Soil 3:Weathering, Soil, and Mass Movements

Pedalfer

This soil type is present in much of the eastern half of the United States, most often in forested areas. The B horizon in pedalfers contains large amounts of iron oxide and aluminum-rich clays, giving it a brown to red-brown color.

Pedocals

are found in the drier western United States in areas that have grasses and brush vegetation.

Organisms 2

Plants are the main source of organic matter in soil. Animals and microorganisms also contribute. Microorganisms, including fungi, bacteria, and single-celled protozoans, play an active role in decomposing dead plants and animals. Because organic matter releases nutrients when it decays, it contributes to soil fertility.

Laterite 3

Plants that die in a tropical rain forest decompose rapidly because the bacterial activity is high in hot and wet climates. As a result, laterite contains almost no organic matter. The roots of living rain forest plants quickly absorb the nutrients released during decomposition. So, even though the vegetation may be dense, the soil itself contains few available nutrients. Most of the nutrients in a tropical rain forest are present in the plants themselves. Today, large areas of tropical rain forests are being cleared for timber and to provide land for agriculture.

How does the U.S. Department of Agriculture classify soil texture?

based on the percentages of clay, silt, and sand

Laterites

form in hot, wet tropical areas.

Humus is an important part of soil. What is humus?

the decayed remains of organisms

regolith

the layer of rock and mineral fragments that nearly everywhere covers Earth's surface

A soil profile shows how soil varies between _____.

the surface and the parent material beneath it

Pedalfers

usually form in temperate areas that receive more than 63 cm of rain each year.

In good-quality surface soil, approximately what percentage of the volume is made of mineral and organic matter?

50 percent

The Soil Profile 2

A vertical section through all of the soil horizons is called a soil profile. In others the soil horizons are quite distinct. Mature soils usually have three distinct soil horizons. From the surface downward, these horizons are called the A, B, and C horizons.

How Water Erodes Soil 2

After flowing as a thin sheet for a short distance, the water forms tiny streams called rills. As more water enters the rills, they erode the soil further, creating trenches known as gullies. Although most dislodged soil particles do not move far during each rainfall, large quantities eventually make their way downslope to a stream. The stream transports these soil particles, which are now called sediment, and eventually deposits them.

Controlling Erosion

Although we cannot completely eliminate soil erosion, we can significantly slow it by using soil conservation measures. In the past, a misunderstanding of the composition of rain forest soil has led to the destruction of millions of acres, leaving only severely leached, unproductive land. Conservation measures include steps taken to preserve environments and protect the land. These measures include planting rows of trees called windbreaks, terracing hillsides, plowing along the contours of hills, and rotating crops. Preserving fertile soil is essential to feeding the world's rapidly growing population.

The Nitrogen Cycle 1

Ammonia provides nitrogen in a form plants can use. These nitrogen-fixing bacteria live both in soil and in the roots of legumes. (Legumes are plants such as peanuts, beans, and clover that have nitrogen-fixing bacteria in nodules on their roots.

Sediment Deposition 1

Another problem caused by excessive soil erosion is the deposition of sediment. Rivers that accumulate sediment must be dredged to remain open for shipping. As sediment settles in reservoirs, they become less useful for storing water, controlling floods, and generating electricity.

The Nitrogen Cycle 3

Another type of soil bacteria, called denitrifying bacteria, continues the cycle. These bacteria break down nitrates and release nitrogen gas into the atmosphere.

Pedocal

Because chemical weathering is slower in dry climates, pedocals generally contain less clay than pedalfers. Pedocals contain abundant calcite, or calcium carbonate, and are typically a light gray-brown.

C Horizon

Between the B horizon and the unaltered parent material is the C horizon, which contains partially weathered parent material. While the A and B horizons barely resemble the parent material, the C horizon does.

Climate

Climate has the greatest effect on soil formation. Variations in temperature and precipitation influence the rate, depth, and type of weathering. For example, a hot, wet climate may produce a thick layer of chemically weathered soil. In the same amount of time, a cold, dry climate might produce only a thin layer of mechanically weathered debris. The amount of precipitation also influences soil fertility by affecting the rate at which nutrients are removed from the soil.

Soil Types

Climate is the most important factor in soil formation. Climate also has a major effect on the type of soil that forms. Three common types of soil are pedalfer, pedocal, and laterite.

Soil Texture 2

For example, point A, near the left-center part of the diagram, represents a soil composed of 40 percent clay, 10 percent silt, and 50 percent sand. Such a soil is called a sandy clay. In soils called loam, which occupy the central part of the diagram, neither clay, silt, nor sand is dominant.

Laterite 4

However, laterite is one of the poorest soils for agriculture. Because laterite contains little organic matter and few nutrients, it cannot nourish crops. The nutrients it does have are soon washed out by the plentiful rainwater that filters through the soil. In only a few years, the soil in a freshly cleared area may be completely useless for growing crops. Without trees or crop plants to anchor the soil and shield the ground from the full force of heavy rains, the soil erodes quickly.

How do human activities affect rates of erosion?

Human activities, such as farming, logging, and construction, have greatly accelerated erosion as the natural vegetation that protects topsoil is removed from areas where these activities are performed.

Slope 2

In contrast, flat areas have little erosion and poor drainage. The waterlogged soils that form in flat areas are typically thick and dark. The dark color results from large amounts of organic matter.

Rates of Erosion 1

In the past, soil eroded more slowly than it does today because more land was covered by trees, grasses, and other plants. However, human activities, such as farming, logging, and construction, that remove natural vegetation have greatly accelerated erosion. Without plants, soil is more easily carried away by wind and water.

How does the climate in which pedalfers are found differ from of the climate in which pedocals are found?

Pedalfers are found in temperate climates, and pedocals are found in dry climates.

Why are laterites considered deep soils?

Laterites form in warm, wet climates, so chemical weathering is extensive.

The Nitrogen Cycle 2

Most ammonia is consumed by other bacteria, called nitrifying bacteria. Nitrifying bacteria produce compounds called nitrites and nitrates, which are made up of nitrogen and oxygen. Nitrate is the most common source of nitrogen for plants.

Soil Texture 1

Most soils contain particles of different sizes. Soil texture refers to the proportions of different particle sizes. To classify soil texture, the U.S. Department of Agriculture has established categories based on the percentages of clay, silt, and sand in soil. The diagram in the figure shows how the percentages differ for each category.

What is the difference between residual soil and transported soil?

Residual soil forms on bedrock. Transported soil forms on unconsolidated deposits and is carried into a location.

Rates of Erosion 2

Scientists can estimate the rate of erosion due to water by measuring the amount of sediment in rivers. These estimates indicate that before humans appeared, rivers carried about 9 trillion kg of sediment to the oceans each year. In contrast, the amount of sediment currently transported to the sea by rivers is about 24 trillion kg per year.

Laterite 1

So laterites are usually deeper than soils that develop over a similar period in temperate areas. The large quantity of water that filters through these soils removes most of the calcite and silica. Iron oxide and aluminum oxide are left behind. The iron oxide gives laterite a distinctive orange or red color.

How Water Erodes Soil 1

Soil erosion is a natural part of the constant recycling of Earth materials known as the rock cycle. Water, wind, and other agents move soil from one place to another .Every time it rains, raindrops strike the soil surface with surprising force. Each drop acts like a tiny bomb, blasting soil particles off the surface. Water flowing across the surface then carries away the dislodged particles. Because thin sheets of water move the soil particles, this process is called sheet erosion.

Soil Formation

Soil forms through the complex interaction of several factors. The most important factors in soil formation are parent material, time, climate, organisms, and slope. Although these factors all interact, we'll examine them separately.

Soil Structure 1

Soil particles usually form clumps that give soils a particular structure. Soil structure determines how easily a soil can be cultivated and how susceptible it is to erosion. Soil structure also affects the ease with which water can penetrate the soil. This, in turn, influences the movement of nutrients to plant roots.

Soil Erosion 1

Soils are just a tiny fraction of all Earth materials, yet they are a vital resource. Because soils are necessary for the growth of rooted plants, they are the foundation of the human life-support system. However, soils are among our most abused resources. The loss of fertile topsoil is a growing problem as human activities disturb more of Earth's surface.

Organisms 3

Some bacteria also aid soil fertility. In the nitrogen cycle, these bacteria convert nitrogen gas into nitrogen compounds that plants can use. Organisms require nitrogen in order to make amino acids, the building blocks of proteins. Nitrogen gas is common in the atmosphere. But most living things cannot use nitrogen gas in their cells.

Sediment Deposition 2

Some sediments are contaminated with agricultural pesticides. When these chemicals enter a river or lake, they endanger organisms that live in or use the water, including humans. Sediments also contain soil nutrients, which may come from natural processes and from added fertilizers. Excessive nutrient levels in lakes stimulate the growth of algae and plants. This can accelerate a process that eventually leads to the early death of the lake.

Soil Texture 3

Texture strongly influences a soil's ability to support plant life. Sandy soils may drain and dry out too quickly, while clay-rich soils drain very slowly. Plant roots often have difficulty penetrating soils that contain a high percentage of clay and silt. Loam soils are usually best for plant growth. They retain water better and store more nutrients than do soils composed mainly of clay or sand.

A Horizon

The A horizon is commonly known as topsoil. Its upper part consists mostly of organic matter, including loose leaves and partly decomposed plant structures. It is teeming with insects, fungi, and microorganisms. The lower part of the A horizon is a mixture of mineral matter and organic matter.

B Horizon

The B horizon, or subsoil, contains fine clay particles washed out of the A horizon by water that filters through pore spaces. In some soils, the clay that accumulates in the B horizon forms a compact, impenetrable layer called hardpan. The B horizon is the lower limit of most plant roots and burrowing animals.

Slope 4

The direction a slope faces also affects soil formation. In the temperate zone of the Northern Hemisphere, south-facing slopes receive much more sunlight than do north-facing slopes. Consequently, soils on south-facing slopes are usually warmer and drier.

Slope 3

The direction a slope faces also affects soil formation. In the temperate zone of the Northern Hemisphere, south-facing slopes receive much more sunlight than do north-facing slopes. Consequently, soils on south-facing slopes are usually warmer and drier.

Parent Material 3

The longer a soil has been forming, the thicker it becomes. The parent material largely determines the characteristics of young soils. As weathering continues, however, the influence of the parent material can be overshadowed by the other factors, especially climate.

Time

The longer a soil has been forming, the thicker it becomes. The parent material largely determines the characteristics of young soils. As weathering continues, however, the influence of the parent material can be overshadowed by the other factors, especially climate.

What are the major components of soil?

The major components of soil are mineral matter, or broken-down rock; organic matter, or humus; air; and water.

Parent Material 2

The nature of the parent material influences soils in two ways. First, it affects the rate of weathering and the rate of soil formation. Because unconsolidated deposits are already partly weathered, they provide more surface area for chemical weathering. Therefore, transported soil usually develops more rapidly than residual soil develops. Second, the chemical makeup of the parent material affects the soil's fertility. Fertility influences the types of plants the soil can support.

Soil Composition 2

The percentage of organic matter in soil varies greatly. Certain bog soils are composed almost entirely of organic matter. Desert soils may contain only a tiny amount. In most soils, organic matter, or humus, is an essential component. It is an important source of plant nutrients and increases the soil's ability to retain water. Poor soils can be enriched with the addition of humus.

The Soil Profile 1

The processes that form soil operate from the surface downward. Soil varies in composition, texture, structure, and color at different depths. These variations divide the soil into zones known as soil horizons.

What are the factors that affect the rate of soil erosion?

The rate of soil erosion depends on soil characteristics and on factors such as climate, slope, and type of vegetation.

Rates of Erosion 4

The rate of soil erosion depends on soil characteristics and on factors such as climate, slope, and type of vegetation. In many regions, including about one-third of the world's croplands, soil is eroding faster than it is being formed. This results in lower productivity, poorer crop quality, and a threatened world food supply.

Slope 1

The slope of the land can vary greatly over short distances. Such variations can result in very different soil types. Many of the differences are related to the amount of erosion and the water content of the soil.

From the surface downward, how does soil vary with depth?

The soil becomes rockier, less fertile, and more like the parent material.

Soil Composition 1

The soil has four major components: mineral matter, or broken-down rock; organic matter, or humus, which is the decayed remains of organisms; water; and air. The proportions of these components vary in different soils.

Parent Material 1

The source of the mineral matter in soil is known as the parent material. Notice in the figure that parent material may be either bedrock or unconsolidated deposits, such as those in a river valley. The soil that forms on bedrock is called residual soil. The soil that forms on unconsolidated deposits is called transported soil. Its parent material was moved from another location by gravity, water, wind, or ice.

.Organisms 1

The types of organisms and how many there are in a soil have a major impact on its physical and chemical properties. In fact, scientists name some soils—such as prairie soil, forest soil, and tundra soil—based on the soils' natural vegetation.

Soil Composition 3

The water and air components of soil are also vital for plant growth. Soil water provides the moisture needed for chemical reactions that sustain life. Soil water provides nutrients in a form that plants can use. And air is the source of the carbon dioxide plants use to produce sugar during photosynthesis.

Characteristics

Weathering produces a layer of rock and mineral fragments called regolith, which covers nearly all of Earth's land surface. Soil is the part of the regolith that supports the growth of plants. Three important characteristics of soil are its composition, texture, and structure.

Laterite 2

When dried, laterite becomes very hard and practically waterproof. For centuries, people in portions of South and Southeast Asia have made bricks by digging up laterite, shaping it, and allowing it to harden in the sun. Ancient structures built of laterite bricks, such as the one shown in the figure, are well preserved even today.

Rates of Erosion 3

Wind generally erodes soil much more slowly than water does. During a prolonged drought, however, strong winds can remove large quantities of soil from unprotected fields. That's exactly what happened during the 1930s in the part of the Great Plains that came to be known as the Dust Bowl.

Where would soil be expected to erode the fastest?

a hillside in a forest soon after the trees had been cut down

What factor accounts for most of the differences between three common types of soil: pedalfer, pedocal, and laterite?

climate

Along with parent material and time, the most important factors in soil formation are _____.

climate, organisms, and slope

Which properties describe pedocal, a type of soil common in the western United States?

dry, light gray-brown in color, and abundant in calcite

Where is the soil most likely to be very thin and not fertile?

on a steep slope where the climate is dry

Humans cause soil to erode more rapidly when they _____.

remove natural vegetation