Weathering and

Soil Formation

Importance of Soil• Soil is one of Earth’s most precious

resources. It provides the support and nourishment that plants need in order to grow. In turn, plants provide food for animals and people.

How is sand different from soil?

• Sand is a type of soil and both are formed by the erosion of rocks.

• Soil is different from sand because it contains organic matter such as the remains of plants and animals.

• Plants don’t grow as easily in sand as they can in soil because it doesn’t have the organic material in like in the soil’s hummus.

Soil Formation• As well as rock fragments, SOIL

LAYERS contain air, water, and plant and animal remains.

Soil Formation• Soil is formed as rock is weathered

by ice, frost, wind, and water. • Plants take root among the rock

fragments and bind them together. • When plants die, they fertilize the

soil.

Types of soil• There are three main types of soil:

–Clay–Sandy–Loams

• There are also other types of soil, depending on the type of underlying rock and the climate and vegetation.

Loams• Loams are a mixture of clay, sand,

and silt, and are more fertile than other soils.

Clay soil

• Clay soils are usually sticky and waterlogged.

silt

• Chalky soils are thin and dry

Top Soil

• Acidic peaty soils are made up of rotting plants.

Living things in the soil• Living things play an important role in

helping to recycle nutrients (nourishing minerals) that enrich the soil.

• When plants and animals die, their remains are broken down by scavenging creatures, such as beetles, microscopic bacteria, and fungi. This releases minerals into the soil. The minerals fertilize plants so they can grow, and so the cycle of life begins again.

Soil has measurable properties, such as color, texture, pore space, and chemistry.

Plants, microorganisms, and animals affect soil characteristics.

Soil is a mixture of weathered rock, organic matter, water, and air.

Soil layers• Soil scientists divide the soil into

layers from the surface down to the underlying bedrock. This is called a soil profile. The layers in a soil profile are known as horizons. The depth of each horizon varies among different types of soil.

Soil layers• A layer of dark, fertile humus made of rotting

plants lies at the soil’s surface. • Underneath, the topsoil contains plant roots,

and plant and animal remains that bacteria and fungi are helping to rot down.

• The subsoil contains fewer plant and animal remains but has plenty of minerals washed down from the layers above.

• Below are rock fragments, then solid bedrock.

Soil layers / Soil Profile

Humus - The decayed organic matter in soil.

Bedrock - the solid unweathered rock that lies beneath the loose surface deposits of soil,

Soil Destruction• Soil is essential to life and takes a

long time to form, but it can be destroyed very quickly.

• It is difficult or impossible to replace soil that has been lost.

Human activities affect soil.Farming, construction development, and mining are a few human activities that affect soil.

Human activities affect soil.

Soil conservation practices help keep soil from blowing or washing away.

Factors that affect soil formation

• Climate• Rock Type (Parent Material)• Slope of Land• Vegetation Type• Time• Trails/Burrows

Climate• Precipitation, temperature, wind,

and sunlight are the major climatic factors affecting soil formation.

• Energy (sunlight) and precipitation (rain) strongly influence physical and chemical reactions on parent material. Climate also determines vegetation cover which in turn influences soil development.

Rock Type (Parent Material)• Parent material refers to the

composition (organic and mineral material) in which soil formation begins.

• The composition of the soil is determined by the parent material.

Rock Type (Parent Material)• Mineral matter includes partially

weathered rock, ash from volcanoes, sediments moved and deposited by wind and water, or ground up rock deposited by glaciers.

• Mechanical and chemical weathering acts on the solid and massive rocks of the earth’s crust to produce particles; sand, silt, and clay.

Slope of land• Topography is the hilliness, flatness,

or amount of slope (gradient) of the land.

• Soils vary with slope primarily because of the influence of moisture and erosion.

Slope of land• In many areas, moist, poorly drained

soils are located in low areas, and depressions of the land. In contrast, soils in sloping areas can be drier and well drained. These soils tend to be moderately and well developed.

• Erosion can remove all or part of the topsoil and subsoil, leaving a weakly developed soil.

Vegetation types• Plants affect soil development by supplying

upper layers with organic matter, recycling nutrients from lower to upper layers, and helping to prevent erosion.

• In general, deep rooted plants contribute more to soil development than shallow rooted plants because the passages they create allow greater water movement, which in turn aids in drainage.

Time• Generally, the longer soil

materials stay in one place, the deeper and more developed (mature) the soil becomes.

Time• The age of a soil must be considered in

thousands and even millions of years since it may take hundreds of years to form one inch of soil from parent material.

• The length of time that climate, plants and animals act on a given parent material with a specific slope determines the degree of development.

Trails• Recreational activities have the potential for

compacting soil. Hiking trails which concentrate foot traffic frequently create flat structure near the surface.

Burrows• As animals dig through the soil, they break it up,

permitting more air and water to enter. They mix the organic matter throughout the soil.