense mat of fibrous roots, some of which die each year. This keeps the organic matter content high and the soil color dark. In a forest, much of the organic material is above ground in the trees. When the leaves fall or the tree dies, the material falls to the soil where it creates a surface layer of organic matter that does not mix with deeper layers. As a result, forest soils have less organic matter than prairie soils and are lighter in color. The type of trees also influences the soil. Compared to hardwoods (deciduous trees), softwood (conifer) foliage is acidic and resistant to decay, therefore their soils tend to be thinner, lower in organic matter, and more acidic. Deserts, with very sparse vegetation, have the least organic matter (figure 2-15).

Vegetation also affects the location of nutrients and other ions in the soil. Plants absorb ions in the roots and carry them to the tops, where they are returned to the soil surface when leaves drop. This recycles ions from deeper in the soil to the surface and helps reduce their loss from leaching. Deep tree roots, for instance, extract ions from deep in the soil, leaving the surface horizon of forest soils enriched in ions.

[FIGURE 2-14 OMITTED]

[FIGURE 2-15 OMITTED]

We tend to stress vegetation as the main living factor in soil formation, but other life impacts soil as well, such as burrowing animals that bring subsoil to the surface, earthworms that create large, deep pores and speed organic matter decay, or nitrogen-fixing bacteria. These other organisms are covered in greater detail in chapter 5.

Topography

Topography, or the soil's position in the landscape, influences soil development mainly by affecting water movement. Water runs off slopes, making them drier, and collects in low areas, making them moremoist. This, in turn, affects leaching, chemical reactions, and types of vegetation. Slope effects vary according to a number of characteristics: steep or south-facing slopes are drier than gentle or north-facing slopes, and the top portion of a slope is drier than the bottom portion of one.

If enough water runs off a slope, it may carry away soil as fast as it is formed. Thus, soil may be thin on a slope and thick at its base. The effect of topography is most obvious in rolling fields, where sloped areas are light brown from topsoil loss while lower areas are black from accumulating topsoil and organic matter (figure 2-16).

Because running water tends to carry off smaller particles, soils in lower areas may be finer than those of higher areas. Depressions may also intersect the water table at least part of the year, keeping them wet for long periods.

Time

Soils change over time, undergoing an aging process. Initially, a thin layer of soil forms on the parent material. Such a young, immature soil takes as little as a hundred years to form from well-weathered parent materials under warm, humid conditions. Under other conditions, it may take hundreds of years.

[FIGURE 2-16 OMITTED]

Weathering of the young soil continues, and many generations of plants live and die, so the young soil becomes deeper and higher in organic matter. If there is enough rainfall, leaching begins to carry some material deeper into the soil, creating the soil profile described later in this chapter.

As soils age, biological processes tend to increase the nitrogen content, while leaching tends to reduce phosphorus. Thus, young soils tend to be low in nitrogen but high in phosphorus, while older soils are the opposite. Mature soils are generally productive, but as soils continue to age, they become more severely weathered, more highly leached, and often less productive. In general, as soil ages it becomes deeper, develops distinct layers, and becomes more acidic and leached.

However, the aging process is not static. Time zero for a soil usually begins when some dramatic event like landslides or glaciers changes everything and resets the clock. Such events can happen at any time. A soil might age through the years until it reaches some steady state and remains unchanged thereafter, but this is rare. Soils can erode away, be buried, or even become the parent material for a new soil. If soil factors change, the direction of soil development can be deflected into a new path. For instance, if forest invades prairie, the soil embarks on a new path towards a forest-type soil.

Humans

Humans may be considered just another living entity that modifies soil, but their action can be so rapid, dramatic, and different from other life that they might be considered a separate, sixth soil formation factor. Very few soils have been unaffected by human activities. Effects may be as subtle as the deposition of air pollutants distant from any human habitation to as massive as earthmoving during road construction. The latter resets the time clock for this new soil material to zero, and the earth moved by the machinery is the parent material for this new soil. Chapter 19 describes traits of urban soils, those most modified by humans.

The Soil Profile

Soils change over time in response to their environment, represented by the soil-forming factors. Soil scientists have classified the causes of those changes into four soil-forming processes:

ADDITIONS: materials may be added to the soil; some examples are fallen leaves, wind-blown dust, alluvium, and man-made materials like air pollutants and compost.

LOSSES: materials may be lost from the soil, as a result of deep leaching, erosion from the surface, or as gases filtering out of the soil.

TRANSLOCATIONS: materials may be moved within the soil, by leaching deeper into (but not out of ) the soil, being carried upward with evaporating water, or by being moved by animals like ants or earthworms.

TRANSFORMATIONS: materials may be altered in the soil; for example, organic matter decay, weathering of minerals to smaller particles, or chemical reactions.

Each of these processes occurs differently at different depths. For instance, organic matter tends to be added at or near the surface, not deep in the soil. Some material moves from high in the soil to be deposited lower. As a consequence, different changes occur at different depths, and horizontal layers develop as a soil ages (figure 2-17).

[FIGURE 2-17 OMITTED]

These layers are known as soil horizons, visible wherever the earth is dug deep enough to expose them. The soil profile is a vertical section through the soil extending into the unweathered parent material and exposing all the horizons. Each horizon in the profile differs in some physical or chemical way from the other horizons.

In a very young soil, weathering and plant growth produce a thin layer of mineral particles and organic matter atop parent material. The thin layer of soil is labeled the A horizon, a surface mineral horizon enriched with organic matter. The parent material below the A horizon of this young soil is termed the C horizon. It is defined as a subsurface mineral layer only slightly affected by soil-forming processes. Thus, this young soil has an AC soil profile.

As the young soil ages, the soil increases in depth. In addition, clay-sized particles and certain chemicals leach out of the A horizon (figure 2-18), moving downward in the profile to create a new layer, the B horizon.

Master Horizons. The A, B, and C horizons are known as master horizons. They are part of a system for naming soil horizons in which each layer is identified by a code: O, A, E, B, C, and R. These horizons are shown in figure 2-19, and are described as follows.

O The O horizon is an organic layer made of wholly or partially decayed plant and animal debris. The O horizon generally occurs in undisturbed soil, because plowing mixes the organic material into the soil. In a forest, fallen leaves, branches, and other debris make up the O horizon.

A The A horizon, called topsoil by most growers, is the surface mineral layer where organic matter accumulates. It is darker than the horizons below. Over time, this layer loses clay, iron, and other materials to leaching. This loss is called eluviation. Materials resistant to weathering, such as sand, tend to remain in the A horizon as other materials leach out. The A horizon provides the best environment for the growth of plant roots, microorganisms, and other life.

E The E horizon, the zone of greatest eluviation, is very leached of clay, chemicals, and organic matter. Because the chemicals that color soil have been leached out, the E layer is very light. Many soils have no E horizon; it is mostly likely to occur under forest vegetation in sandy soils in high rainfall areas.

B The B horizon, or subsoil, is often called the "zone of accumulation" where chemicals leached out of the A and E horizon accumulate. The word for this accumulation is illuviation (figure 2-18). The B horizon has a lower organic matter content than the topsoil and often has more clay. The A, E, and B horizons together are known as the solum, the portion of the soil profile most affected by soil-forming processes and that usually contains most plant roots.

C The C horizon lacks the properties of the A and B horizons. It is the soil layer little touched by soil-forming processes and is usually the parent material of the soil. It may also include very soft, weathered bedrock that roots can penetrate.

R The R horizon is underlying hard bedrock, such as limestone, sandstone, or granite. It may be cracked and fractured, allowing some root penetration. The R is identified only if near enough the surface to intrude into soil.

[FIGURE 2-18 OMITTED]

[FIGURE 2-19 OMITTED]

Subdivisions of the Master Horizons. As soils age, they may develop more horizons than the basic master horizons. Some of these layers are between the master horizons both in position and properties. These transitional layers are identified by the two master letters, with the dominant one written first. Thus, an AB layer lies between the A and B horizons and resembles both, but is more like the A than the B. Figure 2-19 shows these layers.

A soil layer can be further identified by a lowercase letter suffix that tells some trait of the layer. Appendix 4 lists these suffixes but two will serve as examples here--the Ap and Bt. An Ap layer is a surface layer disturbed by humankind, so that the old layers were mixed up. For instance, plowing would mix up an O, A, and AB horizon if they were all in the top eight inches. The Ap horizon is the same as the plow layer, the top seven or eight inches of soil in a plowed field. A Bt horizon is a B horizon in which clay has accumulated, usually by illuviation.

Further subdivisions are noted by a number following the letters. Thus, one could have a soil with both a Bt1 and a Bt2 horizon. This means that the Bt horizon of the soil has two distinct layers in it.

Now for an example. Figure 2-17 has the profile ApE-Bt-C. The top seven inches are an old plow layer or Ap. A strong, light-colored E horizon extends from the seven- to fourteen-inch depth, showing a leaching of clays and chemicals. Those clays then settled in the Bt horizon, lying between fourteen and twenty-two inches deep. Below this is the C horizon of sand and gravel. Notice the rodent hole in the E horizon.

SUMMARY

Soils form from minerals broken up by the action of weathering and plant roots and from the addition of decaying plant parts. Young soils continue to age--growing deeper, being leached by rainfall, developing layers, and changing over time. This soil-forming process involves the addition, loss, translocation, or transformation of soil materials, and is governed by the five factors of parent material, climate, life, topography, and time. Some authorities would add humans as a sixth factor in soil formation. Over time soil deepens and develops recognizable horizons, and may finally become severely weathered and highly leached.

Residual soils develop directly from bedrock (igneous, sedimentary, or metamorphic). Most mineral soils come from parent materials moved from one area to another by ice, water, wind, or gravity. Organic soils are composed of decaying plants. Each type of parent material is responsible for a different soil.

Parent materials are acted on by climate and living organisms. Soils develop quickly in warm areas with high rainfall, then age into heavily weathered soils low in organic matter. In cooler regions, organic matter accumulates and weathering is less extreme. In arid climates, sparse plant growth inhibits the formation of organic matter. Grassland soils tend to be high in organic matter, forest soils lower, and dryland soils lowest of all.

Topography affects soil formation by changing water movement and soil temperature. Low areas often have deep, rich soils that drain slowly. Erosion causes thin soils on slopes.

Time is a factor because soil development is a continuing process. Young soils tend to be thin with little horizon development. Mature soils are deeper and productive with several recognizable horizons. Old soils are severely weathered, highly leached, and less productive.

Soil profiles, which develop over time, are divided into master horizons. These, in turn, may also contain layers. Each layer is named by a code system that identifies its position in the profile and provides some information about it.

REVIEW

1. Name the five soil factors and give an example of the effect of each on soil formation.

2. Draw a soil profile containing seven distinct horizons in the correct order, and label them. Indicate topsoil, subsoil, and solum. Hint: You will use more than just the regular master horizons, and there are many possible configurations.

3. Describe the major parent materials and vegetation as well as climate that contributed to the soils of your state. Describe how they influenced your soils.

4. What do alluvial fans, floodplains, deltas, and terraces have in common? How are they different?

5. Would you be likely to read a soil description that includes an At or As horizon? Explain your answer.

6. Discuss the four soil-forming processes and give examples of each.

7. Consider the five soil-forming factors. Which are most likely to account for soil variation on a local scale? Which are more likely to operate on a larger scale? Explain your answer.

8. Running water removes soil from the surface. Which of the four soil-forming processes does this exemplify, and how might the soil-forming factors affect it?

9. Organic matter tends to increase from west to east in the United States because of increasing rainfall. Yet, some of the highest organic matter soils are in the plains states, which are relatively dry. Explain why.

10. A case study: Eighteen thousand years ago, during the peak of the Wisconsin Glaciation, northern Kentucky was probably free of ice. However, the climate was cold, and the likely vegetation was taiga, which is relatively open conifer forest. How do you think today's soils of northern Kentucky would compare with those of 18,000 years ago? Use information found in this chapter.

ENRICHMENT ACTIVITIES

Study the history of the soils in your state or vicinity.

1. Dig a soil pit and study the soil profile. See if you can name the layers.

2. Obtain samples of common soil-forming rocks and minerals. Find more information about each from a simple field guide to rocks and minerals. What plant nutrients does each contain (see chapter 10 for a list)? Using one of the several available laboratory exercises, experiment with the various weathering processes. For instance, try to scratch feldspar with quartz, and vice versa. Which is harder?

3. To observe the effects of freezing on physical weathering, pat a handful of clay soil into a ball. Inject water into the ball with a syringe, then freeze overnight. Observe the results.

4. This Web site from Alberta discusses soil-forming processes: <http://www.rr.ualberta.ca/Courses_Teaching/SOA/process. html>. It has one additional factor not included here. Which one is it and how would you fit it into this text's five factors?

5. This chapter skimmed over the complex deposits left behind by glaciers that became parent materials for many states. Type the search phrase "glacial landforms" into your favorite Internet search engine and find out more.
FIGURE 2-5 Composition of several igneous and sedimentary rocks
of Minnesota, according to the Minnesota Geological Survey.
Dashes mean only trace amounts, and the starred number was
estimated by the author.

                          Gray
MINERALS                Granite         Basalt

% quartz                   64             49
% feldspars, others        20             20
% calcite, dolomite        7              15

ELEMENTS              Pounds per Ton of Rock

Calcium                    69            150
Potassium                  66             17
Magnesium                  36             66
Iron                       23             35
Phosphorus                 5              5
Manganese                  --             3

                        Hinckley     Platteville
MINERALS               Sandstone      Limestone

% quartz                   94            7.5
% feldspars, others        2              --
% calcite, dolomite        5              90

ELEMENTS              Pounds per Ton of Rock

Calcium                    --            704
Potassium                  --             --
Magnesium                  --             18
Iron                      15 *
Phosphorus                 --             --
Manganese                  --             --

FIGURE 2-6 Relationships of some common sedimentary,
igneous, and metamorphic rock.

Rocks

Sedimentary   Igneous   Metamorphic
Sandstone               Quartzite
Limestone               Marble
Shale                   Slate
              Granite   Gneiss
              Basalt    Schist

Sedimentary   Igneous   Main Components
Sandstone               Quartz sand
Limestone               Calcite
Shale                   Feldspar clays
              Granite   Quartz, mica, feldspar
              Basalt    Feldspar, mica, olivine
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Author:Plaster, Edward J.
Publication:Soil Science & Management
Date:Jan 1, 2003
Words:5974
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