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Characterizing Soil Horizons Physical Properties of Soils

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Solids Voids Avenues Storage Distribution Movement Interactive Media Minerals Organic matter Reactivity Idealized Surface Soil

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Parent Material to Soil Parent Material Bedrock Additions Losses Translocations Transformations Bedrock Differentiation

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Soil Horizons Roughly parallel layers in the soil with varying composition and properties

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Criteria for Characterizing Soil Horizons ColorTextureDensityStructure Organic matter MineralogyChemistry

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Value Chroma Hue Soil Color dominant spectral color; related to the wavelength of light. Related to the proportions of red to yellow. related to total amount of light reflected. measure of the strength of spectral color

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Hue = 10 YR Value = 6 Chroma = 3 Munsell Color 10 YR 6/3

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Physical Criteria for Delineating Horizons ColorTextureStructureDensity

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The Soil Mineral Component: Texture the size of soil particles

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Soil texture refers to the relative amounts of three distinct size separates comprising the soil mineral component. SandSiltClay Sizes classes of particles

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Soil Texture Diameter Sand (2.0 – 0.05 mm)Quartz Silt (0.05 – 0.002 mm)Quartz /Feldspars/mica Clay (<0.002 mm)Secondary minerals Dominant Minerals sandsiltclay 100% Quartz Distribution Class

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Importance of Soil Texture Soil Porosity Particle Surface Area (Distribution of particle sizes) Water/Gas Movement Reactivity

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Soil Porosity Porosity – the total volume of soil pores - the distribution of pore sizes Sand Silt Clay

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Texture, Pore Sizes, and Water Large particles yield large pore spaces Small particles yield small pore spaces Water moves rapidly and is poorly retained in Coarse-textured sandy soils. Water moves slowly and is strongly retained in Fine-textured, clayey soils.

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Sands Clays/iron Rapid Slow Water Retained Poor Retention

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Surface Area and Particle Size

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Each face is 4 mm 2 Each face is 1 mm 2 6 faces x 4 mm 2 = 24 mm 2 6 faces x 1mm 2 x 8 cubes = 48 mm 2 If each of the resulting cubes was divided similarly, the surface area would increase 16 times more 2 mm 2 mm 2 mm 1 mm

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Surface Area Specific Surface Area = Surface Area mass cm 2 g units Interface with the environment nutrients gasses O.M. water microorganisms

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Specific Surface Area Separate SSA (cm 2 /g) Sand 30 Silt 1500 Clay3,000,000 16 g of clay

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Soil A 100 g soil Soil B 95 g sand 4 g silt 1 g clay 90 g sand 5 g silt 5 g clay 95 g sand x 30 g/cm 2 = 2850 cm 2 4 g silt x 1500 g/cm 2 = 6000 cm 2 1 g clay x 3 M g/cm 2 = 3 M cm 2 Total = 3,008,850 cm 2 90 g sand x 30 g/cm 2 = 2700 cm 2 5 g silt x 1500 g/cm 2 = 7500 cm 2 5 g clay x 3 M g/cm 2 = 15 M cm 2 Total = 15,010,200 cm 2

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Soil Horizons Texture Clay Content Surface Area Potential Reactivity

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Soil Textural Classes

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Soil Textural Triangle

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Sand<10% Loamy sand10 – 15% Sandy loam15 – 20% Sandy clay loam20 – 35% Sandy clay35 – 55% Clay> 50% Florida Soils clay

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70% sand, 20% silt, 10% clay

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60% sand, 10% silt, 30% clay

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Measuring Soil Texture

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Texture-by-Feel Relative amounts of 3 soil separates: Sand, Silt, and Clay Grittysmooth plastic

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Texture-by-Feel Soil Sand No Ball Loamy Sand No Ribbon Grittiness, Smoothness Low Clay Medium Clay High Clay Increasing Ribbon Length Field Analysis

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Texture by Feel Sand = Gritty Silt = Smooth Clay = Sticky, Plastic

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Laboratory Analysis of Soil Texture

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Laboratory Analysis Sedimentation – Sand, Silt, and Clay Fraction gravity drag

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Sand Silt Clay Sand Silt Clay sand silt Sedimentation

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Quantifying Sedimentation Rates

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Stokes’ Law Velocity V(cm/s) = g (d p -d L ) D 2 18ų g = gravity d p = density of the particle d L = density of the liquid ų = viscosity of the liquid K V = D 2 K = 11,241 cm -1 sec -1 1 cm · sec

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Stokes’ Law V = K D 2 K = 11,241 cm -1 sec -1 Sand:D = 1 mm0.1 cm V = 11,241 x (0.1) 2 = 112.4 cm/sec 1 cm · sec X cm 2

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Stokes’ Law V = K D 2 K = 11,241 cm -1 sec -1 clay: D = 0.002 mm0.0002 cm V = 11,241 x (0.0002) 2 = 0.00045 cm/sec 1 cm · sec X cm 2

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sand silt clay

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Sedimentation 1 minuteSand settles out 4 hoursSilt settles out sand 1 min. silt 4 hr. suspension The density of a soil suspension decreases as particles settle out.

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Stem Scale Bulb weight t = 0 t = 1 min Hydrometer Method hydrometer 0 g/L Pure distilled water (18 o C) = 0 g/L

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Hydrometer Method Time = 0 secdensity = 40 g/L Add 40 g soil to 1 liter of water Time = 1 min. density = 10 g/L Sand settled = 40 g– 10 g= 30 g Sand (%) = 30 g sand = 0.75 = 75% 40 g soil

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Time = 4 hrs density = 4 g/L Hydrometer Method What is being measured? Clay content = 4 g Silt % = 100% - (75% + 10%) 100% - 85% = 15 % Clay % = 4 g clay = 0.10 = 10% 40 g soil

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Sand = 75% Silt = 15% Clay = 10% Sandy Loam Hydrometer Method

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Reactivity, Water Movement

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Next: Density, Structure, Porosity

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Chapter 4: Soil Architecture and Physical Properties

Chapter 4: Soil Architecture and Physical Properties

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