Soils, Chapter 10 © Brooks/Cole Publishing Company / ITP

Chapter Overview Questions What is soil? How is soil formed? What are the characteristics of soil? What is the importance of soil as a resource?

SOIL: A RENEWABLE RESOURCE Soil is a slowly renewed resource that provides most of the nutrients needed for plant growth and also helps purify water. Soil formation begins when bedrock is broken down by physical, chemical and biological processes called weathering. Mature soils, or soils that have developed over a long time are arranged in a series of horizontal layers called soil horizons.

Weathering Weathering- when rocks are exposed to air, water, certain chemicals or biological agents that degrade the rock. Physical weathering- the mechanical breakdown of rocks and minerals. Water gets into cracks in rock, where it expands when it freezes. Growing plant roots can pry rock sections apart.

Weathering Chemical weathering- the breakdown of rocks and minerals by chemical reactions. Rain water contains carbonic acid, which wears away limestone

Erosion Erosion- the physical removal of rock fragments from a landscape or ecosystem. Wind, water, ice transport and living organisms can erode materials. Deposition- the accumulation or depositing of eroded material such as sediment, rock fragments or soil. Soft rocks, such as sedimentary rocks, erode more easily than metamorphic and igneous rocks.

Soil Resources Soil- weathered rock mixed with organic material (humus), mineral nutrients, microorganisms, water and air Soil is important because it Is a medium for plant growth Serves as a filter for water Is a habitat for living organisms Serves as a filter for pollutants

Ecosystem Services of Soil

Factors That Determine the Formation of Soil Parent material- what the soil is made from influences soil formation Climate- what type of climate influences soil formation Topography- the surface and slope can influence soil formation Organisms- plants and animals can have an effect on soil formation Time- the amount of time a soil has spent developing can determine soil properties.

Wood sorrel Oak tree Organic debris builds up Lords and ladies Dog violet Rock fragments Grasses and small shrubs Earthworm Millipede Fern Moss and lichen Honey fungus O horizon Mole Leaf litter A horizon Topsoil B horizon Bedrock Subsoil Immature soil Regolith Young soil C horizon Figure 3.23 Natural capital: soil formation and generalized soil profile. Horizons, or layers, vary in number, composition, and thickness, depending on the type of soil. (Used by permission of Macmillan Publishing Company from Derek Elsom, Earth, New York: Macmillan, 1992. Copyright © 1992 by Marshall Editions Developments Limited) Pseudoscorpion Mite Parent material Nematode Root system Actinomycetes Red Earth Mite Fungus Mature soil Bacteria Springtail Fig. 3-23, p. 68

Soil Horizons O horizon – surface litter layer. Contains organic material in various stages of decomposition (leaves, twigs, etc.) A horizon – top soil. Contains humus and mineral particles. Where overlying organic material mixes with underlying inorganic material B horizon – subsoil. Zone of accumulation of metals and nutrients C horizon – least weathered portion of profile, same composition as parent material Bedrock or parent material – the rock material from which soil is derived.

Soil Horizons As soils form, they develop characteristics layers.

Layers in Mature Soils Infiltration- the downward movement of water through soil. Leaching- dissolving of minerals and organic matter in upper layers carrying them to lower layers. The soil type determines the degree of infiltration and leaching.

Animation: Soil Profile PLAY ANIMATION

Soil Profiles of the Principal Terrestrial Soil Types Figure 3-24

Physical Properties of Soil Texture- the percentage of sand, silt and clay the soil contains. Soils vary in the size of the particles they contain, the amount of space between these particles, and how rapidly water flows through them.

Soil Triangle Soil texture can be determined using the soil triangle

Physical Properties of Soil Soil texture determines porosity, permeability and structure of soil Porosity- measure of volume of pores per volume of soil Permeability- rate at which water and air move from upper to lower soil layers Structure- way in which soil particles are organized and clumped together

Physical Properties of Soil Porosity- the ratio of the volume of pore space in a unit of material to the total volume of material. Which has the highest porosity?

Physical Properties of Soil Permeability- how quickly the soil drains (which depends on its texture) Sand has large, loosely-packed particles and drains quickly. Clay, with its platy particles, drains much more slowly.

Chemical Properties of Soil pH- measure of how acidic or basic a substance is. It determines plants’ ability to take up nutrients from soil Cation exchange capacity- the ability of a soil to adsorb and release cations (positively charged mineral ions) Soil bases- calcium, magnesium, potassium and sodium Soil Acids- aluminum and hydrogen

Biological Properties of Soil Many organisms are found in the soil including fungi, bacteria, protozoans, rodents and earthworms. 23

Soil type affects vegetation: trees are growing on soil formed from sandstone that holds water

Differences in rocks, soil and microclimates determine the plants that live on two sides of the same mountain.

Soil Degradation Soil degradation- loss of some or all of soil’s ability to support plant growth Causes: Soil erosion Soil compaction (machines, foot traffic or overgrazing) Salinization- excess salts added to soil through incorrect irrigation practices Desertification- transformation of arable land into desert or unproductive land

Review Questions How do soils form? What are the roles of soils in ecosystems? How do a soil’s physical and chemical properties influence its role as a medium for plant growth?