Soil Horizons Master Horizons O A O horizon E A horizon E horizon B B horizon C horizon C R horizon W horizon R W Master Horizons O horizon – predominantly organic matter (litter and humus) A horizon – zone of organic matter accumulation E horizon – zone of eluviation (loss of clay, Fe, Al) B horizon – zone of accumulation (clay, Fe, Al, CaC03, salts…) – forms below O, A, or E horizon Master Horizons C horizon – little or no pedogenic alteration, unconsolidated parent material, soft bedrock R horizon – hard, continuous bedrock O horizon A horizon E horizon B horizon B horizon B horizon B horizon C horizon R horizon Transitional Horizons contains properties of horizon above and horizon below – AB – mostly A horizon, but some B horizon – BA – mostly B horizon, but some A horizon – BE – mostly B horizon, but some E horizon – BC – mostly B horizon, but some C horizon – CB – mostly C horizon, but some B horizon – C/B – intermingled bodies of C and B horizon material, majority is C horizon material Transitional Horizons EB BC Transitional Horizons BE BC Horizon Suffixes Horizon Criteria Suffix a highly decomposed organic matter (O) b buried genetic horizon (any) c concretions or nodules (any) e moderately decomposed organic matter (O) g strong gleying (any) h illuvial organic matter accumulation (B) i slightly decomposed organic material (O) k pedogenic carbonate accumulation (B or C) Horizon Suffixes Horizon Criteria Suffix o residual sesquioxide accumulation (B) p plow layer or other artificial disturbance (A) r weathered or soft bedrock (C) s illuvial sesquioxide accumulation (B) ss slickensides (B) t illuvial accumulation of silicate clay (B) v plinthite (B) w weak color or structure (B) x fragipan (B) Subhorizon Examples Ap Ap Bt Bt Btx Alfisol Ultisol Subhorizon Examples O A E Bh A Bs Bt Bw Bk C Spodosol Aridisol Subhorizon Examples Ap A1 Bo A2 Bk Mollisol Oxisol Subhorizon Examples Oe Oa C Histosol Entisol Subhorizon Examples Plinthic Kandiudult Numerical Suffixes A1 used to denote A2 subdivisions within a master horizon Bt1 Bt2 Bt3 C1 C2 Numerical Suffixes Ap A C1 Bt1 C2 Bt2 C3 BC Entisol Discontinuities changes in parent material or mode of deposition colluvium over residual limestone soil over sandstone bedrock – A, E, Bt1, 2Bt2, 2Bt3, 2BC, 2C, 3R by convention, 1 is understood, but not shown Discontinuities Bw Ap Bt1 2R Bt2 2Bt1 colluvium over residuum loess over limestone residuum Boundaries distinctness – distance through which one horizon grades into another abrupt (0-2.5cm), clear (2.5-5cm), gradual (5-15cm), diffuse (>15cm) topography – lateral undulation and continuity of the boundary between horizons smooth (planar), wavy (width>depth), irregular (depth>width), broken (discontinuous) Boundaries abrupt smooth abrupt wavy Boundaries clear smooth clear irregular Boundaries gradual smooth diffuse smooth Boundaries irregular broken SOIL COLOR SOIL COLOR Both use Munsell Notation Munsell notation (cont.) 7.5 YR 4 / 3 The number before the slash is the Value. The number after the slash is Value indicates the lightness of a the Chroma. color. The scale of value ranges Chroma describes how the “intensity” from 0 for pure black to 8 for pure of a color. The scale ranges from 1 to white. 8. For neutral colors, chroma is 0. Color Mechanics 1. Break the ped. If it is dry, moisten it slightly with water from your water bottle. 2. Stand with the sun over your shoulder so that sunlight shines on the color chart and the soil sample you are examining. 3. Compare the color of the inside surface with the soil color chart. Note: Sometimes, a soil sample may have more than one color. Record the colors and indicate (1) the matrix (dominant color) and (2) other colors (mottles or redox). Color Mechanics Viewing Conditions ? • very cloudy days • foggy days • early morning • late afternoon • wintertime conditions. • smoky conditions • indoor artificial light • nighttime ?????? Inferred Soil Characteristics Aquic Conditions soils with aquic conditions are those that undergo continuous or periodic saturation or reduction Saturation endosaturation – soil is saturated with water in all layers from the upper boundary of saturation to a depth of 200 cm or more from the mineral surface – groundwater saturation episaturation – water table is perched on top of an impermeable layer Formation of Redoximorphic Features Anaerobic conditions – soil is saturated so most all pores are filled with water, absence of oxygen Prolonged anaerobiosis – changes the chemical processes in the soil Reduction of Fe and Mn oxides – results in distinct soil morphological characteristics most are readily observable changes in soil color Soil Color and Oxidation / Reduction In subsoil horizons, Fe and Mn oxides give soils their characteristic brown, red, yellow colors Soil Color When reduced, Fe and Mn are mobile and can be Coating of Fe2O3 stripped from the soil particles Mineral grain (gray) Leaving the characteristic mineral grain color Gray Soil Red Soil Remove Fe – usually a “grayish” color Types of Redoximorphic Features Redox Concentrations – Masses – Pore Linings – Nodules and Concretions Redox Depletions – Depleted Matrix Reduced Matrix Redox Concentrations Bodies of apparent accumulation of Fe-Mn oxides – Masses – Pore Linings ped faces root channels – Nodules and Concretions Soft Masses Soft bodies – frequently in the soil matrix – variable shape – can usually be removed from the soil “intact” Soft Masses in Sand The masses have diffuse reddish boundaries Depleted Matrix Dominant color of the soil is “gray” Commonly used to identify hydric soils Describing Redoximorphic Features Concentrations and Depletions – Describe type, color, abundance and location (i.e. along macropores or within matrix) contrast can be obtained from color charts Reduced Matrix – Describe reduced matrix color, oxidized color, and time for color change to occur Mottles - Quantity few <2% common 2 to <20% many (>20%) Mottles - Size fine -- <2 mm medium -- 2 to 5 mm coarse -- 5 to 20 mm very coarse – 20 to 76 mm extremely coarse -- >76 mm Redox Concentrations Hard Fe/Mn nodule in matrix (likely relict) Pore linings on root Pore linings on ped surface channel Soft Fe mass in matrix Hard Fe/Mn concretion in matrix Hard Fe/Mn nodule in matrix (likely contemporary) Adapted from Fig 1, Vepraskas 1995 Schematic illustration showing different kinds of redox concentrations and their relationship to soil macropores and matrices Interpretation Problems Redoximorphic features do not occur in all soils Low amounts of soluble Organic Carbon High pH Cold temperatures Low amounts of Fe Aerated groundwater Rate of Feature Formation A 2 mm thick Fe depletion around a root channel ranged from less than 1 to greater than 100 years depending upon how long reducing conditions occurred and how much Fe was in solution each day – Recently constructed wetlands should have redox depletions evident within a couple of years if wetland hydrology is present during the “growing season” Age of Features Redox features do not always indicate current hydrologic condition – commonly found in drained (historic) wetlands – can be relict of past climates relict features may have sharp edges and abrupt boundaries with the soil – relict nodules and concretions are often rounded – contemporary features should have diffuse boundaries and / or be associated with ped faces or root channels REVIEW •1 We describe color using Munsell Notation to accurately convey what we see to someone else. •2 “Good” viewing conditions are critical to accurate reading of a color. •3 If the sun is low colors are redder. •4 Cloudy days make it difficult to read low chroma colors. •5 Color is a critical characteristic when working with redoximorphic features •6 The location of color within the “soil fabric” can separate relict features from current conditions; the age of color. •7 Observable soil colors, accurately described, leads to critical inferred characteristics. Soil Texture proportion by weight of sand, silt, and clay estimated in the field or measured in the laboratory placed into a texture class NOTE: – soil texture is only the fine-earth fraction (< 2mm) – particle size distribution is fine-earth plus rock fragments (>2mm) Particle Sizes Sand – 2 mm to 0.05 mm – very coarse sand – 1 to 2 mm – coarse sand – 0.5 to 1 mm – medium sand – 0.25 to 0.5 mm – fine sand – 0.10 to 0.25 mm – very fine sand – 0.05 to 0.10 mm Silt – 0.05 to 0.002 mm Clay -- < 0.002 mm (<2m) Relative Sizes of Particles beachball frisbee dime Silt Clay (feels floury) (feels sticky) (< 0.002 mm) (0.05 - 0.002 mm) Sand (feels gritty) (2.00 - 0.05 mm) Texture Classes Class Abbrev. Class Abbrev. very coarse sand VCOS fine sandy loam FSL coarse sand COS very fine sandy loam VFSL sand S loam L fine sand FS silt loam SIL very fine sand VFS silt SI loamy coarse sand LCOS sandy clay loam SCL loamy sand LS clay loam CL loamy fine sand LFS silty clay loam SICL loamy very fine sand LVFS sandy clay SC coarse sandy loam COSL silty clay SIC sandy loam SL clay C Texture 12 texture classes Triangle Texture Flowchart Rock Fragments -- Sizes SPHERICAL OR CUBELIKE – gravel (2 – 75 mm diameter) GRAVELLY – cobbles (75 – 250 mm diameter) COBBLY – stones (250 – 600 mm diameter) STONY – boulders (> 600 mm diameter) BOULDERY FLAT – channers (2 – 150 mm long) CHANNERY – flagstones (150 – 380 mm long) FLAGGY – stones (380 – 600 mm long) STONY – boulders (> 600 mm long) BOULDERY Rock Fragments -- Roundness Texture Modifiers Fragment