Concepts of Basic Soil Science Concepts

Soil Composition and Formation

• Volume Composition: An ideal surface soil consists of $50\%$ solid material ($45\%$ to $48\%$ mineral, $2\%$ to $5\%$ organic matter) and $50\%$ pore space ($25\%$ air, $25\%$ water).
• Weathering Processes: Soil forms from parent material via physical weathering (freeze-thaw) and chemical weathering (carbonic acid reacting with minerals).
• Leaching: Water carries fine clay particles ($< 0.002\,mm$) from surface layers to subsoil, making surface textures coarser over time.

Soil Horizons and the Profile

• Master Horizons:
  • O Horizon: Organic surface layer (forest soils).
  • A Horizon: Mineral surface soil, high in organic matter; called Ap if plowed.
  • E Horizon: Light-colored, eluviated zone of maximum leaching.
  • B Horizon: Subsoil; zone of accumulation (iron-coated clays, etc.); described as Bt if high in clay.
  • C Horizon: Partially weathered parent material (e.g., saprolite).
  • R Layer: Hard bedrock.

Soil Physical Properties

• Texture: Determined by relative amounts of sand ($0.05\,mm$ to $2.0\,mm$), silt ($0.002\,mm$ to $0.05\,mm$), and clay ($< 0.002\,mm$).
• USDA Textural Triangle: Used to define $12$ primary classes (e.g., clay loam) based on particle percentages.
• Structure/Aggregation: Soil particles group into peds including Granular, Blocky, Platy, Prismatic, or Structureless (Massive/Single Grain).
• Porosity: The volume percentage of soil not occupied by solids. Macropores ($> 0.05\,mm$) facilitate air/water movement, while micropores ($< 0.05\,mm$) retain water.

Soil Organic Matter (SOM)

• Components: Plant/animal residues and stable humus.
• Influencing Factors: Higher in grassed areas, poorly drained soils, and fine-textured soils; lower in tilled or sandy soils.
• Benefits: Improves aggregation, water infiltration, and nutrient supply through mineralization.

Soil-Water Relationships

• Field Capacity: Water remaining after gravitational drainage stops.
• Permanent Wilting Percentage: Threshold where plants can no longer extract water.
• Plant-Available Water: The interval between field capacity and permanent wilting percentage.
• Drainage and Color: Bright red/yellow indicates well-drained (oxidized) conditions; gray/mottled colors indicate saturation and anaerobic (reduced) conditions.

Soil Chemical Properties

• pH: Logarithmic measure of $H^+$ activity ($0$ to $14$). Mid-Atlantic soils often range $5.5$ to $6.5$.
• Cation Exchange Capacity (CEC): The soil's ability to hold and exchange positively charged ions ($Ca^{2+}, Mg^{2+}, K^{+}, Na^{+}, NH_4^{+}, Al^{3+}, H^{+}$). Units are $\text{cmol}+/\text{kg}$ or $\text{meq}/100\,g$.
• Adsorption Strength: Cations are held with varying strength: $Al^{3+} \gg Ca^{2+} > Mg^{2+} > K^{+} = NH_4^{+} > Na^{+}$.
• Base Saturation: Percentage of CEC occupied by basic cations ($Ca, Mg, K, Na$). High saturation ($> 50\%$) prevents pH decline.
• Buffering Capacity: Resistance to pH change, which increases with higher clay and organic matter content.

Soil Survey and Mapping

• Purpose: Conducted by USDA-NRCS to map soil types and interpret usage for agriculture, conservation, and engineering.
• Key Terminology:
  • Soil Series: Basic unit of classification.
  • Soil Phase: Subdivision of a series based on features like slope or erosion.
  • Map Units: Delineated areas on maps, generally named for the dominant soil series and slope phase.