The Nature of SoilChapter 7, Section 2

The Formation of Soil What is Soil? Mixture of weathered rock,

decayed organic matter, mineral fragments, water, and air.

Formation of Soil How does

soil form?

Weathering breaks down rocks and minerals into fragments.

Plants and animals add organic material to the soil. May take thousands of years to

form. May range in thickness from a few

centimeters to 60 meters thick.

Composition of Soil Rock and

Mineral Fragments

Mostly small particles of sediment such as clay, silt, and sand.

Some larger rocks can be present.

Composition of Soil Organic

Material

Dead and Decaying Plants Make up the majority of organic

material. Leaves, stems, roots, etc.

Dead and decaying animals and microorganisms

Composition of Soil Organic

Material

Humus Dark colored material formed from

decayed plants and animals. Decay process is due to bacteria and

fungi in the soil.

Serves as a nutrient source for plants. Mixed with other components of soil

by burrowing rodents, worms, and insects. Good-quality surface soil has about

50% humus.

Composition of Soil Water and

Air

Fill the spaces between the particles of soil.

Soil Profile Horizons Different layers of soil.

Typical soils have an A, B, and C horizon.

All the horizons form a soil profile.

Soil Profile A Horizon Top layer of soil.

Also known as topsoil. Contains a higher percentage of

humus. Generally dark and fertile. May be covered with litter.

Litter consists of leaves, twigs, and other organic material. Helps prevent erosion and holds water.

Soil Profile B Horizon Contains material moved down

from the A horizon through leaching. Leaching is the removal of minerals

that have been dissolved in water.

Soil Profile C Horizon Consists of partially weathered

rock. Coarser than other horizons. Contains less humus. Similar to the parent rock found

beneath the horizon.

Soil Types What affects

formation of soil and soil type?

Glaciation Climate Type of parent rock Type of vegetation Slope Time

Soil Types Glacial

Deposits

Unsorted mass of clay, silt, sand, and boulders.

Creates very fertile soil.

Soil Types Climate Desert soils

Contain little organic material. Thinner than soils in wet climates

Prairie soils Thick, dark A horizons due to the

large number of grasses. Temperate Forest soils

Thinner A and B horizons enriched in minerals due to leaching.

Soil Types Parent rock Affects the type of soil that

develops. Can also affect the vegetation that

grows in the area.

Soil Types Slope Steep slopes

Soil horizons are often poorly developed.

Bottomland Soils are often thick, dark, and full

of organic material.

Soil Types Time At first, soil characteristics are

determined by the characteristics of the parent rock.

Over time, soil resembles the parent rock less and less.