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SOILS Soil Natural body that occurs on the land surface, occupies space, and is characterized by one or both of the following: • Horizons or layers, or . . . • The ability to support rooted plants in a natural environment – Upper limit is air or shallow (>2.5 m) water – Lower limit is either bedrock or the limit of biological activity – Lower limit for classification set at an arbitrary 2 m Soil Common soil volumes are composed of 45% mineral material, 5% organic material, 25% air, and 25% water. The 50% pore space is important for allowing water to move through the soil profile. Factors That Influence Soil Development • • • • • Climate Parent material Topographical relief Organisms Time Key Soil Properties Properties that are important to hydric soil development and recognition: • • • • • • Color Organic matter Texture Horizonation Drainage Permeability 1 Soil Color Aspects of Soil Color Soil color is due to the organic matter content and the presence and chemical form of Fe and Mn. Fe is responsible for the red, yellow, and brown colors. • Hue • Value • Chroma 10R 5/8 Hue Hue Red 0 2.5R 5R 7.5R Additional hues important to very wet soils are blue-green (BG) and green-yellow (GY). Soils with those colors are called Gleyed. 10R Yellow-Red 0 2.5YR 5YR 7.5YR 10YR Yellow 0 2.5Y 5Y 7.5Y 10Y Chroma Value • 10/0 - Pure White The Lightness or Darkness of Color “Neutral” Color /0 • 5/0 - “Gray” “Pure” Color /2 /4 /6 /8 Increasing strength of color • 0/0 - Pure Black Increasing grayness 2 Chroma Soil Color Chroma is the most important aspect of color for hydric soil determinations. Typical hydric soils have a chroma of 1 or 2 immediately below the A horizon. Zerochroma chips have no color, are neutral gray, and are given a hue of N (neutral). Munsell Soil Color Book, 7.5YR page Reading Soil Colors Color Patterns in Soils • Optimum conditions – Natural light – Clear, sunny day – Midday – Light at right angles – Soil moist Abundance and Size of Mottles • Few -- less than 2% • Common -- 2 to 20% • Many -- more than 20% • • • • Fine -- < 2 mm Medium -- 2 to 5 mm Coarse -- 5 to 20 mm V. Coarse -- > 20 mm • Matrix (predominant) color • Mottle colors (describe mottle abundance, size, and contrast) Contrast of Mottles • Contrast refers to the degree of visual distinction between associated colors – Faint -- evident only on close examination – Distinct -- readily seen – Prominent -- contrast strongly 3 Two Categories of Soil Material Differ in organic matter content: • Organic horizons • Mineral horizons Two Categories of Soil Material Two Categories of Soil Material Organic horizons • Consist of decomposed organic material • An organic soil has at least 16 inches of decomposed organic material in the upper 32 inches, or any thickness over bedrock Relative Sizes of Soil Particles Mineral horizons Sand (0.05-2.00 mm) • Primarily sand, silt, and clay, with varying amounts of organic matter Silt (0.002-0.05 mm) Clay (<0.002 mm) Major Horizon Designations Soil Texture Triangle Surface Organic Layer O horizon Surface Mineral Layer A Horizon Subsoil B Horizon Underlying Material C Horizon Bedrock R Horizon 4 Horizon Terminology Oi Oe Oa Ap A1 A2 A E AB Surface Layer Subsurface Layers Surface Soil BA Transition Layer Bt A horizon Bw horizon Subsoil AC Layer of Accumulated Lime Bk BC C1 Underlying Material C2 Bedrock R Cr C horizon 2C Permeability • A measure of the ability of water or air to move through the soil profile • Permeability is “measured” in inches per hour – Permeability is actually an estimated property based primarily on texture and structure – Measured values are saturated hydraulic conductivity Permeability A horizon E horizon Bh horizon Bs horizon Capillary Fringe • Based upon permeability • The zone above the free water table that is effectively saturated – Water held at tension – Theoretical values much higher than “real life” – Difficult to measure 5 Drainage Classes • • • • • • • Drainage Classes Excessively drained Somewhat excessively drained Well drained Moderately well drained Somewhat poorly drained Poorly drained Very poorly drained 6