12.2 Soil Key Ideas: Soil is made of weathered rock and organic material. Climate and other factors affect the composition of the soil.

What Is Soil? Soils is uppermost layer at the surface of the earth. It consists of loose, weathered material, and a certain amount of organic matter. The soil is the support of life at the surface of the Earth. Terrestrial plants would not exist in the absence of soil.

How Soil Forms The first step in the formation of the soil is when the bedrock is weathered and turns into loose material. The soil whose parent material is the bedrock beneath the soil is called residual soil. In some areas the weathered material is brought from other areas by transporting agents, like glaciers, or river floods. Soils formed from transported materials are called transported soils.

1. The weathering of the bedrock is the first step in soil formation

How Soil Forms The loose material resulted from the weathering of the bedrock is enriched with organic material consisting of decaying plants and animals. The organic matter gives the soil fertility, which is the ability of soils to support plant life. The more organic matter the soil has, the more nutrients it contains, and the plants grow better.

Soil Profile To study soils, scientists analyze a cross section through the whole height of the soil, named soil profile. A mature soil has typically 3 main layers, called soil horizons, usually labeled as: - A Horizon (topsoil) - B Horizon (subsoil) - C Horizon (weathered material)

Soil Horizons

Soil Horizons

Soil Horizons Characteristics A Horizon (topsoil) is usually gray or black in color, because it contains decomposed organic matter, named humus. The humus gives off nutrients necessary for plants growth. B Horizon (subsoil) is red or brown from iron oxides and clay that have been washed down from the topsoil. C-Horizon is made of weathered parent material, such as rock fragments. The bedrock lies right beneath the C horizon.

Soil Composition The soil has mainly two parts: a) Rock material, consisting of: -sand -silt -clay b) Organic matter, which comes from decomposed plants and animals.

Soil Composition The amounts of rock fragments and organic matter, as well as the chemicals contained in each type of soil, vary greatly. There is a science that studies nothing but soils, called Pedology. Based on their components and chemistry, the scientists identified several thousands of types of soils.

Factors Affecting Soil Formation The soil formation depends greatly on the combination of natural conditions in a certain area. The most important factors determining the type of soil are; -Time -Topography (flat versus sloped terrain) - Climate (temperature, rainfall, etc) - Plants and animals, etc.

Factors Affecting Soil Formation Time: It takes over 200 years to form an inch of soil rich in organic matter. The soil formed on a slope is poor in organic matter, because it is washed off by the rain water flowing downhill, while the soils formed on flat ground are rich in nutrients. A humid climate with lots of rainfall has poor soils, because the rain carries away the nutrients.