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Soils Basics 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 Nonsoil • • • • • • Badlands Beaches Rubble lands Rock outcrops Glaciers Deepwater habitats Factors That Influence Soil Development • • • • • • Climate Parent material Topographical relief Organisms Time Humans Landscape Position • Critically influences water flow and soil formation • Most wetlands, even groundwater seeps, are on some sort of concave surface Hill Slope Elements and Curvature After Pennock et al., 1987 Slope Block Overland and Throughflow: Convergent landscapes Contour Upslope Potential hydric soil zone Runoff Infiltration Percolation Divergent Upslope Convergent Throughflow “Epiaquic” “Endoaquic” Modified from Pennock et al., 1987 Soil Catena Summit Backslope Footslope Toeslope Key Soil Properties Properties that are important to hydric soil development and recognition: • • • • • • Color Organic matter Texture Horizonation Drainage Permeability Aspects of Soil Color • Hue • Value • Chroma 10R 5/8 Coloring Agents in Soil • Organic matter – OM will mask all other coloring agents. • Iron (Fe) – brown colors are the result of Fe oxide stains coating individual particles • Manganese (Mn) – resulting in a very dark black or purplish black color • Lack of coatings – Color of the mineral soil grains (stripped) Soil Color Coating of Fe2O3 Mineral grain (gray) Remove Fe Bright Gray Soil Soil “Bright-colored” soil is bright because the gray-colored mineral grains are coated with a thin layer of “paint” formed by Fe oxides. Stripping the paint off the particles leaves the mineral grains exposed. Hue Red 0 2.5R 5R 7.5R 10R Yellow-Red 0 2.5YR 5YR 7.5YR 10YR Yellow 0 2.5Y 5Y 7.5Y 10Y Value • 10/0 - Pure White The Lightness or Darkness of Color • 5/0 - “Gray” • 0/0 - Pure Black Chroma “Neutral” Color /0 “Pure” Color /2 /4 /6 Increasing strength of color Increasing grayness /8 What is important? • Chroma is most important, with value close behind • Hue is also important – Gley page hue is present – Red parent material soils – Gray parent materials Soil Color Munsell Soil Color Book, 7.5YR page Munsell® or Earth Colors ® • The Munsell notation system is a system for recording color. • The Earth Colors and Munsell books use the same colors. – Differences are from personal interpretation of color, fading of pages over time, smudging of chips with use, and quality control from the factories. Reading Soil Colors • Optimum conditions – – – – – Natural light Clear, sunny day Midday Light at right angles Soil moist Color Patterns in Soils • Matrix (predominant) color • Color of redoximorphic features (mottles) • Contrast, abundance, location, and size of redox features Redox Feature Matrix Contrast • Contrast refers to the degree of visual distinction between associated colors – Faint -- evident only on close examination – Distinct -- readily seen at arms length – Prominent -- contrast strongly Several indicators require distinct or prominent contrast! Defining Distinct Redox • Different Munsell page (hue), OR • Value differs by more than 2 (i.e. 10YR 3/6 vs 10YR 6/6), OR • Chroma differs by more than 1 (i.e. 10YR 4/4 vs 10YR 4/6) Abundance and Size of Redox • Few -- less than 2% • Common -- 2 to 20% • Many -- more than 20% • Fine -- < 5 mm • Medium -- 5 to 15 mm • Coarse -- > 15 mm Several indicators require at least 2% abundance From Munsell Color Chart & Field Book Two Categories of Soil Material Differences in organic matter content: • Organic horizons • Mineral horizons 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 Two Categories of Soil Material Mineral horizons • Primarily sand, silt, and clay, with varying amounts of organic matter Soil Texture - Relative Sizes of Soil Particles Sand (0.05-2.00 mm) Silt (0.002-0.05 mm) Clay (<0.002 mm) Soil Texture Triangle Loamy Sands? • • • • • 70-87% sand, 0-30% silt, and 0-15% clay, loose and single-grained, most individual grains can be seen and felt, are slightly cohesive when moist, and fragile casts can more readily be formed with them vs. sands Specifically, What Kind of Loamy Sand? • Loamy fine sand. 50 percent or more* fine sand; or less than 50 percent* very fine sand and a total of less than 25 percent* very coarse, coarse, and medium sand. OR • Loamy very fine sand. 50 percent or more* very fine sand. • *by weight, not volume How to Know… • • • • • Accurate texturing procedure Soil survey information may help Fine sands are slightly gritty Very fine sands will feel smooth Sieve analyses • - If >50% (by weight of total soil) lvfs, then loamy • - If <50% (by weight of total soil) lfs, then sandy Texturing Standards Crops and Soils Club Phone: 715-425-3395 Department of Plant & Earth Science University of WI – River Falls 410 South Third St., River Falls, WI 54022-5001 • A. Texture kit contains samples of 10 soils, listing percentage of sand, silt and clay and includes a soil textural triangle. – $25. • B. Sand-size card – Samples of USDA vfs, fs, s, cos, vcos mounted on a 4 1/2 x 7 1/4 6-hole punch card. $8.00 Major Horizon Designations Surface Organic Layer O horizon Surface Mineral Layer A Horizon Subsoil B Horizon Underlying Material C Horizon Bedrock R Horizon Horizon Terminology Oi Oe Oa Ap A1 A2 A E AB Surface Layer Subsurface Layers Surface Soil BA Transition Layer Bt Subsoil AC Layer of Accumulated Lime Bk BC C1 Underlying Material C2 Bedrock R Cr 2C A horizon Bw horizon C horizon A horizon E horizon Bh horizon Bs horizon 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 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 Capillary Fringe Largest Pores Not Filled Saturated All Pores Filled with Water