Comprehensive Study Guide for Soil Surveys and Land Use Planning

Learning Objectives for Soil Surveys and Land Use Planning

• Comprehensive Goals:     * Achieve an understanding of soil texture and the soil classification system.     * Gain knowledge of the three phases of soil.     * Learn the specific characteristics of the soil profile and individual horizons.     * Understand the modern concept of precision agriculture.     * Understand the principles and processes of land use planning.

Introduction to Soil Science

• Societal Importance of Soil:     * People value soil because it is the primary support system for plants that provide food, fibers, and drugs.     * Soil functions as a natural filter for water.     * Soil plays a critical role in recycling wastes.

• Soil Distribution and Coverage:     * Soil exists as a continuum across the Earth’s surface.     * Exclusions to this continuum include areas of bare rock, regions of perpetual frost, deep water bodies, or the bare ice of glaciers.

• Defining Soil Thickness:     * In a practical sense, the thickness of soil is defined by the rooting depth of plants.

• Soil Formation Processes:     * Soil is formed through the physical or chemical breakdown of rock.     * The formation process is heavily dependent on two primary factors: temperature and the availability of water.

• Determinants of Soil Type:     * The specific soil type at any given location is determined by five key factors:         1. Climate.         2. Geologic parent material.         3. Biota and Organisms.         4. Topography and Landscapes.         5. Time.     * The influence of climate on soil type increases as time passes, which simultaneously reduces the initial influence of the parent material and the local topography.

Composition and Phases of Soil

• The Natural Body of Soil:     * Soil is defined as a natural body occupying space on the land surface comprised of:         * Solids: Consisting of minerals and organic matter.         * Liquid: Water.         * Gases: Air.

• Characterization Metrics:     * Soil is characterized by horizons (layers) that are distinguishable from the parent material due to additions, losses, transfers, and transformations of energy and matter.     * It is also characterized by its ability to support rooted plants within a natural environment.

• The Three Phases of Soil:     * Saturated Soil: Contains only water and solid particles.     * Dry Soil: Contains only air and solid particles.     * Moist/Wet Soil: Contains a mixture of all three components: water, air, and solids.

• Variability:     * Soil is highly complex and variable across both space and time.     * Chemical, biological, and physical properties differ significantly from one location to another.

Soil Surveys and Historical Context

• Functions of Soil Surveys:     * Surveys produce detailed maps and soil descriptions.     * They provide vital information regarding both the soil components and the surrounding landscapes.     * While soil is a critical part of land assessment, it is not the only factor considered in land management.

• Land Use Planning:     * Operates on various scales to optimize land use.     * Goal: Meet current societal needs while protecting land resources for future generations.

• Historical Timeline:     * Nations have historically used land use surveys to create inventories for national planning.     * Formal observations of soil differences began thousands of years ago, documented by scholars from Chinese, Greek, Hebrew, and Roman civilizations.

• The Work of Soil Surveyors:     * Surveyors study soils directly on the landscapes where they occur.     * Tasks include description, geo-encoding, classification, and mapping of soil types and properties.     * Chemical Testing: Surveyors use a $10\%$ Hydrochloric Acid ($HCl$) solution to detect the presence of calcium carbonate in soils and rocks.

• Statistical Scope in the United States:     * Surveyors have designated more than $100,000$ map units.     * Soils have been classified into more than $13,000$ soil series.     * Soil survey reports are generally made available on a county-by-county basis.

Soil Classification Systems

• Map Requirement: Soils must be classified before they can be effectively mapped.

• United States Classification Standards:     * The USDA-NRCS Soil Taxonomy system is used for categorical classification.     * The USDA-NRCS Soil Classification (Texture) system is used for physical property classification.

• Categorical Levels of Soil Taxonomy:     1. Order (Highest level).     2. Suborder.     3. Great Group.     4. Subgroup.     5. Family.     6. Series (Most specific level).

Detailed Descriptions of the Twelve Soil Orders

• Alfisols:     * Features: Moderately leached soils with relatively high native fertility. They have a subsurface horizon where clays have accumulated.     * Formation: Primarily under forest or mixed vegetative cover in semiarid to moist areas.     * Global/US Coverage: Makes up about $10\%$ or $10.1\%$ of the world’s ice-free land surface and $13.9\%$ of US land.

• Andisols:     * Features: Formed in volcanic ash or ejecta. Dominated by glass and short-range-order colloidal weathering products (allophane, imogolite, and ferrihydrite).     * Properties: High water- and nutrient-holding capacity; highly productive.     * Global/US Coverage: $1\%$ of the world’s ice-free surface; $1.7\%$ of US land.

• Aridisols:     * Features: Dry soils common in deserts with little organic matter. Too dry for mesophytic plants.     * Formation: Low moisture restricts weathering. Accumulate materials like gypsum, salt, and calcium carbonate.     * Global/US Coverage: $12\%$ of the world’s ice-free surface; $8.3\%$ of US land.

• Entisols:     * Features: Soils of recent origin showing little or no evidence of pedogenic horizon development. Usually only have an A horizon.     * Occurrence: Areas of recently deposited parent materials, dunes, steep slopes, or flood plains.     * Global/US Coverage: $16\%$ to $18\%$ of the world’s ice-free surface; $12.3\%$ of US land.

• Gelisols:     * Features: Soils of very cold climates containing permafrost within $2\,m$ of the surface. Show evidence of cryoturbation (frost churning) and ice segregation.     * Global/US Coverage: $9\%$ to $9.1\%$ of the world’s ice-free surface; $8.7\%$ of US land.

• Histosols:     * Features: Organic soils (bogs, moors, peats, mucks) consisting of plant remains. No permafrost.     * Behavior: Most are saturated year-round. If drained, they decompose rapidly and may subside dramatically.     * Global/US Coverage: $1\%$ to $1.2\%$ of the world’s ice-free surface; $1.6\%$ of US land.

• Inceptisols:     * Features: Soils of semiarid to humid environments with weak to moderate horizon development. Often limited by cold, waterlogging, or lack of time.     * Global/US Coverage: $15\%$ to $17\%$ of the world’s ice-free surface; $9.7\%$ of US land.

• Mollisols:     * Features: Deep, dark-colored surface horizons rich in organic matter. They are base-rich and highly fertile.     * Formation: Characteristically form under grass in climates with seasonal moisture deficits (e.g., steppes).     * Global/US Coverage: $7\%$ of the world’s ice-free surface; $21.5\%$ of US land.

• Oxisols:     * Features: Highly weathered soils of tropical/subtropical regions. Rich in iron ($Fe$) and aluminum ($Al$) oxide minerals with indistinct horizons.     * Properties: Stable over long periods; low natural fertility and low capacity to retain lime/fertilizer.     * Global/US Coverage: $7.5\%$ to $8\%$ of the world’s ice-free surface; $0.02\%$ of US land.

• Spodosols:     * Features: Stripped of organic matter combined with aluminum (and sometimes iron) from the surface and deposited in the subsoil.     * Appearance: Light gray eluvial horizon over a reddish-brown or black subsoil.     * Occurrence: Coarse-textured deposits under coniferous forests in humid, cool regions. Acidic and infertile.     * Global/US Coverage: $4\%$ of the world’s ice-free surface; $3.5\%$ of US land.

• Ultisols:     * Features: Extensively weathered acidic soils in humid areas. Subsoil is clay-enriched and dominated by quartz, kaolinite, and iron oxides.     * Properties: Nutrients concentrate in the upper few inches; low capacity to retain lime/fertilizer.     * Global/US Coverage: $8\%$ to $8.1\%$ of the world’s ice-free surface; $9.2\%$ of US land.

• Vertisols:     * Features: High content of expanding clay minerals. Undergo volume changes with moisture, causing cracks to open and close.     * Behavior: Swell when wet, transmit water very slowly; fairly high natural fertility.     * Global/US Coverage: $2\%$ to $2.4\%$ of the world’s ice-free surface; $2\%$ of US land.

Soil Profile and Master Horizons

• The Soil Profile Structure:     * O Horizon: The top organic layer. Composed mostly of leaf litter and humus (decomposed organic matter).     * A Horizon (Topsoil): Located below O and above E. Dark color due to mixed humus and minerals. Where seeds germinate and roots grow.     * E Horizon (Eluviation layer): Light-colored leaching layer beneath A and above B. Primarily sand and silt; minerals and clay are lost as water drips through.     * B Horizon (Subsoil): Located beneath E and above C. Contains clay and mineral deposits (iron, aluminum oxides, calcium carbonate) received from upper layers via mineralized water.     * C Horizon (Regolith): Beneath B and above R. Consists of slightly broken-up bedrock. Very little organic material; plant roots do not penetrate here.     * R Horizon (Bedrock): The unweathered rock layer beneath all other horizons. Can be igneous, sedimentary, or metamorphic.

• Standard Horizon Usage:     * Most soils contain three major horizons: A, B, and C.     * Some have an O horizon on the surface (which can sometimes be buried).     * The E horizon is specifically designated for horizons showing significant mineral loss.

Soil Texture and Measurement

• Definition of Soil Texture:     * Texture is determined by the specific proportions of sand, silt, and clay.     * Particles larger than sand (> 2\,mm) are removed before determination.

• Particle Size Classifications (Grain Diameter $d$):     * Clay: $d \le 0.002\,mm$     * Silt: 0.002 < d \le 0.05\,mm     * Sand: 0.05 < d \le 2\,mm     * Gravel: d > 2\,mm

• Terminology for Large Particles:     * If gravel or larger particles comprise more than $15\%$ of the soil, adjectives like "gravelly" or "stony" are added to the texture name.

• Determination Methods:     * Grain-size Distribution: Often presented as a cumulative-frequency plot mapping grain diameter against the weight fraction of grains with smaller diameters.     * Sieve Analysis: Used for particles larger than $0.05\,mm$.     * Sedimentation: Used for measuring grain sizes smaller than $0.05\,mm$.