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Diagnostic Horizons

Diagnostic Horizons. special horizons used in classification. EPIPEDON surface diagnostic horizon A, E, sometimes upper part of B 2. SUBSURFACE diagnostic horizons always B. The Epipedons : 5 common and 2 anthropogenic. 5 common:. 1.Mollic epipedon. Soft Thick (>18 cm) Dark

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Diagnostic Horizons

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  1. Diagnostic Horizons special horizons used in classification

  2. EPIPEDON surface diagnostic horizon A, E, sometimes upper partof B 2. SUBSURFACE diagnostic horizons always B

  3. The Epipedons: 5 common and 2 anthropogenic

  4. 5 common:

  5. 1.Mollic epipedon • Soft • Thick (>18 cm) • Dark • Chroma < 3.5 • > 1% OM • Grasslands • Strong structure • 50% Base Saturation Holdredge, Nebraska

  6. 2.Umbric epipedon • Similar to mollic • <50% base saturation less Ca, Mg (higher rainfall or parent material)

  7. 3.Ochric epipedon • “wimpy A” • Paler • chroma>3.5 • value >3.5 • Low OM • Hard, massive when dry

  8. Common in arid soils

  9. 4.Melanic epipedon • Black • Chroma≤2 • High OM • Volcanic ash • > 30 cm thick • Light in density

  10. 5.Histic epipedon • Organic materials • >30% OM if >60% clay; >20% if no clay • 20 – 60 cm thick • Peat or muck • In poorly drained areas • saturated for at least 1 month

  11. 2 Anthropogenic Epipedons:

  12. 1.Plaggen epipedon • >50 cm thick • Produced by manuring for long periods of time; sod used for animal bedding • None in US • artifacts

  13. 2.Anthropic • Human-made mollic horizon through addition of OM with artificially high phosphate content • Formed under long term cultivation and fertilization • Terra Preta (black earth) of Amazonia

  14. Subsurface diagnostic horizons

  15. Accumulation of clay (2) Accumulation of Salt (3) Accumulation of Humus (1) Loss of Materials (2) Hardpans (2) Other (1)

  16. Accumulation of clay: • Argillic • Natric

  17. Argillic horizon • Accumulation of silicate clays as “clay skins” (argillans) • Same as Bt

  18. Natric horizon Layer of accumulated clay and sodium hard and impermeable Columnar or prismatic srtucture

  19. Accumulation of salts: • Calcic • Salic • Gypsic

  20. Calcic horizon carbonates

  21. Salic horizon • Soluble salts • more soluble than gypsum • “salty” horizon

  22. salinization • Forms salic horizons • high evapotranspiration rates cause salts dissolved in soil water to migrate upwards

  23. Gypsic horizon • Gypsum (calcium sulfate) • gypsum has precipitated out of solution

  24. Accumulation of humus: • Spodic

  25. Spodic horizon • Illuvial accumulation of aluminum and organic carbon • acidic

  26. Loss of materials: • Albic • Glossic

  27. Albic horizon • Clay and free iron oxides have been removed • light in color • E

  28. Glossic • Degraded argillic or natric horizon, from which clay and iron oxides are removed

  29. Paleogroundwater caused iron depletions and concentrations; plinthite

  30. Hardpans • Duripan • fragipan

  31. duripan • Hardpan cemented by silica • root penetration is impeded

  32. fragipan • Minerally-cemented hardpan • Dense • hard when dry; brittle when wet • no roots

  33. Other: Oxic horizon • tropical climates • highly weathered layer of only Fe and Al oxides and 1:1 clay minerals • low pH • low fertility

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