A generalised soil profile is a vertical cross-section of the soil, revealing distinct layers called horizons. These horizons are differentiated based on their physical, chemical, and biological properties. The primary distinction lies between organic horizons, dominated by decomposed plant and animal matter, and mineral horizons, composed mainly of weathered rock material.

Organic Horizons (O Horizon):

• Composition: Primarily composed of organic matter in various stages of decomposition. This includes fresh litter (leaves, twigs, dead plants, animal remains) at the surface, partially decomposed organic material (fibric, hemic), and highly decomposed, stable organic matter (humus).
• Location: Typically found at the very top of the soil profile, above the mineral horizons. It is most prominent in forest soils but can also be found in grasslands and wetlands.
• Color: Usually dark brown to black due to the high concentration of organic carbon.
• Structure: Often loose, crumbly, and porous, especially in the upper parts. It lacks the distinct granular or blocky structures common in mineral horizons.
• Density: Generally has a very low bulk density compared to mineral horizons because of its high organic content and porosity.
• Water Retention: Possesses a very high water-holding capacity due to the spongy nature of organic matter.
• Nutrient Content: Acts as a significant reservoir of essential plant nutrients (nitrogen, phosphorus, sulfur) released through decomposition.
• Biological Activity: Teems with a diverse array of microorganisms (bacteria, fungi), invertebrates (earthworms, insects), and plant roots, which are crucial for decomposition and nutrient cycling.
• Formation: Formed by the accumulation and decomposition of plant and animal residues on the soil surface.

Mineral Horizons (A, E, B, C Horizons): Mineral horizons are primarily composed of weathered rock fragments and secondary minerals, with varying amounts of organic matter mixed in.

• A Horizon (Topsoil):

  • Composition: A mixture of mineral particles (sand, silt, clay) and significant amounts of decomposed organic matter (humus) that has been incorporated into the mineral matrix.
  • Location: Directly beneath the O horizon (if present) or at the surface in cultivated soils.
  • Color: Darker than underlying mineral horizons due to humus content, but generally lighter than the O horizon.
  • Structure: Often granular or crumbly, indicating good aggregation and biological activity.
  • Biological Activity: High biological activity, including roots, earthworms, and microorganisms, which mix organic matter with mineral particles.
  • Nutrient Content: Generally fertile due to the presence of both mineral nutrients and organic matter.

• E Horizon (Eluvial Horizon):

  • Composition: Primarily mineral particles, characterized by the loss (eluviation) of clay, iron, aluminum oxides, and organic matter due to leaching.
  • Location: Found below the A horizon in some soils, particularly in forested areas with high rainfall.
  • Color: Lighter in color (often grayish or whitish) than the A and B horizons because of the removal of darker materials.
  • Structure: Often platy or single-grained.
  • Nutrient Content: Generally lower in nutrients due to leaching.

• B Horizon (Subsoil / Illuvial Horizon):

  • Composition: Dominated by mineral particles, characterized by the accumulation (illuviation) of materials leached from above, such as clay, iron and aluminum oxides, and sometimes humus or carbonates.
  • Location: Below the A or E horizon.
  • Color: Often reddish, yellowish, or brownish due to the accumulation of iron oxides. Can be lighter if carbonates accumulate.
  • Structure: Typically blocky, prismatic, or subangular blocky, indicating denser packing and less biological mixing.
  • Density: Higher bulk density than A or O horizons.
  • Nutrient Content: Varies depending on the accumulated materials; can be a source of some nutrients but often less fertile than the A horizon.

• C Horizon (Parent Material):

  • Composition: Unconsolidated, weathered rock material from which the overlying soil horizons developed. It has undergone little to no soil-forming processes.
  • Location: Below the B horizon.
  • Color: Reflects the color of the parent material.
  • Structure: Massive or single-grained, lacking distinct soil structure.
  • Biological Activity: Very little biological activity.
  • Nutrient Content: Reflects the mineral composition of the parent material.

In summary, organic horizons are defined by their high organic matter content, dark color, and intense biological activity, serving as a nutrient and water reservoir. Mineral horizons, conversely, are characterized by their mineral composition, varying colors and structures based on weathering and accumulation processes, and generally lower organic content, forming the bulk of the soil's structural framework.