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)

• Also known as the zone of leaching

• The most fertile soil layer — organic material and minerals are well mixed, giving it a typically dark colour

• Soluble minerals and clay particles are leached downwards from this layer by water moving through the soil

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

B horizon (subsoil)

• Also known as the zone of accumulation

• Accumulates minerals, clay, and soluble compounds (such as iron oxides and calcium carbonate) that have been leached down from the A horizon

• Contains little organic material, but is often rich in nutrients and important for deeper plant roots

• Example: rich in iron and aluminium oxides in tropical soils, giving them a distinctive red colour

C horizon (weathered parent material)

• Composed of weathered rock fragments from which the soil above develops

• Least affected by soil-forming processes — few roots and little organic matter

• Example: the type of rock here (e.g. granite, limestone, sandstone) influences the texture and mineral composition of the soil above

[INSERT IMAGE — new four-horizon soil profile diagram (O / A / B / C)
Image caption: Generalised soil profile showing the O, A, B, and C horizons. Organic matter is concentrated in the O and A horizons, making them the critical layers for vegetation growth.

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

Protection from soil erosion

• A range of land-management practices can reduce soil erosion and protect long-term soil productivity, including:

  • Strip cropping — alternating strips of different crops to keep the soil covered year-round

  • Terracing — step-like platforms cut into steep slopes to slow runoff

  • Contour plowing — plowing along the contours of slopes rather than up and down

  • No-till and conservation tillage — leaving crop residue on fields and minimising soil disturbance

  • Cover crops — plants such as clover or rye grown between cash-crop harvests to anchor the soil

  • Windbreaks — rows of trees or shrubs along field edges to reduce wind speed

• For full coverage of these conservation methods, with examples and diagrams, see Unit 5: Intro to Sustainable Agriculture — Soil conservation methods

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