Soil Formation and Development

• Soil formation and soil development are different processes.
• Soil formation refers to the creation of soil, specifically the factors and processes of formation from unconsolidated parent material.
• Soil development refers to the changes in the soil profile because of leaching, translocation of colloids, accumulation of organic matter, and continued rock and mineral weathering.
• Hans Jenny determined the five factors that control the formation of soils:
  • Parent material
  • Climate
  • Topography/relief
  • Biotic (vegetation and animals) factor or living organisms
  • Time
• The relationship of all the factors is: Soil = f (parent material) [p], climate [c], relief/topography [r], living organisms[l], time[t] or Soil = f (clorpt)

Parent Material

• Parent material determines both the mineralogy and the nutrition status of the soil.
• There are two categories of parent materials:
  • Residual/sedentary: Rocks and minerals that are original to a location that gave rise to soil via physical and chemical processes.
  • Transported:
    • Alluvial: Transported by rivers.
    • Colluvial: Transported by gravitational action.
    • Aeolian: Wind deposits.
    • Lacustrine: Lake deposits.
    • Marine: Sea or ocean bottom deposits.

Climate and Living Organisms

• Climate and living organisms are the active factors that determine the degree and rate of soil formation.
• The most important climatic parameters are temperature and precipitation.
  • Temperature changes cause expansion and contraction that can crack hard rock.
  • Temperature directly affects the amount of organic matter produced.
  • Organic matter production increases as temperature increases, provided there is rainfall for adequate plant growth.
• Rainfall affects weathering and the amount of organic matter production and decomposition.
• As rainfall increases, the possibility of erosion losses increases.
• When rainfall increases, organic matter production increases provided the temperature is high enough for plant growth.

Topography

• Topography, or relief, of the location of the parent material influences the effect of the active factors.
• Soil formation is less extensive in steep slopes where soil disturbance (erosion) and the degree of downward water flow are low.

Soil Genesis

• The study of soil formation is called soil genesis.
• Understanding soil genesis requires knowledge of the beginning of soil to interpret soils for specific uses.

Weathering

• Weathering is a chemical, physical, and biological process that results in the degradation of parent material and subsequent soil formation.
• It refers to the breakdown and changes in rocks and sediments at or near the Earth's surface by biological, chemical, and physical agents.
• Secondary minerals, like clay minerals, are synthesized during weathering.
• Weathering produces unconsolidated material (parent material) from which new soil is formed, provides plant nutrients, and results in the formation of secondary minerals.
• Rocks are the starting point for the weathering processes.

Basic Processes of Weathering

• Mechanical/Physical Process:
  • Temperature changes (expansion and contraction and the freezing and thawing of water).
  • Wind effects.
  • Force exerted by plant roots.
• Chemical Process
  • Hydrolysis: The breakdown of rock by acidic water to produce clay and soluble salts.
  • Hydration: The chemical bonds of a mineral change as it interacts with water.
  • Acid solution: Carbonic acid flows through cracks in rocks, causing some of the rock to dissolve.
  • Oxidation-reduction:
    • Oxidation: Process of oxidation rock breakdown occurs due to the disturbance caused by the addition of oxygen.
    • Reduction: When oxidized minerals are placed in an environment where oxygen is absent, reduction takes place.
  • Dissolution: Slightly acidic solutions on rocks create pits and holes and can act to enlarge and widen preexisting fractures.

Soil Development

• Soils are variable from one location to another, having different physical appearances and properties.
• These properties are a result of the chemical and physical processes of the environment.

Soil Horizon

• Soil horizon is a layer of soil roughly parallel to the land surface that differs from adjacent genetically related layers in physical, chemical, and biological properties.
• Soil Profile: A vertical section of soil through all of its horizons, extending into parent material.

Master Horizons

• O: Organic material at the surface, such as leaves, twigs, branches, and dead grass. Usually seen in forested soils and wetlands.
• A: The horizon where decomposed organic matter accumulates. It has a dark color and the area where most biological activity takes place. If the field is plowed this horizon extends to the depth of cultivation and becomes the plow layer.
• E: The zone of maximum removal or leaching, also eluviation. Maximum clay and chemical losses occur here, and is generally light-colored.
• B: Horizon where materials from the A and E horizons collect. Known as the zone of eluviation, containing maximum level of clay and chemicals.
• C: Layer of nearly unaltered mineral material; contains no biological activity and no eluviation. Considered the lower limit of the soil and called the soil's parent material.
• R: The underlying bedrocks, such as limestone, sandstone, or granite.

Processes Involved in Horizon Differentiation:

• Addition to soil
• Losses from the soil
• Translocation within the soil
• Transformations within the soil

Additions

• Water as precipitation, condensation, or runoff
• O2 and CO2 from the atmosphere
• N, Cl, and Sulfur from the atmosphere and precipitation
• Organic matter from biotic activities
• Materials from sediments
• Energy from the sun

Losses

• Water by evapotranspiration
• N by denitrification
• C as CO2 from oxidation of organic matter
• Soil by erosion
• Energy by radiation
• Water and material in solution or suspension

Translocations

• Clay, organic matter, iron oxides, and chemicals by water
• Nutrients circulated by plants
• Soluble salts in water
• Soils by animals

Transformations

• Decomposition of organic matter
• Reduced particle size by weathering
• Mineral transformations (primary to secondary)
• Clay and organic matter reactions

Soil Pedon

• Soil profiles do not give any information on irregularities in horizon thickness.
• A soil pedon is a three-dimensional soil body that represents the smallest volume, generally 1 to 10 meters across and hexagonal in shape.
• Polypedons are put together into groups known as soil series.
• There are over 14,000 soil series throughout the world, each with its own chemical and physical properties that affect management.

Diagnostic Horizons

• Are active in certain types of soil development and have specific soil characteristics indicative of certain classes of soils.
  • Epipedons: Diagnostic horizons that occur at the surface.
  • Diagnostic Subsurface Horizon: Diagnostic horizons below the surface.

Common Diagnostic Horizons

• Mollic horizon (A): Thick, dark-colored, good structure.
• Histic (O): Very high in organic matter content, wet during some part of the year.
• Oa: Most highly decomposed organic matter; very few fibers are identifiable.
• Oe: Intermediate degree of decomposition.
• Oi: Least degree of decomposition, with readily identifiable fibers.
• Argillic horizon (Bt): Silicate clay accumulation.
• Albic Horizon (E): Light colored; clay, Fe, and Al oxides most likely removed.
• Oxic Horizon (Bo): Highly weathered; primary mixture of Fe or Al oxide, rust-colored.
• Spodic Horizon (Bh): Organic matter, Fe and Al oxide accumulation.
• Calcic Horizon (Bk): Accumulation of CaCO3 or CaMgCO3.
• Duripan (Bqm): Hardpan, strongly cemented by silica.
• Fragipan (Bx): Brittle pan, usually loamy textured, dense.