Formation of Soil Review

*This is a great review for tomorrow's quiz on soil formation. Please know that some of this information is "enriched" material and will not be on the quiz. 

Student Expectation

The student is expected to explore and record how soils are formed by weathering of rock and the decomposition of plant and animal remains.

Key Concepts

• Weathering is the process that breaks down Earth’s rocks into smaller and smaller pieces over time.
• Soils are made up of small pieces of weathered rock.
• Soil contains many substances including decomposed plant and animal remains.
• The materials in soil, or soil type, are different in different areas.

Fundamental Questions

• What is weathering?
• What is soil made of?
• How are soils different in different areas?

Key Concept 1: Weathering is the process that breaks down Earth’s rocks into smaller and smaller pieces over time.

All soils initially come from rocks, called “parent material”. Weathering agents, such as water, plant roots, wind, ice, temperature changes, plant and animal activity, and chemical processes, break the parent material into smaller pieces, and erosion carries the sediments to new locations. Water seeps into the cracks in rocks and breaks them apart. Temperature changes expand and crack the rocks further. Acidic water also seeps into rocks and breaks them down even more. Below the surface growing plant roots can split rocks, too. On the surface, blowing wind and rushing water constantly weather and erode rock into smaller pieces.

Key Concept 2: Soils are made up of small pieces of weathered rock.

Soil is made of four materials: small pieces of weathered rock or minerals (rock, sand, clay, and silt), air, water, and organic material (matter from dead plants and animals). There are thousands of soil types, but all soils are some combination of sand, silt, and clay particles. Of the particles, sand is the largest, silt is the middle-sized particle, and clay is the smallest. The texture of these three particles can be felt by rubbing the soil between the fingers. Sandy soil feels gritty, silt soil feels floury or silky; and clay soil feels slick and sticky. There are four major types of soil.
Sandy soil is made of quartz grains that appear light brown. When you roll slightly wet, sandy soil in your palm, usually no ball forms and it crumbles. Sand particles are the largest of the three rock particles and create large spaces between the grains. This causes water to flow too quickly through the soil, washing away important nutrients. Sandy soil, without humus, cannot retain water or nutrients essential to seedlings, which is not good for growing crops. Sandy soils are found in deserts and near coastal areas.
Silt soil is also made of quartz minerals, but is grey in color, with grains that are much smaller than sand, and feel silky smooth. Small spaces between sediments allow water and nutrient retention. Silt soils that have some humus are found in the flood plains surrounding rivers and are considered good for farming.
Clay soil particles appear red, are much smaller than silt, are sticky when wet, and have very little pore space to let water flow, but do retain plant nutrients. When wet clay soil dries, it shrinks and cracks. Clay can also become dense, hard, and brittle, making it difficult for plant roots to penetrate. Wet clay soil easily forms a sticky ball in the hand.
Loamy soil is an ideal soil that appears dark brown or black from rich organic materials and has a balance of all three particle sizes: sand, clay, and silt. Loamy soil, also known as topsoil is preferred for farming.

Key Concept 3: Soil contains many substances, including decomposed plant and animal remains.

Soil is actually made of four materials: small pieces of weathered rock or minerals (rock, sand, clay, and silt), air, water, and organic material (matter from dead plants and animals). When plants and animals die and decompose (decay), this organic matter makes nutrient-rich material called humus, which mixes with inorganic material (rock particles, minerals, and water) to form soil. The humus layer of soil is usually the dark rich top soil. Soil micro-organisms (e.g., bacteria and fungi) are responsible for this decomposition, which breaks down animal and vegetable material into nutrient elements that can be used by growing plants.

Key Concept 4: The materials in soil (or soil type) are different in different areas.

Landforms, climate, rainfall, plants, and animal activity affect the soil found in a particular regions of the United States. Forest soil is rich in topsoil and humus where decayed plant and animal material help retain water and nutrients that allow forest trees, other plant life, and animals to flourish.Desert soil, however, lacks topsoil and humus due to sandy particles that do not retain water or nutrients. Only specially adapted plants, such as cactus and sagebrush, are suited to this sandy soil.Prairie soil of the Midwestern United States is fertile, dark, loamy soils that are a balance between the three particle sizes. A variety of weathered materials eroded into this basin represent the diverse parent material that contributed to the rich soil that supports American farmlands. Southern soil is dominated by red clay. The red color results from the iron minerals that have rusted into iron oxides in the warm, damp climate of the South.