Soil forms slowly as a result of mechanical and chemical processes. Section 3: Soil Soil forms slowly as a result of mechanical and chemical processes. K What I Know W What I Want to Find Out L What I Learned

Essential Questions How does soil form? What are the different soil horizons in a soil profile? What factors affect soil formation? Soil Copyright © McGraw-Hill Education

Vocabulary Review New organism soil residual soil transported soil soil profile soil horizon Soil Copyright © McGraw-Hill Education

Soil Formation Soil is the loose covering of weathered rock particles and decaying organic matter, called humus, overlying the bedrock on Earth’s surface, and serves as a medium for the growth of plants. Copyright © McGraw-Hill Education Soil

Soil Formation Soil development The soil-development process often begins when weathering breaks solid bedrock into smaller pieces, which continue to weather and break down further. Copyright © McGraw-Hill Education Soil

Soil Formation Soil development Worms and other organisms help break down organic matter and add nutrients to the soil as well as create passages for air and water. Copyright © McGraw-Hill Education Soil

Soil Layers During the process of soil formation, layers develop in the soil. Most of the volume of soil is formed from the weathered products of a source rock, called the parent material. Soil Copyright © McGraw-Hill Education

Soil Layers A soil whose parent material is the local bedrock is called residual soil. Transported soil is soil that develops from parent material that has been moved far from its original location, usually by agents of erosion. Soil Copyright © McGraw-Hill Education

Soil Layers Soil profiles A soil profile is a vertical sequence of soil layers. New soils that have not yet developed distinct layers are called undeveloped soils. Mature soils are those that have distinct layers, sometimes after tens of thousands of years. Soil Copyright © McGraw-Hill Education

Soil Layers Soil profiles A distinct layer within a soil profile is called a soil horizon. There are typically four major soil horizons in mature soils: O, A, B, and C. Soil Copyright © McGraw-Hill Education

Soil Layers Soil profiles The O-horizon is the top layer of organic material and is composed of humus and leaf litter. The A-horizon is a layer of weathered rock combined with a rich concentration of dark brown organic material. The B-horizon, also known as the zone of accumulation, is a red or brown layer enriched over time by clay and minerals deposited by water flowing from above, or percolating upward from below. The C-horizon contains little or no organic matter and is often made of broken-down bedrock. Soil Copyright © McGraw-Hill Education

Factors of Soil Formation Five factors influence soil formation: climate, topography, parent material, biological activity, and time. These factors combine to produce different types of soil, called soil orders, that differ by region. The five factors of soil formation result in 12 different soil orders. Soil Copyright © McGraw-Hill Education

Factors of Soil Formation Climate Climate is the most significant factor controlling the development of soils because it also affects the weathering of rocks. Temperature, wind, and the amount of rainfall determine the type of soil that can develop. Soil Copyright © McGraw-Hill Education

Factors of Soil Formation Topography Topography, which includes the slope and orientation of the land, affects the type of soil that forms. In the northern hemisphere, slopes that face south receive more sunlight than other slopes, allowing more vegetation to grow. Soil Copyright © McGraw-Hill Education

Factors of Soil Formation Parent material If the soil is residual, it will have the same chemical composition as the local bedrock. If the soil is transported, the minerals in the soil are likely to be different from those in the local bedrock. Soil Copyright © McGraw-Hill Education

Factors of Soil Formation Biological activity Organisms including fungi and bacteria, as well as plants and animals, interact with soil. Different types of organisms in a soil can result in different soil orders. Soil Copyright © McGraw-Hill Education

Factors of Soil Formation Time The effects of time alone can determine the characteristics of a soil. After tens of thousands of years of weathering, most of the original minerals in a soil are changed or washed away. Soil Copyright © McGraw-Hill Education

Visualizing Soil Orders The five factors of soil formation determine how the soil orders are distributed across the United States. Soil profiles of three soil orders from different parts of the country are shown. Each soil profile has soil horizons expressed differently. Soil Copyright © McGraw-Hill Education

Add link to Animation from p. 181 here. Visualizing Soil Orders Concepts In Motion FPO Add link to Animation from p. 181 here. Soil Copyright © McGraw-Hill Education

Soil Texture The relative proportions of particle sizes determine a soil’s texture, which affects a soil’s capacity to retain moisture and therefore its ability to support plant growth. A soil textural triangle is used to determine a soil’s texture. Soil Copyright © McGraw-Hill Education

Soil Fertility Soil fertility is the measure of how well a soil can support the growth of plants. It is affected by topography, availability of minerals and nutrients, the number of microorganisms present, the amount of precipitation available, and the level of acidity. Soil Copyright © McGraw-Hill Education

Soil Color The minerals, organic matter, and moisture in each soil horizon determine its color. Scientists use the Munsell System of Color Notation to describe soil color in components of hue (color), value (lightness or darkness), and chroma (intensity). Soil Copyright © McGraw-Hill Education

Review Essential Questions Vocabulary How does soil form? What are the different soil horizons in a soil profile? What factors affect soil formation? Vocabulary soil residual soil transported soil soil profile soil horizon Soil Copyright © McGraw-Hill Education