AY Honors/Soils/Answer Key

Soils

Skill Level

1

Year

2006

Version

22.11.2024

Approval authority

General Conference

1

Define what "soil" means?

Soil is the naturally occurring, unconsolidated or loose covering of broken rock particles and decaying organic matter (humus) on the surface of the Earth, capable of supporting life. In simple terms, soil has three components: solid, liquid, and gas. The solid phase is a mixture of mineral and organic matter. Soil particles pack loosely, forming a soil structure filled with voids. The solid phase occupies about half of the soil volume. The remaining void space contains water (liquid) and air (gas). Soil is also known as earth: it is the substance from which our planet takes its name.

2

Where on earth is all soil located?

Soil on the Earth is located only on its surface to a depth of about 1.5 meters.

3

What are 5 key factors in soil formation?

Climate

Climate is the average and variations of weather in a region over long periods of time. It affects soil formation in that it provides the forces (water, wind, etc.) necessary to erode rock into soil.

Organisms

Organisms are living creatures (including plants, animals, fungi, etc.). Organisms are active agents of soil formation, providing organic soil components or converting it into soil.

Terrain

Terrain is used as a general term in physical geography, referring to the lie of the land. This is usually expressed in terms of the elevation, slope, and orientation of terrain features. Terrain affects surface water flow and distribution.

Parent material

Parent material means the underlying geological material (generally bedrock or a superficial or drift deposit) in which soil horizons form. Soils typically get a great deal of structure and minerals from their parent material. Parent materials are made up of consolidated or unconsolidated mineral material that has undergone some degree of physical or chemical weathering.

Time

All of the forces that create soil require time in which to operate. When leaves fall from trees, they are not instantly converted into soil. Similarly, erosion is a process that occurs over time.

4

Define the following terms:

Soil Horizons

4a

A-Horizon

The A Horizon is the top layer of the soil horizon. The technical definition of an A Horizon may vary, but it is most commonly described in terms relative to deeper layers. A Horizons may be darker in color than deeper layers and contain more organic material, or they may be lighter but contain less clay or sesquioxides. The A is a surface horizon, and as such is also known as the zone in which most biological activity occurs. Soil organisms such as worms, nematodes, fungi, and many species of bacteria is concentrated here, often in close association with plant roots. Thus the A-horizon may be referred to as the "biomantle". However, since biological activity extends far deeper into the soil, it cannot be used as a chief distinguishing feature of an A Horizon.

4b

B-Horizon

B Horizons are commonly referred to as ‘subsoil’, and consist of mineral layers which may contain concentrations of clay or minerals such as iron or aluminium, or organic material. In addition, they are defined by having a distinctly different structure or consistence to the A horizon above and the horizons below. They may also have ‘stronger’ colours (i.e. higher chroma) than the A horizon.

4c

C-Horizon

C Horizons are simply named so because they come ‘after’ A and B within the soil profile. These layers are little affected by soil forming processes, and their lack of pedological development is one of their defining attributes. C Horizons may contain lumps of unweathered rock, rather than being comprised solely of small fragments as in the solum. ‘Ghost’ rock structure may be present within these horizons.

4d

Organic layer

The upper soil horizons containing mostly organic matter.

4e

Mineral layer

The lower soil horizons containing little organic matter.

4f

Clay

Soil composed primarily of fine-grained minerals, which show plasticity through a variable range of water content, and which can be hardened when dried and/or fired.

4g

Leaching

Leaching is the loss of mineral and organic solutes due to percolation. It is a mechanism of soil formation.

4h

Silica

Silica is the chemical compound silicon dioxide, SiO₂. It is most commonly found in nature as sand or quartz.

4i

Humus

Humus refers to any organic matter which has reached a point of stability, where it will break down no further and might, if conditions do not change, remain essentially as it is for centuries, if not millennia

4j

Soil Profile

The different soil horizons stacked one atop the other makes up the soil profile.

4k

Parent Material

The rock from which mineral soil components was formed.

4l

Pedologist

A pedologist is a scientist who studies soil in its natural environment. Pedology (from Greek: πέδον, pedon, "soil"; and λόγος, logos, "study") is the study of soils in their natural environment. It is one of two main branches of soil science, the other being edaphology which is concerned with influence of soils on living things, particularly plants.

5

Define the term "Soil Classification". Why are soils classified?

Soil classification deals with the systematic categorization of soils based on distinguishing characteristics as well as criteria that dictate choices in use.

Soil classification is a dynamic subject, from the structure of the system itself, to the definitions of classes, and finally in the application in the field. Soil classification can be approached from the perspective of soil as a material and soil as a resource.

For soil resources, experience has shown that a natural system approach to classification, i.e. grouping soils by their intrinsic property (soil morphology), behavior, or genesis, results in classes that can be interpreted for many diverse uses.

Criteria are designed to guide choices in land use and soil management.

6

Discuss three differences between the following soil types:

6a

Desert

Aridisols (or desert soils) are a soil order in USA soil taxonomy. Aridisols (from the Latin aridus, for “dry”) form in an arid or semi-arid climate. Aridisols dominate the deserts and xeric shrublands which occupy about one third of the Earth's land surface. Aridisols have a very low concentration of organic matter. Water deficiency is the major defining characteristic of Aridisols. Also required is sufficient age to exhibit sub-soil weathering and development. Imperfect leaching in Aridisols often results in one or more subsurface soil horizons in which suspended or dissolved minerals have been deposited: silicate clays, sodium, calcium carbonate, gypsum or soluble salts. These subsoil horizons can also be cemented by carbonates, gypsum or silica. Accumulation of salts on the surface can result in salinization.

6b

Temperate

Brown earths are mostly located between 30° and 55° north of the Equator. The largest expanses cover western and central Europe, large areas of western and trans-Uralian Russia, the east coast of America and eastern Asia. Here, areas of brown earth soil types are found particularly in Japan, the Koreas, China, eastern Australia and New Zealand.

They generally have three horizons: the A, B and C horizon. Horizon A is usually a brownish colour, and over 20 cm in depth. It is composed of mull humus (well decomposed alkaline organic matter) and mineral matter. It is biologically active with many soil organisms and plant roots mixing the mull humus with mineral particles. As a result, the boundary between the A and B horizons can be ill defined in unploughed examples. Horizon B is mostly composed of mineral matter which has been weathered from the parent material, but it often contains inclusions of more organic material carried in by organisms, especially earthworms. It is lighter in colour than the A horizon, and is often weakly illuviated (leached). Due to limited leaching only the more soluble bases are moved down through the profile. Horizon C is made up of the parent material, which is generally permeable and non- or slightly acidic, for example clay loam.

6c

Tropical

Oxisols are an order in USDA soil taxonomy, best known for their occurrence in tropical rain forest, 15-25 degrees north and south of the Equator. Some oxisols have been previously classified as laterite soils.

The main processes of soil formation of oxisols are weathering, humification and pedoturbation due to animals. These processes produce the characteristic soil profile. They are defined as soils containing at all depths no more than 10 percent weatherable minerals, and low cation exchange capacity. Oxisols are always a red or yellowish color, due to the high concentration of iron(III) and aluminium oxides and hydroxides. In addition they also contain quartz and kaolin, plus small amounts of other clay minerals and organic matter.

The word "oxisol" comes from "oxide" in reference to the dominance of oxide minerals such as bauxite. In the FAO soil classification, oxisols are known as ferralsols.

Scientists originally thought that the heavy vegetation of tropical rain forests would provide rich nutrients, but as rainfall passes through the litter on the forest floor the rain is acidified and leaches minerals from the above soil layers. This forces plants to get their nutrition from decaying litter as oxisols are quite infertile due to the lack of organic matter and the almost complete absence of soluble minerals leached by the wet and humid climate.

7

Examine a 2-foot vertical section of soil. Label the different types of organic matter found, identify the different soil horizons, and mark the transition from the soil layer to the mineral layer.

Find a place where you have permission to dig a 2-foot deep hole. In the United States, call before you dig. Digger's Hotline, One-call or Miss Utility are services that allow construction workers to contact utility companies, who will then denote where underground utilities are located via color-coding those locations. As required by law and assigned by the FCC, the 8-1-1 telephone number will soon be used for this purpose across the United States.

Failure to call such a number ahead of time may result in a fine or even a charge against the person or company, particularly if such negligence causes a major utility outage or serious accident, or an evacuation due to a gas leak. Hitting a water main may also trigger a boil-water advisory and local flooding.

A few utilities are permanently marked with short posts or bollards, mainly for lines carrying petroleum products.

Once you have clearance, get your shovel and start the hole. If you are lucky, perhaps someone you know is already digging a hole, and if so, ask permission to come in and make your observations. People dig holes for all kinds of reasons: footers for building a deck, trenches for laying pipe or cable, foundation for a new building, or to add to an existing one. But if no one you know is digging a hole, you may have to get out the shovel yourself.

You can label the required features with craft sticks and a pen. Simply write a label on the craft stick with the pen, and insert it into the soil near the feature you wish to label. Alternatively, you could take a photograph of the vertical section and then import it into a word processor. Then add the labels to the photo digitally.

8

Draw, photograph, or collect and correctly label 5 different soil types.

Listed below are a few types of soil you might collect. For a more complete listing of soil types, see the Types of soil category in Wikipedia.

Sandy soil

Sandy soil consists of relatively large particles (sand). It has a gritty texture. Sandy soil, if mixed well with organic matter, is a good medium for growing plants as long as it is kept moist. Because the particles are so large, water and air easily penetrate the soil and reach the roots of plants. However, this same quality allows the water to drain away almost immediately (which is why it must be kept moist if used for agriculture).

Silty soil

Silty soil is like sandy soil except that it contains finer particles and more nutrients. It has a darker color as well. Silty soil is one of the best soils for agriculture because like sandy soil, it allows water and air to penetrate to the roots, but unlike sandy soil, it hold the water longer.

Clay soil

Clay soil is made of very fine particles. While it is replete with minerals, plants may have a difficult time extracting them because clay is so impermeable. Wet clay will clump together when wadded into a ball in the palm of the hand.

Loamy Soil

Loamy soil is a mixture of sand, silt, and some clay. It is considered the ideal soil for agriculture, as it has good drainage and plenty of plant nutrients.

Peaty Soil

Peat is an accumulation of partially decayed vegetation matter. Peat forms in wetlands or peatlands, variously called bogs, moors, muskegs, pocosins, mires, and peat swamp forests.

Peat forms when plant material, usually in marshy areas, is inhibited from decaying fully by acidic and anaerobic conditions. It is composed mainly of marshland vegetation: trees, grasses, fungi, as well as other types of organic remains, such as insects, and animal corpses.

Peat is soft and easily compressed. Under pressure, water in the peat is forced out. Upon drying, peat can be used as a fuel. Peat is also dug into soil to increase the soil's capacity to retain moisture and add nutrients. This makes it important agriculturally, for farmers and gardeners.

Chalky Soil

Chalky soil is highly alkaline. It is composed primarily of the mineral calcite. Because chalk is porous it can hold a large volume of ground water, providing a natural reservoir that releases water slowly through dry seasons.

References

• http://soils.usda.gov/
• http://www.fao.org/WAICENT/FAOINFO/AGRICULT/AGL/agll/wrb/newkey.stm#DURISOLS
• Bloom, Arthur L. (1978). Geomorphology: A Systematic Approach of Late Cenozoic Landforms. Prentice-Hall, Inc., Englewood Cliffs, New Jersey. ISBN 0-13-353086-8
• Press, Frank and Raymond Siever. (1978). Earth.W.H. Freeman & Company. San Francisco. ISBN 0-7167-0289-4.