HOMEWORK• Text Book: Read pages 264-266. Do

questions 1-5 on page 266.• Review Book: Read pages 83-85. Do

questions 17-23 on page 85.• Write out questions and answers.

HOW DO SOILS FORM?

How Do Soils Form?• Soil is a mixture of weathered rock,

microorganisms, and organic remains that usually covers the bedrock.

• The composition of the soil depends on the rocks from which they weathered and the local climate.

How is soil created from rock?• Physical weathering breaks solid rock into small

particles.• Chemical weathering changes the minerals,

often increasing the clay content.• Plants and animals add organic materials in the

form of waste products and the remains of dead organisms.

• Therefore, soil is the result of mechanical and chemical weathering and biological activity over long periods of time.

How do we classify where a soil is formed?

• The material from which a soil is formed is called a its Parent Material.

• Based on the soil’s parent material, soil can be classified as either a residual or transported soil.

Soil Composition• The parent bedrock determines what

kinds of minerals a soil contains.• The parent rock and climatic conditions

of an area determine the length of time it takes for soil to form.

Describing the Soil Types• Because climatic conditions are the main

influence on soil development, soils are often classified based on the climates in which they form.

• The four major types of soil are polar, temperate, desert, and tropical.

Soil TypesPolar Soils– Polar soils form at high latitudes and high

elevations in places such as Greenland, Canada, and Antarctica.

– These soils have good drainage but no distinct horizons because they are very shallow, sometimes only a few centimeters deep.

Soil TypesTemperate Soils– Temperate soils vary greatly and are able to

support such diverse environments as forests, grasslands, and prairies.

– The specific amount of rainfall in an area determines the type of vegetation that will grow in temperate soils.

– Grasslands, which have an abundance of humus, are characterized by rich, fertile, soils.

Soil TypesDesert Soils– Deserts receive low levels of precipitation.– Desert soils often have a high level of

accumulated salts and can support only a limited amount of vegetation.

– Desert soils have little or no organic matter and a very thin A horizon, but they often have abundant nutrients.

Soil TypesTropical Soils– Tropical areas experience high temperatures

and heavy rainfall, leading to the development of intensely weathered and often infertile soil.

– The intense weathering combined with a high degree of bacterial activity leave tropical soils with very little humus and very few nutrients.

– These soils experience much leaching of soluble materials, such as calcite and silica, but they have high concentrations of iron and aluminum.

Soil Types

Soil Profiles• A soil profile is the vertical sequence of

soil layers. • A soil horizon is a distinct layer, or zone,

within a soil profile. • There are three major soil horizons: A, B,

and C.

Soil Profiles• A soil profile is a vertical cross section of

the soil that displays the horizons. • Soil horizons are layers within the profile

that result from soil forming processes. • The horizons can differ in texture,

structure, color, development, and parent material.

• The horizons are named based on their textural, color, and structural properties.

Soil Horizons– Horizon A contains high concentrations of

organic matter and humus. – Horizon B contains subsoils that are enriched

with clay minerals. – Horizon C, below horizon B and directly above

solid bedrock, contains weathered parent material.

Soil Horizons• In mature soils, three distinct zones or soil

horizons can be seen in the soil profile. • The O horizon has a very high

accumulation of organic matter above the mineral portion of the soil.

• The A horizon, called topsoil, has a high accumulation of organic matter and tends to be very dark in color because it usually contains Humus.

• Humus is organic matter that forms from decayed plants and animals.

Soil Horizons• The B horizon begins with the subsoil. The

subsoil is usually red or brown from the iron-oxides that formed in the A horizon and were washed down into the B horizon. It is a zone of clay accumulation.

• The C horizon is composed of weathered unconsolidated parent material.

Typical New York State

Section Assessment1. Match the following terms with their

definitions.___ residual soil___ transported soil___ soil profile___ soil horizon

A. the vertical sequence of soil layers

B. soil located above its parent material

C. a distinct layer, or zone, within the vertical sequence of soil layers

D. soil that has been moved to a location away from its parent bedrock

CADB

Section Assessment2. What differences would you expect to

find between soil profiles taken from a slope and a valley floor?

Soils on slopes tend to be thin, course, and infertile, whereas soils formed in valleys tend to be thick and fertile.