Characteristics of Soil (College Board AP® Environmental Science): Study Guide

Soil formation

• Soil formation is the process by which rocks break down and organic material accumulates to create soil over time

• It is influenced by a combination of biological, physical, and chemical processes

Factors affecting soil formation

Parent material

• The underlying rock from which soil develops

• Determines the mineral composition and texture of the soil

Climate

• Temperature and precipitation influence the rate of weathering and organic decomposition

• Warm, wet climates promote faster soil formation, while cold, dry climates slow it down

Organisms

• Plants, animals, and microorganisms contribute organic material and aid in decomposition

• Earthworms and microbes help mix and aerate the soil, improving fertility

Topography

• Factors like slope influence drainage and erosion

• Steep slopes tend to have thin soils due to erosion, while valleys accumulate deeper soils

Time

• Soil formation is a slow process, taking hundreds to thousands of years to develop significant layers

• Older soils are typically more developed with distinct layers, while young soils are less complex

Soil characteristics

Soil texture

• Determined by the proportion of sand, silt, and clay in the soil

• Affects water retention, drainage, and nutrient availability

Soil structure

• Refers to how soil particles are arranged into aggregates

• Influences aeration, root penetration, and water movement

Soil porosity and permeability

• Porosity: the amount of space between soil particles, affecting water and air movement

• Permeability: how easily water passes through soil

Soil color

• Can indicate organic matter content and mineral composition

• Darker soils tend to have higher organic matter, while red or yellow soils indicate iron oxides

Soil categorization & soil horizons

What are soil horizons?

• Soil horizons are distinct layers of soil that develop over time due to biological, chemical, and physical processes

• These layers differ in composition, texture, color, and organic material

• The vertical arrangement of horizons forms a soil profile, which helps categorize soil types

Major soil horizons

O horizon (organic layer)

• Also known as the litter layer

• Composed of decomposed plant and animal material (humus)

• Found mostly in forested areas with high biological activity

• Example: Thick in temperate forests, thin or absent in deserts

A horizon (topsoil)

• The most fertile soil layer, rich in minerals and organic material

• Supports plant growth due to its nutrient content

• Example: Agricultural soils depend on a healthy A horizon for crop production

E horizon (eluviation layer)

• Also known as the leaching layer

• A leached layer where minerals and nutrients are washed downward

• Light in color due to loss of clay, iron, and organic material

• Example: Common in heavily weathered soils found in forests

B horizon (subsoil)

• Accumulates minerals, clay, and nutrients leached from above layers

• Less organic material than the A horizon but still important for plant roots

• Example: Rich in iron and aluminum in tropical soils

C horizon (weathered parent material)

• Composed of weathered rock fragments from which the soil develops

• Least affected by soil-forming processes

• Example: Affects soil texture and mineral composition depending on rock type

R horizon (bedrock)

• Solid rock (parent material) beneath all soil layers

• Provides the foundation for soil formation over thousands of years

Importance of soil horizons in categorization

• Soil profiles help determine soil fertility, drainage, and land use suitability

• Different ecosystems and climates produce unique soil horizon arrangements

  • Example: Grassland soils have deep A horizons, while tropical soils have thin topsoil layers due to rapid decomposition

• Understanding soil horizons is important for activities like agriculture and construction

Soil erosion & protecting soils

What is soil erosion?

• Soil erosion is the process by which soil is removed from one location and transported to another by natural forces such as wind and water

• Erosion reduces soil fertility, depletes organic matter, and can lead to desertification

• Human activities like deforestation, overgrazing, and unsustainable agriculture accelerate erosion

Causes of soil erosion

Water erosion

• Rainfall and surface runoff: Heavy rains dislodge soil particles and carry them away

• Rivers and streams: Flowing water erodes riverbanks and transports sediments downstream

• Example: The Mississippi River carries vast amounts of sediment due to upstream erosion

Wind erosion

• Strong winds remove loose soil, especially in dry and exposed areas

• Example: The Dust Bowl of the 1930s resulted from poor land management and severe drought in the Great Plains region of the US

Protecting soils from erosion

Strip cultivation

• Planting crops in alternating strips or bands, leaving natural vegetation between the strips

• Reduces soil erosion by trapping water, slowing down runoff and increasing infiltration while still allowing for crop production in the cultivated strips

Terracing

• Creates step-like structures on steep slopes to reduce water runoff and retain soil

• Example: Terraced farming in Southeast Asia minimizes soil erosion on mountainsides

Contour Plowing

• Plowing along the natural contours of the land (instead of directly up and down slopes) slows water runoff and reduces soil loss

• Minimises soil erosion by reducing length and speed of water flow downhill

Conservation tillage

• Crop residues are left on fields to protect the surface

• Helps prevent wind and water erosion while maintaining soil moisture

Cover crops

• Plants such as clover or rye are grown between harvests to hold soil in place

• Roots anchor the soil and prevent erosion during off-seasons

Windbreaks

• Rows of trees or shrubs planted along field edges to reduce wind speed and protect soil

• Example: Farmers in the Great Plains use windbreaks to combat soil loss

Soil protection and water quality

• Healthy soils act as natural filters

  • This means they can remove pollutants and sediments from water before it reaches groundwater or surface water sources

  • Example: Wetlands and forested soils naturally filter excess nutrients, improving water quality in rivers and lakes

• This means that protecting soils is an effective way of protecting water quality