Soil horizon Study Guide

📖 Core Concepts Soil horizon – a layer parallel to the soil surface that differs in physical, chemical, and biological traits from layers above and below. Master horizons – denoted by capital letters (O, A, E, B, C, R); lowercase letters, numbers, and suffixes add detail. Pedogenesis – the suite of soil‑forming processes that create true horizons; layers without these processes are simply “layers.” Solum – the portion of the profile that includes all horizons affected by pedogenesis (typically O, A, E, B). Diagnostic horizons – specific horizons (or combinations) used by classification systems to name a soil type. 📌 Must Remember Sequence of major horizons (most common order): O → A → E → B → C → R. O horizon – ≥20 % organic C, dominated by decomposing plant material. A horizon – mineral topsoil with humus; may be subdivided A1 (dark, high activity), A2 (paler), A3 (transition). E horizon – eluviated, light‑colored, loss of clay, iron, Al, organic matter. B horizon – illuviated, accumulation of clay, Fe/Al oxides, carbonates, silica, etc. C horizon – minimally altered parent material (weathered rock, saprolite). R horizon – unweathered bedrock, cannot be excavated by hand. Suffix meanings: b – buried horizon c – mineral concretions/nodules d – root‑restricting layer g – gleyed (bluish‑gray, reduced) h – organic matter accumulation k – carbonate accumulation t – clay accumulation x – fragipan (dense, water‑impeding) e – bleached (strong eluviation, AUS) p – ploughed A horizon (AUS) 🔄 Key Processes Pedogenesis (horizon formation) Eluviation: upward/downward movement of fine particles/solutes → creates E horizon. Illuviation: deposition of eluviated material into lower horizon → builds B horizon. Horizon notation construction Start with master letter (e.g., B). Add numeric prefix for lithic breaks (2C). Add numeric suffix for minor internal variation (B21). Append suffix letters for specific features (Bt, Bkc, Bs). Combined horizons Write dominant horizon first, then secondary (e.g., AB, BA). 🔍 Key Comparisons O vs. A O: ≥20 % organic C, mostly undecomposed plant material. A: mineral matrix + humus, supports highest biological activity. E vs. B E: leached, light‑colored, loss of clay/Fe/Al. B: accumulation of those same materials (clay, Fe, Al, carbonates). WRB vs. USDA classification WRB: uses broad set of diagnostic horizons, integrates climate/vegetation. USDA: emphasizes epipedons and specific horizon modifications (e.g., argillic, calcic). C vs. R C: weathered parent material, some pedogenic alteration possible. R: solid, unweathered bedrock, no pedogenic alteration. ⚠️ Common Misunderstandings All dark layers are A horizons – darkness alone isn’t enough; organic content and biological activity define A. E horizon always present – only in well‑developed, older soils; many profiles lack an E. C horizon = “soil” – C is parent material; true soil horizons are those altered by pedogenesis (O, A, E, B). Suffix “k” means “kaolin” – it actually marks carbonate accumulation, not a specific clay mineral. 🧠 Mental Models / Intuition “Up‑and‑Down” model: Think of the profile as a sandwich where eluviation removes goodies from the upper layer (E) and illuviation drops them into the lower layer (B). Letter‑Hierarchy: Capital letters = “major rooms”; lowercase/suffixes = “furnishings” that tell you what’s inside each room. 🚩 Exceptions & Edge Cases Human‑altered soils may lack distinct horizons (e.g., deep ploughing, earthworks). Buried soils retain a full or partial profile beneath newer deposits; notation includes “b” suffix. Australian suffix “e” for bleached horizons is not used in WRB or USDA systems. 📍 When to Use Which Identify horizon letter first (O, A, E, B, C, R). Add numeric prefix only when a lithic discontinuity is evident (e.g., 2C). Apply suffixes to convey specific features: Use t for clay‑rich B horizons → indicates argillic‑type accumulation. Use k when carbonates dominate → suggests calcic or petrocalcic horizons. Use g for gleyed conditions → signals water‑logged, reduced environment. Choose classification system based on exam focus: For WRB‑style questions, emphasize diagnostic horizons and climate‑vegetation links. For USDA‑style questions, focus on epipedons, suffixes, and numerical subscripts. 👀 Patterns to Recognize Color + Texture shift → likely a transition (A1 → A2 → A3 or B2). Bleached, siliceous layer sandwiched between darker horizons → classic E horizon. Dark, organic‑rich top layer with “O” → look for ≥20 % organic C. Presence of iron/red staining in B → iron oxide accumulation (common in B2). Abrupt textural change with concretions → suffix c (mineral nodules). 🗂️ Exam Traps Choosing “C” because it’s the deepest layer – remember R may be deeper; C is defined by minimal pedogenic change, not depth. Assuming any light‑colored layer is an E horizon – only true if leaching of clay/Fe/Al is documented. Confusing suffix “k” (carbonates) with “t” (clay) – they indicate different accumulations; mixing them up changes horizon classification. Selecting “O” for any organic‑looking surface – must meet the ≥20 % organic C threshold. Overlooking combined horizons (AB, BA) – the dominant horizon’s properties are primary; the secondary can’t be ignored. --- Use this guide for rapid recall before your exam – focus on the letters, suffixes, and the “up‑and‑down” movement of materials!