GLG 110

Part 1

Chapter 3

Soils and Environment

3.1 Introduction to Soils

            Soil:      solid earth material that can support plant life (scientific definition)

                        Solid earth material that can be removed without blasting (engineer definition)

            Soil is important for land-use planning, waste management, and natural hazard management

3.2 Soil Profiles

            Weathering is physical and chemical breakdown of rocks and minerals

            Result of weathering is soil: transported (moved away from source) or residual (stays in place)

            Soil-forming factors are: parent material, climate, topography, time, organic processes

            Soil is an open system

            Mature soil forms layers = a soil profile

                        O-horizon: mostly organic material (humus)

                        A-horizon: zone of leaching

                        B-horizon: zone of accumulation

                        C-horizon: partially altered bedrock

                        R: unaltered bedrock

            Soil color is usually due to composition (e.g. organic-rich soil is black or dark)

            Soil texture depends on the relative abundance of sand, silt, and clay particles

            Soil structure (arrangement of particles in soil) depends on soil-forming processes

3.3 Soil Fertility

            Fertility: capacity of soil to supply nutrients to plants

            Soil contains large numbers of living organisms which may supply nutrients

            Fertile soils: e.g. floodplain, glacial deposits

            Poor soils: e.g. tropical rainforest, deserts

3.4 Water in Soil

            Pore spaces in soil are filled with gases and fluids

            Water content of soil is called moisture content

            Saturated soil: pores are full of water

            Unsaturated soil: some pores are not filled with water

            Water can flow vertically and horizontally through soil

            Amount of water in soil is an important factor for soil stability (e.g. in engineering projects)

3.5 Soil Classification

            Soil taxonomy: comprehensive and systematic soil classification

            Classification consists of: (see Table 3.1)

                        Orders, Suborders, Great Groups, Subgroups, Families, Series (=smallest category)

                        Parameters used to classify: morphology, nutrients, organics, color, climate conditions, …

            Classification used in agriculture, and other land-use purposes

            Engineering classification is much simpler

                        Three major soil categories:

                                    Coarse-grained soil

                                    Fine-grained soils

                                    Mostly organic soils

3.6 Engineering Properties of Soils

            Soils above water table have three parts or phases:

                        Soil material

                        Liquid

                        Gas

            Parameters to characterize soil stability:

                        Plasticity: based on water content

                        Soil strength: ability to resist deformation, depends on friction vs. cohesion

                        Sensitivity: changes of soil due to vibration or excavation

                        Compressibility: tendency of soil to consolidate

Erodibility: removal of soil by wind and water

                        Permeability: potential for liquids to flow through soil

                        Corrosion: weathering or chemical decomposition

                        Ease of excavation: pertains to equipment needed to remove or dig into soil

                        Shrink-swell potential: measures the volume increase or decrease due to addition of liquids                    

3.7 Rates of Soil Erosion

erosion: removal of volume, mass, or weight of soil

erosion depends on: properties of soil, land use, topography, climate

how to measure erosion rates:

- actual measurements on slopes over several years

- survey reservoirs several times to see how much water is displaced by sediment

- use the Universal Soil Loss Equation

3.8 Sediment Pollution

sediment is the greatest pollutant (fills in lakes, reservoirs, ponds, canals, etc.)

eroded soil (sediment) is a resource out of place

origin of sediment can be natural (rock and mineral fragments) or caused by

humans (from industry, manufacturing, public waste, etc.)

strategies to reduce sediment pollution include:

- contour plowing of fields

- changes in farming practices

- construction of small dams to trap runoff and sediment

3.9 Land Use and Environmental Problems of Soils

human activity influences runoff, erosion, and sedimentation

most important are: conservation of natural areas of land use and manipulation of

surface water

urbanization:

- causes dramatic changes

- during construction: significant increase in sediment production, slight increase in runoff (flooding potential increased)

- after construction: decrease in sedimentation, but significant increase in runoff (flooding potential increased)

- affect on soils: scraped off and lost, change of soil characteristics due to fill material, dessication due to pumping

- higher possibility of being polluted by human activities

off-road vehicles:

- enormous increase in off-road vehicles in last few years

- impact is significant (considering the large number of ORVs)

- soil erosion, changes in hydrology, damage to plants, and animals

3.10 Soil Pollution

occurs when chemicals get into soil (inadvertently or deliberately)

organic chemicals (pesticides) added to soil

inorganic chemicals (heavy metals) added to soil

in most cases clean up is very expensive to impossible

3.11 Desertification

conversion of land from a productive state to desert

driving forces for desertification include:

- overgrazing

- deforestation

- adverse soil erosion

- poor drainage of irrigated land

- overuse of water supply

desertification is most pronounced during drought periods

in most cases desertification is human induced

major symptoms of desertification include:

- declining groundwater table

- salinization of soil and water

- reduction of surface water features (ponds, lakes, streams)

- high rates of soil erosion

- damage to native vegetation

prevention, minimization, reversal of desertification includes:

- protection and improvement of high-quality land

- protection from overgrazing

- conservation measures for agriculture

- use of appropriate technologies to increase production on high-quality land

- increased land restoration efforts through vegetation management, stabilization of sand dunes, and control of soil erosion

3.12 Soil Surveys and Land-Use Planning

soils determine land use

soil surveys are an important first step to understand soil and its use

soil survey includes:

- soil description

- soil map showing horizontal and vertical soil distribution

- grain size

- moisture content

- shrink-swell potential

- soil strength

rating of soil for particular uses such as:

- housing

- light industry

- septic-tank systems

- roads

- recreation

- agriculture

- forestry

soil characteristics used to determine use:

- slope

- water contents

- permeability

- depth of rock

- susceptibility to erosion

- shrink-swell potential

- bearing strength

- corrosion potential

Review “Some Questions to Think About” on page 84.