Lecture 5: Introduction to Soil Classification; Soil Physical Properties

Lecture 5: Introduction to Soil Classification; Soil Physical Properties

Soil Classification

Argillic Horizon Alfisols Moderately leached soils with a clay-rich B-horizon Typically found in forests Andisols Dominated by short-rangeorder minerals Form in volcanic ash Aridisols Dry and often saline soils Form in arid regions

Histic Epipedon Entisols Little or no evidence of pedogenesis Often sandy or shallow Gelisols Contain permafrost Found in arctic and high alpine regions Histosols Dominantly organic soil material, no permafrost Wetlands, bogs, marshes

Mollic Epipedon Oxic Horizon Inceptisols Soils with altered horizons but no accumulations Young soils, still developing Mollisols Dark surface layer rich in base cations Found in grasslands Oxisols Highly leached soils with iron and aluminum oxides Found in tropical regions

Albic Horizon Spodic Horizon Argillic Horizon Argillic Horizon Spodosols Accumulation of Fe/Al and OM below leached horizon Found in coniferous forests Ultisols Highly leached soils with a clay-rich B-horizon Found in warm, humid regions Vertisols High content of swelling clay; deep cracks when dry Shrink-swell behavior

Soil Orders Vary with the Degree of Weathering

Hypothetical Soil Development Sequence Entisol Inceptisol Alfisol Ultisol Oxisol Increasing Age, Increasing Soil Development, Increasing Weathering

Process of Assigning Soil Orders

Official Definitions More Complex

Lower Levels of Classification Suborder: Soils of an order having a unique property (moisture or temperature regime) Great Group: Soils of a suborder having a type of diagnostic horizon or other feature Subgroup: Soils of a great group sharing another common feature Family: Soils of a subgroup sharing common soil properties Series: Soils of a family sharing all major characteristics, including depth of horizons

Order Suborder Great Group Subgroup Family Series

Soil Series All the soil individuals in the world with a common suit of soil profile properties and horizons are called a soil series Generally unique to a state; rarely global 25,000 soil series named in US MENFRO SERIES The Menfro series consists of very deep, well drained, moderately permeable soils formed in thick loess deposits on upland ridgetops, backslopes and benches adjacent to the Missouri and Mississippi Rivers and their major tributaries. Slopes range from 2 to 60 percent. Mean annual temperature is 56 degrees F, and mean annual precipitation is 36 inches. TAXONOMIC CLASS: Fine-silty, mixed, superactive, mesic Typic Hapludalfs

Taxonomic Class A soil s taxonomic class is a descriptive term that covers all of the basic features of the soil Example: Fine-silty, mixed, superactive, mesic Typic Hapludalfs Fine silty texture in the upper mineral surface horizon Mixture of clay types with high cation exchange capacity Average soil temperature of 8 to 15 C Typic means a standard Hapludalfs Hapludalfs: A moist (-ud-) Alfisol (-alfs) with no other special features (hapl-)

Key Concepts in Soil Classification A system has been created to objectively classify soils This system relies of characteristics of the soil, including diagnostic horizons and physical and chemical properties There are 12 major soil orders: Know these! There are many lower levels of classification The soil series is the most important level for understanding soils at a specific location

Soil Physical Properties

Important Physical Properties Soil Color: Most obvious physical property; provides clues about chemical conditions Soil Texture: Size distribution of soil particles Sand, silt, or clay size particles Proportions of each define 12 soil textural classes Soil Structure: Arrangement of sand, silt, clay, and organic matter in soils Spheroidal, platelike, blocklike, prismlike Soil Density: Particle and bulk density, controlled by texture and porosity

Color

Soils Display a Wide Range of Colors

Munsell Color System Hue: General color Value: Lightness or darkness Chroma: Intensity of color Determined by comparison with color chips in Munsell Soil Color Book

Hue: General color Value: Lightness or darkness Chroma: Intensity of color Munsell Color System

OM Produces Dark Soils

Soils are Typically Darker when Wet

Color Indicates Iron Oxidation State Fe +3 Fe +2 Fe +3 Fe +2

Gleyed Soils