Soil Formation Soils may be formed in place from rock orformed in weathered rock and minerals thathave been transported from where theoriginal rock occurred.Rocks Consist of mixtures of minerals. Igneous Metamorphic Sedimentary Igneous rocks are formed from moltenmagma and contain primary minerals.Sedimentary rocks are formed by depositionand cementation of weathered products.Metamorphic rocks are formed from igneousor sedimentary rocks by high pressure andtemperature. Weathering Physical disintegration Chemical decomposition Physical disintegration causes decrease insize without appreciably alteringcomposition. Differential stresses due toheating and cooling or expansion of icebreak the rock. Abrasion due to watercontaining sediment or wind carrying debrisis another type of physical weathering. Chemical decomposition and synthesis alterchemical composition. Four types ofchemical weathering reactions are:hydrolysis, hydration, acid dissolution andredox (particularly, Fe2+ / Fe3+).Five Factors of Soil Formation Parent material Climate Organisms Topography Time Soils defined -dynamic natural bodieshaving properties derived from thecombined effect of climate and bioticactivities, as modified by topography, actingon parent material over time.Parent Material Geologic material in which a soil forms. Residual sedentary Transported AgentColluvial gravity Alluvial water Marine water Lacustrine water Glacial ice Eolian wind Residual Develops in place from the underlying rock.If soil is young, properties tend to reflecteffect of parent material. For example, Igneous and metamorphic rock, if Siliceous (granite and granite gneiss) acid and sandy Ferromagnesian (basalt and diorite) nonacid and clayey Sedimentary, if Limestone sand or clay impurities lead to sandy or clayey soilSandstone shallow if SiO2 cements but deep if CaCO3 cements particles togetherShale clay minerals in shale give rise to a clayey soil Colluvial Consists of coarse and stony debrisdetached from rocks and carried downslopeby gravity. Alluvial These deposits occur as alluvial fans, flood plains and deltas. Alluvial fan occurs at the discharge of anupland stream into a broader valley below.Coarse textured material. Flood plains are adjacent to streams andrivers. During floods, coarse sediment isdeposited nearest the existing channel andfine sediment further away, resulting in anatural levee. Changes in the course of thestream result in a complex spatial pattern ofalternating coarse and fine sedimentsthroughout the flood plain. If there is achange in grade, the stream may cutthrough existing deposits, thereby formingterraces. Example Mississippi River natural levee showingdifferent soils at different positions on the levee. Complex soilscape on a flood plain. A delta occurs at the mouth of river andmarks the downstream extent of a floodplain. Marine Sediments Unconsolidated marine sedimentsdeposited by streams emptying into oceansmay undergo uplift. Common along Atlanticand Gulf coasts. Vary from sandy to clayey. Glacial Deposits From a series of glaciations during thePleistocene epoch. Each advancing icesheet accumulated a great mass ofunconsolidated material which wasdeposited as glacial drift when the glaciermelted and retreated. Material directlydeposition from the ice is called glacial tilland occurs in formations called moraines.Streams originating in a glacier transportedsediment away and produced outwashplains. Where regional topographyimpounded glacial melt, lakes formed andlacustrine deposits accumulated. Deltas ofcoarser materials occur in what was theinflow region whereas finer materials weredeposited further away. Ice sheet melt and resulting topographical features. Eolian Deposits consisting of silt and some finesand plus clay (loess) blanketed regionsalong the Mississippi and Missouri Rivers. Organic Materials Accumulate in wet places where plantgrowth exceeds the rate of residuedecomposition. Such organic deposits areknown as peat. Typical pattern of peataccumulation: 1) sedimentary (limnic) peat,from aquatic plants, 2) herbaceous(telmatic) peat, from sedges and so forth,then 3) woody (terrestic) peat, from trees.Climate Through effects of precipitation andtemperature, climate affects the rates ofbiological, chemical and physical processesinvolved in soil formation. Effects of climateon soil formation include: High precipitation and low temperatureincrease organic matter in soil.Leaching of soluble materials like CaCO3increases with increasing precipitation. Movement of clay in soil profile increasewith increasing precipitation.Silicate clay and Al and Fe oxide formationincrease with increasing temperature. Generally, high rainfall and temperatureleads to deep weathering and soil leaching. Just contrast weathered profile of humidtropical soils with profile of arid soils fromwhich soluble salts have not been leached. Climate also indirectly influences soilformation by its effect on natural vegetation.For example, trees under humid climate,grasses under semiarid climate and brushunder arid climate.Organisms Living organisms are responsible foraccumulation of organic matter, nutrientcycling and profile mixing. Difference in profiles of soils developedunder grassland and forest vegetationinclude: 1) development of a high organic mattersurface horizon under grass and 2) leachedsubsurface horizon (E) overlying a moreclayey horizon (Bt) under forest.Idealized prairie - forest transition. Forest type, deciduous versus coniferousalso affects soil development because thehigher rate of nutrient cycling in deciduousforest retards leaching of basic cations andsoil acidification. For example, compare twosoils from a biosequence in Louisiana thatdeveloped in loess: Calhoun JeaneretteCover pine / hardwood prairie Solum