NRES 201 Lecture 7 Fall 2014 The Soil Profile NRES 201 The Soil Profile 1 The Soil Profile Today s lecture topics The soil in profile Soil horizons 2 1 NRES 201 Lecture 7 Fall 2014 The Soil Profile The soil in profile What is the soil profile A belowground vertical slice of the soil in place Includes A surface organic layer if present O horizon All mineral layers developed by soil forming processes A E and B horizons Underlying parent material C horizon Source http www soils4kids org 3 Importance of soil horizonation Horizons are a fundamental feature of most soils Their presence reveals which factor dominated soil formation Vegetation type was critical in Illinois Grass formed dark prairie soils Trees formed light forest soils Prairie soil Forest soil Source http www nrcs usda gov 4 2 NRES 201 Lecture 7 Fall 2014 The Soil Profile Rainfall has a huge impact on soil development Poorly developed soils in arid regions Well developed soils in the tropics Arizona Aridisol Hawaiian Oxisol Source Hassett and Banwart 1992 5 Progressive nature of soil profile development Stage 1 Mosses and lichens colonize bare rock O horizon appears Time Source Brady and Weil 2010 6 3 NRES 201 Lecture 7 Fall 2014 The Soil Profile Progressive nature of soil profile development Stage 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 horizons have appeared Time Source Brady and Weil 2010 7 Progressive nature of soil profile development Stage 3 Forest trees now established The O horizon is very distinct The A horizon is thicker darker and more granular A bleached zone E horizon has lost Fe oxides and clay These materials have formed a B horizon The C horizon has thickened Time Source Brady and Weil 2010 8 4 NRES 201 Lecture 7 Fall 2014 The Soil Profile Progressive nature of soil profile development Stage 4 The B horizon has gained more clay and developed a blocky structure As weathering continues The soil becomes more acidic and less productive Clay breaks down to Fe and Al oxides The B horizon develops a red color Time Source Brady and Weil 2010 9 Soil horizons Distinguishing features Color Organic matter content Texture Structure Processes of horizonation Organic matter accumulation Loss eluviation through profile water movement Gain illuviation of material eluviated from other horizons Transformations in place 10 5 NRES 201 Lecture 7 Fall 2014 The Soil Profile Master soil horizons Total of six But some may not be present O organic A mineral E mineral Solum O A E B B mineral Only if highly decomposed C mineral R mineral Source Brady and Weil 2010 11 O horizons Composed of organic matter Usually occur in forests but not in prairies Absent from cultivated soils Source Brady and Weil 2010 12 6 NRES 201 Lecture 7 Fall 2014 The Soil Profile A horizons Topmost mineral horizons Dark colored by humus Granular texture Finer materials lost by Eluviation Erosion Source Brady and Weil 2010 13 E horizons Light colored horizons formed by eluviation of clay Fe Al and or organic matter Coarse textured Common in forest soils but not prairie soils Source Brady and Weil 2010 14 7 NRES 201 Lecture 7 Fall 2014 The Soil Profile B horizons Illuvial horizons characterized by accumulation of Clay Fe Al in humid regions CaCO3 CaSO4 in arid regions Often show blocky prismatic structure Source Brady and Weil 2010 15 C horizons Unconsolidated material underlying the solum A E B May or may not be the soil s parent material Little affected by pedogenic processes Source Brady and Weil 2010 16 8 NRES 201 Lecture 7 Fall 2014 The Soil Profile R layers Consolidated bedrock No weathering Source Brady and Weil 2010 17 Subdivisions within master horizons Separate layers within a single horizon indicated by 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 over glacial till Example O A B 2C 18 9 NRES 201 Lecture 7 Fall 2014 The Soil Profile Transition horizons Uniform mixture dominated by properties of one horizon Composite of separate zones from two horizons Example E B Source Brady and Weil 2010 19 Subordinate distinctions Specific horizon characteristics indicated by lowercase letters following the master horizon designation Source Brady and Weil 2010 20 10 NRES 201 Lecture 7 Fall 2014 The Soil Profile Major subordinate distinctions b buried horizon Can be of natural origin For profiles formed from multiple parent materials As when loess covers a soil formed in glacial till Can also be of human origin From past tillage or manuring practices Or at archaeological sites Source Ab http www texasbeyondhistory net 21 g strong gleying mottling Occurs in the B horizon Indicates fluctuating water table Due to Fe reduction oxidation Beware of a wet basement Bg Source http ohwm blogspot com 22 11 NRES 201 Lecture 7 Fall 2014 The Soil Profile k accumulation of carbonates Common in arid regions Occurs in the B horizon Avoid deep plowing To maintain P availability Bk Source http www isric org 23 n accumulation of sodium Problem in arid regions Occurs in the B horizon Destroys soil structure Bad for crop production Bn Source http wps prenhall com Source http www regyp com au 24 12 NRES 201 Lecture 7 Fall 2014 The Soil Profile p plowing Occurs in the A horizon Zone of Organic matter depletion Lower structural stability Ap Source http nesoil com 25 t accumulation of clay Common in humid regions Especially in forest soils Clay coatings form on peds in the B horizon Bt Shiny clay film Source http www cals uidaho edu Source http www latebloomershow com 26 13 NRES 201 Lecture 7 Fall 2014 The Soil Profile w weakly developed B horizon Typical color and or structure But little if any clay accumulation Bw Source http www nature com 27 x fragipan Hard dense loamy layer that restricts water flow and root penetration Often occurs below the B horizon A problem in southern Illinois Bx Source https www flickr com 28 14 NRES 201 Lecture 7 Fall 2014 The Soil Profile The Soil Profile Today s lecture topics The soil in profile Soil horizons 29 Today s key concepts Horizons form naturally during soil development These horizons reflect the dominant soil forming factor s 30 15