NRES 201 Lecture 7 Outline of Last Lecture I Organisms II Topography III Time IV Weathering Outline of Current Lecture V The soil in profile VI Soil horizons Current Lecture The Soil in Profile What is the soil profile A belowground vertical slice of the soil in place which includes a surface organic layer if present 0 horizon all mineral layers developed by soil forming processes A E and B horizons and underlying parent material C horizon The importance of soil horizonation o Horizons are a fundamental feature of most soils o Their presence reveals which factor dominated soil formation o Vegetation type was critical in Illinois Grass formed dark prairie soils and trees formed light forest soils o Rainfalls have a huge impact on soil development Poorly developed soils in arid regions Well developed soils in the tropics Progressive nature of soil profile development o Stage 1 Mosses and lichens colonize bare rock O horizon appears o Stage 2 Grasses shrubs and stunted trees have rooted in a deepening layer of disintegrated rock and developing soil O horizon has increased A and C horizons have appeared o Stage 3 Forest trees now established The O horizon is very distinct The A horizon is thicker darker and more granular These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute A bleached zone E horizon has lost Fe oxides and clay These materials have formed a B horizon The C horizons have thickened o 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 Soil Horizons Distinguishing features Color Organic Matter Content Texture and Structure Processes of Horizonation Organic matter accumulation loss eluviation through profile water movement gain illuviation of material eluviated from other horizons and transformations in place Master soil horizons o O Organic Usually occur in forests but not in prairies Absent from cultivated soils o A Mineral Topmost mineral horizons Dark colored by humus o E Mineral Light colored common in forest soils but not prairie soils o B Minerals Illuvial horizons characterized by accumulation of Clay Fe Al in humid regions o C Mineral Unconsolidated material underlying the solum may or may not be the soil s parent material o R Mineral Solum O A E B Only if highly decomposed Consolidated bedrock no weathering Transition horizons Uniform mixture dominated by properties of one horizon Subordinate distinctions Specific horizon characteristics indicated by lowercase letters following the master horizon designation o b buried horizon o g strong gleying mottling o k accumulation of carbonates o o o o o n accumulation of sodium p plowing t accumulation of clay w weakly developed B horizon x fragipan
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