AY Honors/Soils/Answer Key

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

a. 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.

b. 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 (ie higher chroma) than the A horizon.

c. 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.

d. Organic layer

The upper soil horizons containing mostly organic matter.

e. Mineral layer

The lower soil horizons containing little organic matter.

f. 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.

g. Leaching

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

h. Silica

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

i. 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

j. Soil Profile

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

k. Parent Material

The rock from which mineral soil components was formed.

l. Pedologist

A pedologist is a scientist who studies soil in its natural environment.

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.

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.