NRES 201 Lecture 7 (Fall 2014): The Soil Profile11NRES 201The Soil Profile2Today’s lecture topics- The soil in profile- Soil horizonsThe Soil ProfileNRES 201 Lecture 7 (Fall 2014): The Soil Profile23The soil in profileWhat is the soil profile?- A belowground vertical sliceof the soil in place- Includes:- A surface organic layer, ifpresent (O horizon)- All mineral layers developed by soil formingprocesses (A, E, and Bhorizons)- Underlying parent material(C horizon)Source: http://www.soils4kids.org4Importance of soil horizonation- Horizons are a fundamental feature of mostsoils- Their presence reveals which factor dominated soil formation- Vegetation type was critical in Illinois- Grass formed dark prairie soils- Trees formed light forest soilsPrairie soil Forest soilSource: http://www.nrcs.usda.govNRES 201 Lecture 7 (Fall 2014): The Soil Profile35- Rainfall has a huge impact on soil development- Poorly developed soils in arid regions- Well developed soils in the tropicsSource: Hassett and Banwart (1992)Hawaiian OxisolArizona Aridisol6Progressive nature of soil profile developmentStage 1- Mosses and lichenscolonize bare rock- O horizon appearsTimeSource: Brady and Weil (2010)NRES 201 Lecture 7 (Fall 2014): The Soil Profile47Progressive nature of soil profile developmentStage 2- Grasses, shrubs, and stunted trees have rooted in a deepening layer of disintegrating rock and developing soil- O horizon has increased- A and C horizonshave appearedTimeSource: Brady and Weil (2010)8Progressive nature of soil profile developmentStage 3- Forest trees now established- The O horizon is very distinct- The A horizon is thicker, darker, andmore granular- A bleached zone (Ehorizon) has lost Fe oxides and clay- These materialshave formed a Bhorizon- The C horizon hasthickenedTimeSource: Brady and Weil (2010)NRES 201 Lecture 7 (Fall 2014): The Soil Profile59Progressive nature of soil profile developmentStage 4- The B horizon hasgained more clayand developed a blocky structure- As weatheringcontinues:- The soil becomesmore acidic andless productive- Clay breaks down to Fe and Aloxides- The B horizondevelops a red colorTimeSource: Brady and Weil (2010)10Soil horizonsDistinguishing features- Color- Organic matter content- Texture- StructureProcesses of horizonation- Organic matter accumulation- Loss (eluviation) through profile water movement- Gain (illuviation) of material eluviated from otherhorizons- Transformations in placeNRES 201 Lecture 7 (Fall 2014): The Soil Profile611Master soil horizons- Total of six- But some maynot be presentO - organicA - mineralE - mineralB - mineralC - mineralR - mineralSource: Brady and Weil (2010)Solum = O† + A + E + B†Only if highly decomposed.12O horizons- Composed oforganic matter- Usually occurin forests butnot in prairies- Absent fromcultivated soilsSource: Brady and Weil (2010)NRES 201 Lecture 7 (Fall 2014): The Soil Profile713A horizons- Topmost mineralhorizons- Dark-colored byhumus- Granular texture- Finer materialslost by:- Eluviation-ErosionSource: Brady and Weil (2010)14E horizons- Light-coloredhorizons formedby eluviation ofclay, Fe, Al,and/or organicmatter- Coarse-textured- Common inforest soils butnot prairie soilsSource: Brady and Weil (2010)NRES 201 Lecture 7 (Fall 2014): The Soil Profile815B horizons- Illuvial horizonscharacterized byaccumulation of:- Clay/Fe/Al in humid regions-CaCO3/CaSO4in arid regions- Often showblocky/prismaticstructureSource: Brady and Weil (2010)16C horizons- Unconsolidatedmaterialunderlying thesolum (A + E + B)- May or may notbe the soil’s parent material- Little affected bypedogenicprocessesSource: Brady and Weil (2010)NRES 201 Lecture 7 (Fall 2014): The Soil Profile917R layers- Consolidatedbedrock- No weatheringSource: Brady and Weil (2010)18Subdivisions within master horizons- Separate layers within a single horizon indicatedby a numeral following the letter designation- Layers differ in color, structure, or texture- Examples: B1, B2, B3- Horizons formed from a different parent material indicated by a numeral preceding the letter designation- Common for soils formed from loess overglacial till- Example: O-A-B-2CNRES 201 Lecture 7 (Fall 2014): The Soil Profile1019Transition horizons- Uniform mixturedominated byproperties of onehorizon- Composite ofseparate zonesfrom twohorizons- Example: E/BSource: Brady and Weil (2010)20Subordinate distinctions- Specific horizon characteristics indicated by lowercase letters following the master horizon designationSource: Brady and Weil (2010)NRES 201 Lecture 7 (Fall 2014): The Soil Profile1121Major subordinate distinctions- b = buried horizon- Can be of natural origin• For profiles formed frommultiple parent materials• As when loess covers asoil formed in glacial till- Can also be of human origin• From past tillage ormanuring practices• Or at archaeological sitesSource: http://www.texasbeyondhistory.netAb22- g = strong gleying (mottling)- Occurs in the B horizon- Indicates fluctuatingwater table- Due to Fe reduction/oxidation- Beware of a wet basementSource: http://ohwm.blogspot.comBgNRES 201 Lecture 7 (Fall 2014): The Soil Profile1223- k = accumulation of carbonates- Common in arid regions- Occurs in the B horizon- Avoid deep plowing• To maintain P availabilitySource: http://www.isric.orgBk24- n = accumulation of sodium- Problem in arid regions- Occurs in the B horizon- Destroys soil structure- Bad for crop productionSource: http://wps.prenhall.comSource: http://www.regyp.com.auBnNRES 201 Lecture 7 (Fall 2014): The Soil Profile1325- p = plowing- Occurs in the A horizon- Zone of:• Organic matter depletion• Lower structural stabilitySource: http://nesoil.comAp26- t = accumulation of clay- Common in humid regions• Especially in forest soils- Clay coatings form on pedsin the B horizonSource: http://www.cals.uidaho.eduBtSource: http://www.latebloomershow.comShiny clay filmNRES 201 Lecture 7 (Fall 2014): The Soil Profile1427- w = weakly developed B horizon- Typical color and/or structure- But little if any clayaccumulationBwSource: http://www.nature.com28- x = fragipan- Hard, dense loamy layer thatrestricts water flow and rootpenetration- Often occurs below the Bhorizon- A problem in southern IllinoisSource: https://www.flickr.comBxNRES 201 Lecture 7 (Fall 2014): The Soil Profile1529Today’s lecture topics- The soil in profile- Soil horizonsThe Soil Profile30Today’s key concepts- Horizons form