Characterizing Soil Horizons Physical Properties of Soils

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

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

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

Criteria for Characterizing Soil Horizons ColorTextureDensityStructure Organic matter MineralogyChemistry

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

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

Physical Criteria for Delineating Horizons ColorTextureStructureDensity

The Soil Mineral Component: Texture the size of soil particles

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

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

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

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

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.

Sands Clays/iron Rapid Slow Water Retained Poor Retention

Surface Area and Particle Size

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

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

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

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

Soil Horizons Texture Clay Content Surface Area Potential Reactivity

Soil Textural Classes

Soil Textural Triangle

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

70% sand, 20% silt, 10% clay

60% sand, 10% silt, 30% clay

Measuring Soil Texture

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

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

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

Laboratory Analysis of Soil Texture

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

Sand Silt Clay Sand Silt Clay sand silt Sedimentation

Quantifying Sedimentation Rates

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

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 = cm/sec 1 cm · sec X cm 2

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

sand silt clay

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.

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

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

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

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

Reactivity, Water Movement

Next: Density, Structure, Porosity