Soil is one of the most important earth materials we encounter each day, but the definition of soil is difficult. Soil Scientists (and most ordinary people): fine-grained, well-weathered earth material that is able to support plant growth focus on the physical and chemical properties Engineers: any earth material that can be removed without blasting focus on particle size and the amount of organic material engineering applications Soils and the Environment S. Hughes, 2003 GEOL g406 Environmental Geology

Environmental Geologists: must understand soil from many perspectives characteristics affect agriculture, engineering, hydrology, natural hazards and other aspects of land use soil development and soil character is crucial to good land use planning. Soils and the Environment Read Table 3.1 (Soil Taxonomy) Understand the meaning of soil types, but do not memorize all of them. Read Table 3.2 (Unified Soil Classification) Learn the definition of each constituent that makes up soil. S. Hughes, 2003GEOL g406 Environmental Geology

Soil Development Soil is an important part of the geologic cycle and soil characteristics are influenced by parent material, climate, topography, weathering, and the amount of time a particular soil has had to develop. Unsurprisingly, variations in climate, parent material, type of weathering and amount of time produce distinct soils that express these variations. As soil develops, weathering creates distinct layers in soil. We call these layers soil horizons, and each soil horizon has distinctive characteristics. Every soil has a soil profile, a list of the horizons that describe a particular soil. S. Hughes, 2003GEOL g406 Environmental Geology

Soil Horizons Materials in a Soil System: Vertical and horizontal movements create a soil profile made up of distinct layers parallel to the surface, which are called soil horizons. Organic top layer (O) Zone of leaching (A and E) Zone of accumulation (B) Weathered rock (C and R) Soil Rock S. Hughes, 2003GEOL g406 Environmental Geology

Soil Horizons O Mostly organic materials, decomposing leaves, and twigs. Often dark brown color. A Mineral and organic materials, light black to brown. Leaching of clay, Fe and Ca. E Light colored materials due to leaching of clay, Ca, Mg, and Fe to lower horizons. Horizons A and E make up the Zone of Leaching. B Enriched in clay, Fe oxides, Silica, carbonate and other material leached from above. This is the Zone of Accumulation. C Partially altered (weathered) parent material, which is either rock or loose sediment. R Unweathered (unaltered) parent material = rock. ~3m S. Hughes, 2003

A soil’s profile depends on its age and its conditions of formation. Soil profile is the primary criteria for soil classification. Soils can be compared in terms of their relative development. Weakly developed soil profiles are generally younger and may have fewer horizons; well-developed soils are generally older and have more horizons. Chronosequences Relative development of a series of soils allows their arrangement in a soil chronosequence. A soil chronosequence gives information about the history of the landscape. The relative development of the soils in a chronosequence tells the investigator about the climate and depositional history of the area. Soil Development S. Hughes, 2003GEOL g406 Environmental Geology

Soil Taxonomy EntisolsEntisols - soils with little or no morphological development VertisolsVertisols - clayey soils with high shrink/swell capacity InceptisolsInceptisols - soils with weakly developed subsurface horizons AridisolsAridisols - CaCO 3 -containing soils of arid environments with moderate to strong development MollisolsMollisols - grassland soils with high base status AndisolsAndisols - soils formed in volcanic ash SpodosolsSpodosols - acid soils with a subsurface accumulation of metal-humus complexes AlfisolsAlfisols - soils with a subsurface zone of silicate clay accumulation and >35% base saturation UltisolsUltisols - soils with a subsurface zone of silicate clay accumulation and <35% base saturation OxisolsOxisols - intensely weathered soils, tropical and subtropical HistosolsHistosols - organic soils (peak, bog, muck) GelisolsGelisols - soils with permafrost within 2 m of the surface S. Hughes, 2003

Soil Texture Texture = relative proportion of sand, silt and clay. Texture classes: Coarse sands, loamy sand and sandy loams with less than 18 % clay, and more than 65 % sand. Medium sandy loams, loams, sandy clay loams, silt loams with less than 35 % clay and less than 65 % sand; the sand fractions may be as high as 82 % if a minimum of 18 % clay is present. Fine clays, silty clays, sandy clays, clay loams and silty clay loams with more than 35 % clay. S. Hughes, 2003GEOL g406 Environmental Geology

Clay (%)Silt (%) Sand (%) 100 % CLAY 100 % SILT100 % SAND Clay SandSilt Clay loam Loam Sandy loam Silt loam See Figure 3.2 in textbook S. Hughes, 2003

Soils are often referred to as being sandy or clayey, or sometimes silty. Different countries use different standards to define sand particle and silt particle sizes. Particle sizes Gravel, Cobbles, and Boulders particles greater than 2 mm diameter Coarse and medium sand particles from 2 mm to 0.2 mm diameter Fine and very fine sand particles from 0.2 mm to mm diameter Silt particles from mm to mm diameter Clay particles less than mm diameter Soil Classification S. Hughes, 2003

Soil Classification WELL SORTEDWELL GRADED S. Hughes, 2003GEOL g406 Environmental Geology

Unified Soil Classification System FINE-GRAINEDCOARSE-GRAINED >50 % larger than mm >50 % smaller than mm Clays Silts Sands Gravels GW = well-graded gravel GP = poorly graded gravel GM = silty gravel GC = clayey gravel SW = well-graded sand SP = poorly graded sand SM = silty sand SC = clayey sand ML = silt MH = micaceous silt OL = organic silt CL = silty clay CH = high plastic clay OH = organic clay PT = peat and muck Mostly Organics Clean (<5 % fines) Dirty (>12 % fines) Clean (<5 % fines) Dirty (>12 % fines) Non-plastic Plastic

Types of water: Water on Earth is known by different terms, depending on where it is and where it came from. Meteoric water = water in circulation. Connate water = "fossil" water, often saline. Juvenile water = water from the interior of the earth. Surface water = water in rivers, lakes, oceans and so on. Subsurface water = groundwater, connate water, soil, capillary water. Groundwater exists in the zone of saturation, and may be fresh or saline. Water in Soils S. Hughes, 2003GEOL g406 Environmental Geology

S. Hughes, 2003

Moisture Content of soil is calculated as follows: W = weight, so that: [(W wet - W dry )/W dry ] x 100 = H 2 O content (%) Moisture content affects the engineering properties and stability of soils. A soil that is stable in dry conditions may become unable to support the structures built on it when saturated with water. Be sure to read the sections of your text describing the engineering properties of soil. Water in Soils S. Hughes, 2003GEOL g406 Environmental Geology

Adhesion and Cohesion S. Hughes, 2003GEOL g406 Environmental Geology

Engineering Properties of Soils Plasticity related to the water content Liquid Limit (LL) water content above which the soil behaves like a liquid Plastic Limit (PL) water content below which the soil is no longer plastic Plasticity Index (PI), PI = LL - PL range of water contents that make the soil behave as a plastic material Low PI (5 %): small change in water content, soil changes from solid to liquid High PI (> 35%): potential to expand and contract on wetting and drying S. Hughes, 2003

Engineering Properties of Soils Expansive Soils high content of swelling clay (montmorillonite) soils swell when water is incorporated between clay plates shrinking occurs when soil is dried damage to building and road foundations Study Table 3.3 in textbook to understand more about soil descriptions and their significant properties. Study the Universal Soil Loss Equation (erosion) : A = RKLSCP S. Hughes, 2003GEOL g406 Environmental Geology

Universal Soil Loss Equation A = RKLSCP A = long-term average annual soil loss for the site R = long-term rainfall runoff erosion factor K = soil erodibility index L = hillslope/length factor S = hillslope/gradient factor C = soil cover factor P = erosion-control practice factor Used to predict the impact of sediment loss on local streams and other resources and to develop management strategies for minimizing impact. S. Hughes, 2003GEOL g406 Environmental Geology

Water in Soils FOREST Precipitation Interception Evapotranspiration Soil Rock Water infiltrates and runs through soil S. Hughes, 2003GEOL g406 Environmental Geology

Water in Soils CLEARCUT Precipitation Little Interception and Evapotranspiration Soil compaction Rock Increased surface runoff and sediment; weaker soil More sediment in channel S. Hughes, 2003GEOL g406 Environmental Geology

Water in Soils Farming Precipitation Less Interception and Evapotranspiration Soil Rock Increased surface runoff and soil erosion from freshly plowed land Increased sediment in channel S. Hughes, 2003GEOL g406 Environmental Geology

Water in Soils URBANIZATION Precipitation Soil Rock Large increase in runoff from urban surfaces and storm sewers S. Hughes, 2003GEOL g406 Environmental Geology

Effect of Land Use on Sediment Yield, eastern U.S. Piedmont Region. S. Hughes, 2003GEOL g406 Environmental Geology

Soils and the Environment Key Terms to Review: weathering soil horizons soil profile development soil chronosequence soil fertility unified soil classification soil strength soil sensitivity liquefaction compressibility erodibility permeability corrosion potential shrink-swell potential expansive soils soil pollution desertification water table soil plasticity index S. Hughes, 2003GEOL g406 Environmental Geology