Chapter 9 Section 3
The Source of Soil
• Soil is a loose mixture of small mineral fragments, organic material, water, and air that can support the
growth of vegetation.
• The rock that breaks down to form a soil is called parent rock.
• Different parent rocks have different chemical compositions. Therefore, soils also differ in chemical
composition.
• Bedrock is the layer of rock beneath soil.
• In some cases, the soil remains above the bedrock that weathered to form the soil.
• Soil that remains above its parent rock material is called residual soil.
• In other cases, the bedrock below the soil is not the soil’s parent rock.
• Soil can be carried away from parent rock by wind, water, ice, or gravity.
• This soil is called transported soil.
Soil Properties
Soil Composition
• Soil composition describe the kinds and relative amounts of materials that the soil contains.
• Soil is made of different percentages of mineral fragments, organic material, water, and air.
• For example, black soil is often rich in organic material.
Soil Texture
• Soil is made of particles of different sizes.
• Some particles are as large as sand, others cannot be seen without a microscope.
• Soil texture describes the relative amounts of soil particles of different sizes.
• Soil texture affects the consistency of soil.
• Consistency describes a soil’s ability to be broken up for farming.
• Soil texture also influences infiltration, or the ability of water to move through soil.
• Soil Fertility
• The ability of soil to hold nutrients and supply nutrients to plants is called soil fertility.
• Nutrients in soil may come from the parent rock, or from humus, the organic material that forms in soil
from the decayed remains of plants and animals.
• Soil Horizons
• Due to the way it forms, soil often ends up in a series of layers.
• Because the layers are horizontal, soil scientists call these layers horizons.
• The top layer of soil is commonly called topsoil.
• Topsoil has more humus than lower layers of soil do.
• The humus is rich in the nutrients that plants need to be healthy.
• Soil pH
• Soils can be acidic, neutral, or basic.
• The pH scale is used to measure how acidic or basic something is.
• The scale ranges from very acidic (0), to neutral (7), to very basic (14).
• The pH of a soil influences how nutrients dissolve in the soil.
• Soils that are too basic or acidic may restrict the uptake of nutrients by plants.
• Soil acidity is influenced by the parent rock, acidity of rainwater, use of fertilizers, and the extent of
chemical weathering.
• Soil and Climate
• Tropical Climates
• Tropical rain forests receive a lot of direct sunlight and a large amount of rain.
• In a tropical rain forest, the fast decay of organic matter provides rich humus to the soil.
• However, constant rain can leach, or remove, nutrients from this soil.
• As a result, many tropical soils are nutrient-poor.
• Another reason that the soils in tropical areas are nutrient-poor is that the lush vegetation quickly uses
up most of the nutrients in the soil.
• Desert and Arctic Climates
• Desert and arctic climates receive little rainfall.
• Thus, little leaching of nutrients happens in soils in these climates.
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However, lack of rain also leads to low rates of chemical weathering, and little plant and animal life.
As a result, soil forms slowly.
Because of the lack of organisms, little humus develops.
Arctic soil has more humus than desert soil does because the cold climate slows the breakdown of
organic materials.
Desert soils can develop high concentrations of salt.
Groundwater in which mineral salts are dissolved seeps into desert soil.
When the water evaporates, the salts are left in the soil. These salts prevent plants from absorbing water
from the soil.
Temperate Forest and Grassland Climates
Much of the continental United States has a temperate climate.
High rates of mechanical weathering cause thick soil layers to form.
Moderate amounts of rain increase chemical weathering without leaching away nutrients.
Temperate soils are some of the most productive soils in the world.
The midwestern area of the United States has earned the nickname “breadbasket” for the many crops
that the soil in this region supports.