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 tosupport plant growth• focus on the physical and chemical propertiesEngineers:• any earth material that can be removed without blasting• focus on particle size and the amount of organic material• engineering applicationsSoils and the EnvironmentS. Hughes, 2003GEOL g406 Environmental GeologyEnvironmental 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 landuse planning. Soils and the EnvironmentRead Table 3.1 (Soil Taxonomy)• Understand the meaning of soil types, but do not memorizeall of them.Read Table 3.2 (Unified Soil Classification)• Learn the definition of each constituent that makes up soil.S. Hughes, 2003GEOL g406 Environmental GeologySoil DevelopmentSoil 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. Wecall 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 GeologySoil HorizonsMaterials in a Soil System:Vertical and horizontal movements create a soil profilemade 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)SoilRockS. Hughes, 2003GEOL g406 Environmental GeologySoil HorizonsO Mostly organic materials, decomposing leaves,and twigs. Often dark brown color.A Mineral and organic materials, light black tobrown. Leaching of clay, Fe and Ca.E Light colored materials due to leaching of clay,Ca, Mg, and Fe to lower horizons. HorizonsA and E make up the Zone of Leaching.B Enriched in clay, Fe oxides, Silica, carbonateand other material leached from above. This isthe Zone of Accumulation.C Partially altered (weathered) parent material,which is either rock or loose sediment.R Unweathered (unaltered) parent material = rock.~3mS. Hughes, 2003A 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.ChronosequencesRelative 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 DevelopmentS. Hughes, 2003GEOL g406 Environmental GeologySoil TaxonomyEntisols - soils with little or no morphological developmentVertisols - clayey soils with high shrink/swell capacityInceptisols- soils with weakly developed subsurface horizonsAridisols-CaCO3-containing soils of arid environments withmoderate to strong development Mollisols- grassland soils with high base statusAndisols- soils formed in volcanic ashSpodosols- acid soils with a subsurface accumulation ofmetal-humus complexesAlfisols- soils with a subsurface zone of silicate clayaccumulation and >35% base saturationUltisols- soils with a subsurface zone of silicate clayaccumulation and <35% base saturationOxisols- intensely weathered soils, tropical and subtropicalHistosols- organic soils (peak, bog, muck)Gelisols- soils with permafrost within 2 m of the surfaceS. Hughes, 2003Soil TextureTexture = relative proportion of sand, silt and clay.Texture classes:Coarsesands, loamy sand and sandy loams with less than 18 % clay, and more than 65 % sand. Mediumsandy 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. Fineclays, silty clays, sandy clays, clay loams and silty clay loams with more than 35 % clay.S. Hughes, 2003GEOL g406 Environmental GeologyClay (%) Silt (%)Sand (%)100 % CLAY100 % SILT100 % SANDClaySand SiltClay loamLoamSandy loamSilt loamSee Figure 3.2in textbookS. Hughes, 2003Soils 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 sizesGravel, Cobbles, and Bouldersparticles greater than 2 mm diameter Coarse and medium sandparticles from 2 mm to 0.2 mm diameter Fine and very fine sandparticles from 0.2 mm to 0.074 mm diameterSiltparticles from 0.074 mm to 0.004 mm diameterClayparticles less than 0.004 mm diameterSoil ClassificationS. Hughes, 2003Soil ClassificationWELL SORTED WELL GRADEDS. Hughes, 2003GEOL g406 Environmental GeologyUnified Soil Classification SystemFINE-GRAINED COARSE-GRAINED>50 % largerthan 0.074 mm>50 % smallerthan 0.074 mmClays Silts Sands GravelsGW = well-graded gravelGP = poorly graded gravelGM = silty gravelGC = clayey gravelSW = well-graded sandSP = poorly graded sandSM = silty sandSC = clayey sandML = siltMH = micaceous siltOL = organic siltCL = silty clayCH = high plastic clayOH = organic clayPT = peat and muckMostly OrganicsClean(<5 % fines)Dirty(>12 % fines)Clean(<5 % fines)Dirty(>12 % fines)Non-plasticPlasticTypes of water:Water on Earth is known by different terms, depending on where it is and where it came from.• Meteoricwater = water in circulation.• Connatewater = "fossil" water, often saline. • Juvenilewater = water from the interior of the earth. •Surfacewater = water in rivers, lakes, oceans and so on.• Subsurfacewater = groundwater, connate water, soil, capillary water.• Groundwaterexists in the zone of saturation, and may be fresh or saline.Water in SoilsS. Hughes, 2003GEOL g406 Environmental GeologyS. Hughes, 2003Moisture Content of soil is calculated as follows:W = weight, so